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Lecture 3.2 Conditional Execution Lecture Overview 1. Control Flow 2. Selection Statements 3. Conditional Execution Pitfalls 2 1. Control Flow 3 Control Flow ◎ Control flow describes the order in which the statements making up a program are executed. ◎ So far all of the Python code we've lo...

Lecture 3.2 Conditional Execution Lecture Overview 1. Control Flow 2. Selection Statements 3. Conditional Execution Pitfalls 2 1. Control Flow 3 Control Flow ◎ Control flow describes the order in which the statements making up a program are executed. ◎ So far all of the Python code we've looked at has had sequential control flow. ◎ However, there are a few different control structures which can result in different kinds of control flow. 4 Control Flow: Sequence (or sequential) ◎ The most basic control structure is a sequence of statements. ◎ Represented by a series of process/input/output elements in a flowchart. ◎ Statements in a sequence are executed in order of appearance. Statement 1 ◎ There is only one possible path for control flow. Statement 2 Statement 3 5 Control Flow: Sequence (or sequential) Example Program: Temperature Converter Start Input number Multiply by 9 Divide by 5 Add 32 Output number # Input tc = input('Enter Celsius: ') x = float(tc) # Processing x = x * 9 # Multiply by 9 x = x / 5 # Divide by 5 x = x + 32 # Add 32 # Output tf = str(x) print('Fahrenheit: ' + tf) Stop 6 Control Flow: Selection ◎ In a selection control structure, a condition determines which statements are executed. ◎ A selection introduces multiple paths for control flow to take, from which one will be selected. ◎ Represented by a decision element in a flowchart. yes no Condition Statement 1 Statement 2 ◎ This allows the program to make a decision. 7 Check Your Understanding Q. Does the flowchart show a sequence or selection control structure? Start Input number Multiply by 20 Divide by 4 Add 10 Output number Stop 8 Check Your Understanding Q. Does the flowchart show a sequence or selection control structure? A. Sequence. There is only one possible path for control flow to take. Start Input number Multiply by 20 Divide by 4 Add 10 Output number Stop 9 Visualising Control Flow ◎ Drawing the control flow over a flowchart can be helpful when reasoning about a program's logic. ◎ When drawn in this way, control flow is a single path from the start element to the stop element. ○ The control flow never splits. ○ When a decision element is encountered, one branch is selected. 10 Control Flow Example: Selection Start 1. Begin at the start element. 2. The first condition determines which path the control flow will take. 3. The condition is not met, so the “no” path is taken. 4. The second condition is met, so the “yes” path is taken. 5. The control flow continues to the stop element. Let age = 78 age < 12? yes no Display “Child” age > 65? no yes Display “Senior” Display “Done.” Stop 11 Check Your Understanding Q. What is the correct control flow for this flowchart? Start Let x = 10 x >= 8? yes no Display “Sad” Display “Happy” Stop 12 Check Your Understanding Q. What is the correct control flow for this flowchart? A. See image. ◎ Control flow can't split. ◎ The condition x >= 8 is met, so the "yes" branch is taken. Start Let x = 10 x >= 8? yes no Display “Sad” Display “Happy” Stop 13 2. Selection Statements 14 Selection Statements ◎ Selection statements are used to define selection control structures. ◎ When coding in Python, indentation MUST be used to group statements under a particular selection statement. 15 Selection Statements ◎ Python has three main selection statements: ○ if statements, which begin a selection control structure by defining a sequence of statements to execute if a condition is met. ○ elif and else statements, which optionally define alternative code paths (or "branches") in the selection control structure. 16 Simple Conditional Execution (if) if Condition: Statement ◎ If the condition is truthy: ○ Execute the statement(s) in the if block. yes Condition no Statement ◎ If the condition is falsy: ○ Do nothing. 17 Example: Simple Conditional Execution Task definition Write a program which subtracts 40% tax from a person's income when their pre-tax income is above $100,000. Display the result. ◎ Input: pre-tax income. ◎ Output: post-tax income. ◎ If pre-tax income is greater than $100,000, then the processing steps are: ○ Calculate tax as 40% of the pre-tax income. ○ Subtract tax from the income. 18 Example: Simple Conditional Execution # File: tax1.py income = float(input('Enter income: $')) if income > 100000: tax = 0.4 * income income = income - tax print(income) ◎ Remember: indentation is important! ◎ If the condition evaluates to: ○ True, then the indented statements are executed. ○ False, then the indented statements are skipped. 19 Conditional Execution - Multiple if statements 20 Conditional Execution - Multiple if statement 21 Conditional Execution - Nested if statements 22 Alternative Execution (if-else) if Condition: Statement 1 else: Statement 2 ◎ If the condition is truthy: ○ Execute the statement(s) in the if block. ◎ If the condition is falsy: ○ Execute the statement(s) in the else block. yes Statement 1 Condition no Statement 2 23 Alternative Execution (if-else) 24 Alternative Execution (if-else) 25 Example: Alternative Execution # File: tax2.py income = float(input('Enter income: $')) if income > 100000: tax = 0.4 * income income = income - tax else: print('No tax applied.') print(income) ◎ Using alternative execution, we can extend our tax program to inform the user when no tax is applied. 26 Chained Conditionals (if-elif-else) if Condition 1: Statement 1 elif Condition 2: Statement 2 else: Statement 3 ◎ elif is a contraction of "else if". ◎ The else clause is optional. ◎ Multiple elif clauses are allowed. 27 Example: Chained Conditionals # File: tax3.py income = float(input('Enter income: $')) if income > 100000: tax = 0.4 * income income = income - tax elif income > 50000: tax = 0.3 * income income = income - tax print(income) ◎ Using chained conditionals, we can extend our original tax program to include a 30% tax bracket. 28 Chained Conditionals (if-elif-else) 29 Chained Conditionals (if-elif-else) 30 Chained Conditionals ◎ Chained conditionals introduce the possibility of having many different branches in the same selection control structure. ○ Only one of these branches will be executed. ◎ If multiple conditions could be satisfied, control flow will go through the branch which appears first. ○ In fact, once a branch is taken the rest of the control structure is skipped---the remaining conditions aren't even evaluated. 31 Check Your Understanding Q. Which print statement will never be executed, regardless of the input? x = float(input()) if x > 25: print('Big') elif x > 50: print('Huge') elif x > 5: print('Medium') else: print('Small') print('Done.') 32 Check Your Understanding Q. Which print statement will never be executed, regardless of the input? A. print('Huge'). Any value of x that could satisfy x > 50 must also satisfy x > 25. Since x > 25 is checked first, print('Huge') can never be executed. x = float(input()) if x > 25: print('Big') elif x > 50: print('Huge') elif x > 5: print('Medium') else: print('Small') print('Done.') 33 3. Conditional Execution Pitfalls 34 Mistake #1: Indentation ◎ Indentation is important---incorrect indentation is a common beginner mistake! ◎ You must use consistent indentation. ○ I strongly recommend using 4 spaces. ◎ Python uses indentation to group statements into blocks. 35 Indentation Rules ◎ Increase indentation after if, elif, and else statements. ◎ Maintain indentation for all statements grouped under the if, elif, or else statement. ◎ Reduce indentation to end the if, elif, or else block. ◎ Blank lines and comment-only lines ignore indentation rules. 36 Example: Incorrect Indentation >>> if x > 2: ... x = x / 2 ... else: ... x = x + 5 File "<stdin>", line 4 x = x + 5 ^ IndentationError: expected an indented block 37 Check Your Understanding Q. Will this Python script result in an error? If not, what is the expected output? 1 2 3 4 5 6 7 price = 90 if price > 100: print('Discount') price = price - 2 print(price) 38 Check Your Understanding Q. Will this Python script result in an error? If not, what is the expected output? A. No error, output 88. ◎ Line 4 is not executed (condition is not true). ◎ Lines 5 and 7 are executed, since they are not part of the conditional (indentation reduced). 1 2 3 4 5 6 7 price = 90 if price > 100: print('Discount') price = price - 2 print(price) 39 Mistake #2: Empty Blocks ◎ An if, elif, or else statement must be followed by at least one indented statement. ○ Comments do not count! Beware! The following program will result in an error: x = 5 if x > 10: # Do nothing... else: print('Small x') 40 The pass Keyword ◎ If you really want to do nothing in a block, use the pass keyword. x = 5 if x > 10: # Do nothing... pass else: print('Small x') ◎ In Python, pass is a statement which does nothing. 41 Mistake #3: Missing Variable Definitions ◎ Take care when creating variables in a conditional and using them afterwards. ◎ If you create (define) a variable in one path, but the control flow does not take that path, the variable will not be defined. ○ Trying to use that variable afterwards will result in an error. ◎ Tip: before using a variable, ensure that it is defined in all possible control flows leading to that point. 42 Example: Missing Variable Definitions Beware! The variable y is not defined, leading to an error. >>> x = 5 >>> if x > 10: ... y = 42 ... >>> print(x + y) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'y' is not defined 43 Example: Missing Variable Definitions ◎ Both of the solutions below ensure that y is always defined before it is used. x = 5 y = 0 if x > 10: y = 42 x = 5 if x > 10: y = 42 else: y = 0 print(x + y) print(x + y) 44 Nested Conditionals ◎ Complex decisions can be handled by nesting if statements. ◎ A nested if statement is an if statement contained within another if, elif, or else block. ◎ In Python, nesting is achieved through deeper levels of indentation. 45 Example: Nested Conditionals Task definition Write a program which prints whether an input number is "negative" or "positive". If the number is positive, also print out whether the number is "even" or "odd". 46 Example: Nested Conditionals Task definition Write a program which prints whether an input number is "negative" or "positive". If the number is positive, also print out whether the number is "even" or "odd". x = int(input()) if x < 0: print('negative') if x > 0: print('positive') if x % 2 == 0: print('even') else: print('odd') ◎ Then nested if/else statement (highlighted) is only encountered when x > 0 is true. 47 Things Can Get Confusing! ◎ Sometimes having a lot of chained and nested conditionals can get confusing. ◎ Using print statements is a good way of tracing control flow. ○ Simply place print statements at various points in the code. ○ When you don't need the print statements anymore, delete them (or comment them out). 48 Example: Tracing Control Flow print('[1]') income = 110000 if income > 50000: print('[2]') tax = 0.3 * income income = income - tax elif income > 100000: print('[3]') tax = 0.4 * income income = income - tax print('[4]') ◎ This program is not working as expected---not enough tax is being applied. ◎ We can add the highlighted print statements to trace control flow and help us debug the issue. print(income) 49 Example: Tracing Control Flow print('[1]') income = 110000 if income > 50000: print('[2]') tax = 0.3 * income income = income - tax elif income > 100000: print('[3]') tax = 0.4 * income income = income - tax print('[4]') Output: [1] [2] [4] 77000.0 ◎ We now know which path the control flow takes. print(income) ◎ This also reveals the source of our error: the conditions are checked in the wrong order. 50 Example: Tracing Control Flow income = 110000 if income > 100000: tax = 0.4 * income income = income - tax elif income > 50000: tax = 0.3 * income income = income - tax print(income) ◎ We can fix the problem by reordering the branches in the selection control structure. ◎ Now that the bug is fixed, we can remove the tracing print statements. 51 Summary 52 In This Lecture We... ◎ Learnt how basic control structures determine control flow. ◎ Expressed conditional execution in Python code using if-elif-else structures. ◎ Explored common pitfalls conditional execution. when dealing with 53 Next Lecture We Will... ◎ Learn how the iteration control structure can be used to execute the same code multiple times. 54 Thanks for your attention! The slides and lecture recording will be made available on LMS. The “Cordelia” presentation template by Jimena Catalina is licensed under CC BY 4.0. PPT Acknowledgement: Dr Aiden Nibali, CS&IT LTU. 55 Lecture 4.1 Iteration I Topic 4 Intended Learning Outcomes ◎ By the end of week 4 you should be able to: ○ Use iteration control structures to repeat the execution of statements, ○ Use a Python range to specify a sequence of numbers, and ○ Perform aggregations like finding the average or maximum value of a sequence. 2 Lecture Overview 1. While Loops 2. Kinds of While Loops 3. Nested Loops and Finishing Early 3 1. While Loops 4 Repeated Actions ◎ Often times you will find that programs require the same action to be performed multiple times. ◎ The data might change, but the fundamental action is the same. 5 Example: Times Table Task Definition Print the times table for a number input by the user. So if the user inputs 7, the output should be: ◎ 7×1=7 ◎ … ◎ 7 × 12 = 84 a = int(input('Times table: ')) print(str(a) + ‘ * 1 = ' + str(a * 1)) print(str(a) + ‘ * 2 = ' + str(a * 2)) print(str(a) + ‘ * 3 = ' + str(a * 3)) print(str(a) + ‘ * 4 = ' + str(a * 4)) print(str(a) + ‘ * 5 = ' + str(a * 5)) print(str(a) + ‘ * 6 = ' + str(a * 6)) print(str(a) + ‘ * 7 = ' + str(a * 7)) print(str(a) + ‘ * 8 = ' + str(a * 8)) print(str(a) + ‘ * 9 = ' + str(a * 9)) print(str(a) + ‘ * 10 = ' + str(a * 10)) print(str(a) + ‘ * 11 = ' + str(a * 11)) print(str(a) + ‘ * 12 = ' + str(a * 12)) ◎ The above code works, but is highly redundant! 6 Repeated Actions 7 Redundant Code is Bad ◎ Having a lot of similar code repeated causes a few problems. ○ Time-consuming to write. ○ Changing the code is error-prone. ○ Inflexible (what if you wanted to show up to 20x). ◎ Fortunately there is a better way using iteration. 8 Repetitive Execution: Iteration ◎ In an iteration control structure, a ◎ Represented by a backwardscondition determines how many pointing arrow in a flowchart. times statements are executed. ◎ Allows the program to execute the same statement(s) multiple times. ◎ Achieved by allowing control flow to return to an earlier point in the program. 9 Repetitive Execution: Iteration Repetition: Repeating a set of actions for a number of times. 10 While Loops 11 While Loops while Condition: Statement ◎ If the condition is truthy: ○ Execute the statement(s) in the while block. ○ Check the condition again. ◎ If the condition is falsy: ○ Exit the while loop. 12 While Loops i = 1 while i <= 4: print (i) i=i+1 Print (“Done!”) # output 1 2 3 4 Done! i = 1 # ---while i <= 4: print (i) i=i+1 initialisation # <--- test (Boolean test) # --- action-during # --- each-loop Print (“Done!”) 13 While loops 14 Example: re-write the Times Table code using While Loops Task Definition Print the times table for a number input by the user. So if the user inputs 7, the output should be: ◎ 7×1=7 ◎ … ◎ 7 × 12 = 84 a = int(input('Times table: ')) print(str(a) + ‘ * 1 = ' + str(a * 1)) print(str(a) + ‘ * 2 = ' + str(a * 2)) print(str(a) + ‘ * 3 = ' + str(a * 3)) print(str(a) + ‘ * 4 = ' + str(a * 4)) print(str(a) + ‘ * 5 = ' + str(a * 5)) print(str(a) + ‘ * 6 = ' + str(a * 6)) print(str(a) + ‘ * 7 = ' + str(a * 7)) print(str(a) + ‘ * 8 = ' + str(a * 8)) print(str(a) + ‘ * 9 = ' + str(a * 9)) print(str(a) + ‘ * 10 = ' + str(a * 10)) print(str(a) + ‘ * 11 = ' + str(a * 11)) print(str(a) + ‘ * 12 = ' + str(a * 12)) ◎ The above code works, but is highly redundant! 15 Example: re-write the Times Table code using While Loops a = int(input('Times table: ')) b = 1 # initialisation while b <= 12: # check condition print(str(a) + ' x ' + str(b) + ' = ' + str(a * b)) b = b + 1 # update condition ◎ Same output as the earlier repetitive code. ◎ Only requires one print statement. ◎ b is a counter variable which keeps track of how many times the loop has repeated. ◎ When b exceeds 12, the loop exits. 16 While loops 17 Check Your Understanding Q. How many times will the program print output? x = 5 while x >= 0: print(x) x = x - 1 18 Check Your Understanding Q. How many times will the program print output? A. Six times. The output will be: x = 5 while x >= 0: print(x) x = x - 1 5 4 3 2 1 0 19 2. Kinds of While Loops 20 Definite While Loops ◎ A definite loop repeats a fixed number of times. ○ The number of repetitions is known before starting the loop. ◎ The improved "times table" example from earlier used a definite loop (it looped 12 times). 21 Indefinite While Loops ◎ An indefinite loop repeats for a number of times which is not obvious before starting the loop. ◎ An indefinite loop could: ○ Finish based on user input (interactive loop). ○ Finish based on a computed value. ○ Never finish (infinite loop). 22 Indefinite While Loops: Interactive Loops ◎ An interactive loop has a condition which decides when to finish repeating based on user input. ◎ Hence the user controls when the loop finishes as it is running. 23 Example: Total Cost Calculator total_cost = 0 item_cost = float(input('Item cost: ')) while item_cost >= 0: total_cost = total_cost + item_cost item_cost = float(input('Item cost: ')) print(total_cost) ◎ This program will run until the user enters a negative number. ◎ The user can decide how many items they want to add. 24 Computed Conditions ◎ A loop can have a condition which is based on a complex computation made by the loop. ◎ The number of repetitions is not obvious before entering the loop. ◎ Can be used for certain mathematical calculations. 25 Example: Collatz Conjecture ◎ The "Collatz conjecture" is a famous mathematical problem: ○ Consider a positive integer n. ○ If the number is even, halve it. ○ If the number is odd, triple it and add 1. ○ The conjecture is that if you repeat this enough times, you will always reach the number 1. ◎ The number of iterations required to reach 1 is called the "total stopping time". 26 Example: Collatz Conjecture Task Definition Write a program which calculates the total stopping time (as defined by the Collatz conjecture) for a number input by the user. n = int(input('Enter n: ')) t = 0 # Repeat until n reaches 1. while n != 1: if n % 2 == 0: # n is even. n = n / 2 else: # n is odd. n = n * 3 + 1 # Increment total stopping time. t = t + 1 print(t) 27 Example: Collatz Conjecture ◎ The calculation in the loop itself determines how many times it will repeat. ○ This is what makes it indefinite. ◎ Fun aside: if you can somehow figure out the total stopping time without a loop, you may be well on your way to solving the Collatz conjecture! n = int(input('Enter n: ')) t = 0 # Repeat until n reaches 1. while n != 1: if n % 2 == 0: # n is even. n = n / 2 else: # n is odd. n = n * 3 + 1 # Increment total stopping time. t = t + 1 print(t) 28 Indefinite While Loops: Infinite Loops ◎ An infinite loop never finishes (it repeats forever). ○ In practice this means until the program crashes or is forced to quit (e.g. shutting down). ◎ This occurs when the condition is always truthy. ◎ If you accidentally run a program containing an infinite loop in a script or interactive session, you can force the program to exit by pressing Ctrl+C. 29 Check Your Understanding Q. How many times will the Python script print output? x = 1 while x * 2 < 10: print(x) x = x + 1 30 Check Your Understanding Q. How many times will the Python script print output? A. Infinite times. ◎ The last line is not part of the while loop since it is deindented. ◎ Since x stays at its initial value of 1. the condition is always true. x = 1 while x * 2 < 10: print(x) x = x + 1 31 Examples: While loops 32 3. Nested Loops and Finishing Early 33 Nested Loops ◎ Like if statements, while loops can be nested. ◎ Nesting loops multiplies the number of times that the innermost loop is repeated. ○ Programs with a lot of nested loops can end up running quite slowly as a result. 34 Example: All Times Tables a = 1 while a <= 12: b = 1 while b <= 12: print(str(a) + ' x ' + str(b) + ' = ' + str(a * b)) b = b + 1 a = a + 1 ◎ Highlighted lines are copied from earlier. ○ e.g. when a = 7, this part prints the 7 times table. ◎ An outer loop has been added. ○ This loops through a = 1, 2, 3, …, 12 35 Finishing the Current Iteration ◎ In Python, a continue statement can be used to finish the current iteration early. ○ Written using the continue keyword. ◎ Control flow will immediately return to the while loop's condition. 36 Finishing the Current Iteration 37 Example: Printing Even Numbers ◎ For each repetition where x % 2 == 1 (i.e. x is odd), the continue statement will be reached. x = 0 while x < 10: if x % 2 == 1: x = x + 1 continue print(x) ◎ The continue statement returns to the top of the while loop immediately. ○ Hence the print statement is skipped. x = x + 1 0 2 4 6 8 38 Finishing the Entire Loop ◎ In Python, a break statement can be used to finish the entire loop early. ○ Written using the break keyword. ◎ Control flow will immediately skip to the code after the loop. 39 Finishing the Entire Loop 40 Example: Total Cost Calculator total_cost = 0 item_cost = float(input('Item cost: ')) while item_cost >= 0: total_cost = total_cost + item_cost item_cost = float(input('Item cost: ')) print(total_cost) ◎ Our previous implementation (shown) has duplicate code. 41 Example: Total Cost Calculator total_cost = 0 while True: item_cost = float(input('Item cost: ')) if item_cost < 0: break total_cost = total_cost + item_cost print(total_cost) ◎ By adding a conditional break statement (highlighted), we can avoid the duplication. ◎ It is now the if statement which controls when the loop finishes. 42 Check Your Understanding Q. How many times will the script print output? x = 0 while x < 5: if x % 2 == 0: x = x + 1 continue y = 0 while y < x: print(y) y = y + 1 x = x + 1 43 Check Your Understanding Q. How many times will the script print output? A. 4 times. ◎ The outer loop repeats 5 times (x=0,1,2,3,4). ◎ Three of the outer loop repetitions exit early (x=0,2,4). ◎ When x = 1, the inner loop prints 1 time. ◎ When x = 3, the inner loop prints 3 times. x = 0 while x < 5: if x % 2 == 0: x = x + 1 continue y = 0 while y < x: print(y) y = y + 1 x = x + 1 44 Summary 45 In This Lecture We... ◎ Learnt how the iteration control structure can be used to execute the same code multiple times. ◎ Expressed iteration in Python code using while loops. ◎ Discovered different kinds of loops and their uses. ◎ Learnt how to finish early using continue and break statements. 46 Next Lecture We Will... ◎ Learn about another kind of loop in Python, the for loop. 47 Thanks for your attention! The slides and lecture recording will be made available on LMS. The “Cordelia” presentation template by Jimena Catalina is licensed under CC BY 4.0. PPT Acknowledgement: Dr Aiden Nibali, CS&IT LTU. 48 Lecture 4.2 Iteration II Lecture Overview 1. For Loops 2. Example Program: FizzBuzz 3. Aggregation 2 1. For Loops 3 For Loops 4 While Loops vs. For Loops ◎ Previously we saw how while loops can be used to perform iteration in Python. ○ The while loop is controlled by a condition. ◎ A common use for iteration is to perform an action for each item in a sequence. ○ e.g. Add numbers from 1 to 10, print all customers. ◎ Python provides for loops as a more convenient way of iterating over a sequence. 5 While Loops vs. For Loops # While loop i = 0 while i < 10: print(i) # For loop for i in range(10): print(i) i = i + 1 ◎ Both code snippets print numbers from 0 to 9. ◎ The for loop version is better. ○ More concise. ○ Easier to read (once we learn about range). ○ Less error prone (can't forget to increment i). 6 For Loops for Variable in Sequence: Statement ◎ While there are items left in the sequence: ○ Assign the next item in the sequence to the iteration variable. ○ Execute the statements in the for block. 7 For Loops ◎ For loops share a lot in common with while loops. ○ You can use continue and break statements to finish early. ○ You can nest them. ○ You must follow the same indentation rules. 8 For Loops 9 For Loops: range () function 10 For Loops- range () function 11 Writing a Python Range ◎ Let's say you have a sequence in mind that you want to express using a Python range. ◎ Firstly you need to check whether this is possible: ○ Are all of the numbers unique integers? ○ Is the sequence an arithmetic sequence (i.e. do items in the sequence increase/decrease by a constant amount)? 12 Valid Range Sequences Valid Range Sequences 🗸 1, 2, 3, 4, 5 🗸 8, 10, 12 🗸 6, 3, 0, -3 🗸 100 Invalid Range Sequences ✗ 1, 4, 9, 16, 25 ✗ 1.1, 2.1, 3.1 ✗ 5, 5, 5, 5 13 Steps For Writing a Python Range 1. Think of the first number in the sequence. This is m. 2. Think of the difference between the second and first item in the sequence. This is step. 3. Think of the last number in the sequence, then add step to it. This is n. The Python code for the sequence is: range(m, n, step) 14 Simplifying Python Ranges ◎ In some instances you can simplify the range. range(m, n, step) ○ If step is 1, you can omit it. ○ If step is 1 and m is 0, you can omit them both. ◎ For example: ○ range(6, 9, 1) simplifies to range(6, 9) ○ range(0, 3, 1) simplifies to range(3) 15 Examples: Writing a Python Range ◎ 0, 1, 2, 3, 4 ○ range(0, 5, 1), or simply range(5) ◎ 1, 2, 3 ○ range(1, 4, 1), or simply range(1, 4) ◎ 0, 2, 4, …, t ○ range (0, t + 2, 2) ◎ y, y-1, y-2, …, y-x ○ range(y, (y-x)-1, -1) 16 Examples: For Loops: 17 Examples: For Loops 18 Examples: For Loops 19 Check Your Understanding Q. Fill in the blank using a Python range such that the program output is 9, 8, 7, 6. for i in ANSWER: print(i) 20 Check Your Understanding Q. Fill in the blank using a Python range such that the program output is 9, 8, 7, 6. A. range(9, 5, -1) 1. The first value is 9. ➢ m=9 2. The first two values are 9, 8. ➢ step = 8 - 9 = -1 3. The last value is 6. ➢ n = 6 + step = 5 for i in ANSWER: print(i) 21 2. Example Program: FizzBuzz 22 What is FizzBuzz? ◎ FizzBuzz is a programming problem often used during software developer job interviews. ◎ Tests knowledge of: ○ Selection and iteration control structures, ○ Boolean expressions, and ○ The modulo operator. ◎ We have learnt about each of these things, so we are now equipped to tackle FizzBuzz! 23 Task Definition Task Definition Write a program that prints the numbers from 1 to 100. But for multiples of three print "Fizz" instead of the number and for the multiples of five print "Buzz". For numbers which are multiples of both three and five print "FizzBuzz". Source: https://imranontech.com/2007/01/24/using-fizzbuzz-to-find-developers-who-grok-coding/ 24 Example Output ◎ The first 16 lines of the expected program output are shown here. 1 2 Fizz 4 Buzz Fizz 7 8 Fizz Buzz 11 Fizz 13 14 FizzBuzz 16 ... 25 Printing Numbers from 1 to 100 ◎ To begin with, let's ignore the fizzing/buzzing and focus on simply printing numbers from 1 to 100. ◎ We know that we can iterate over these numbers using a for loop and a Python range. ◎ How can we express the sequence of numbers from 1 to 100 (inclusive) using a Python range? range(1, 101) 26 Printing Numbers from 1 to 100 for i in range(1, 101): print(i) ◎ The above code will print all integers from 1 to 100, inclusive. 27 Fizzing and Buzzing ◎ Let's now consider the full task definition: Write a program that prints the numbers from 1 to 100. But for multiples of three print "Fizz" instead of the number and for the multiples of five print "Buzz". For numbers which are multiples of both three and five print "FizzBuzz". ◎ We can identify that the program should select one of four different outputs for each number. ○ The number itself (this is the "default" option). ○ "Fizz" (multiple of 3). ○ "Buzz" (multiple of 5). ○ "FizzBuzz" (multiple of 3 and 5). 28 Fizzing and Buzzing ◎ We will need to use a selection structure with four branches. for i in range(1, 101): if Condition 1: print(Output 1) elif Condition 2: print(Output 2) elif Condition 3: print(Output 3) else: print(i) # The "default" option. 29 Fizzing and Buzzing ◎ We are testing conditions based on whether the number is a multiple of other numbers (3 and/or 5). ◎ Some number x is a multiple of another number y if and only if x can be divided by y with no remainder. ○ Recall that the modulo operator calculates the remainder. ○ Hence x is a multiple of y if and only if x % y == 0. 30 Fizzing and Buzzing ◎ Let's pair each output with its condition: ○ multiples of three print "Fizz" ◉ Condition: i % 3 == 0 ◉ Output: "Fizz“ ○ multiples of five print "Buzz" ◉ Condition: i % 5 == 0 ◉ Output: "Buzz“ ○ multiples of both three and five print "FizzBuzz" ◉ Condition: i % 3 == 0 and i % 5 == 0 ◉ Output: "FizzBuzz" 31 First Solution Attempt (Incorrect) for i in range(1, 101): if i % 3 == 0: print("Fizz") elif i % 5 == 0: print("Buzz") elif i % 3 == 0 and i % 5 == 0: print("FizzBuzz") else: print(i) ◎ Can you spot the bug in the code? 32 First Solution Attempt (Incorrect) ◎ The conditions are tested in order of appearance. ◎ Any value of i which satisfies the "FizzBuzz" condition will satisfy the "Fizz" condition. ◎ Since the "Fizz" condition is tested first, the program will output "Fizz" instead of "FizzBuzz". for i in range(1, 101): if i % 3 == 0: print("Fizz") elif i % 5 == 0: print("Buzz") elif i % 3 == 0 and i % 5 == 0: print("FizzBuzz") else: print(i) 33 Second Solution Attempt (Correct) ◎ The bug can be fixed by reordering the branches. ◎ Importantly the most specific condition is tested first. for i in range(1, 101): if i % 3 == 0 and i % 5 == 0: print("FizzBuzz") elif i % 3 == 0: print("Fizz") elif i % 5 == 0: print("Buzz") else: print(i) 34 Check Your Understanding Q. Does the order of the two elif branches matter for getting the correct output? for i in range(1, 101): if i % 3 == 0 and i % 5 == 0: print("FizzBuzz") elif i % 3 == 0: print("Fizz") elif i % 5 == 0: print("Buzz") else: print(i) 35 Check Your Understanding Q. Does the order of the two elif branches matter for getting the correct output? A. No. A value of i which does not satisfy the FizzBuzz condition can never satisfy both the Fizz and Buzz conditions. Therefore the ordering of these conditions does not matter. for i in range(1, 101): if i % 3 == 0 and i % 5 == 0: print("FizzBuzz") elif i % 3 == 0: print("Fizz") elif i % 5 == 0: print("Buzz") else: print(i) 36 3. Aggregation 37 Aggregation ◎ Aggregation is the process of combining values to produce a single summary value. ◎ Some common aggregations include: ○ Sum, ○ Average (mean), and ○ Maximum value. ◎ Using a for loop is a convenient way of aggregating. 38 Aggregation Pattern ◎ A general pattern for aggregation is: 1. Define one (or more) summary variables, using suitable initial values. 2. Write a for loop to update the summary variable(s) using each item in the sequence. 3. Use the summary variable(s) to calculate the final result. ◎ Step 3 is not always necessary---in some cases the summary variable itself is the result. 39 Aggregation: Sum ◎ This program sums the numbers from 1 to 9. total = 0 for x in range(1, 10): total = total + x print(total) ◎ Summary variable: total. ◎ We keep track of the running total as each item in the sequence is considered. 40 Aggregation: Average (Mean) ◎ This program averages the numbers from 1 to 9. ◎ Summary variables: total and count. total = 0 count = 0 for x in range(1, total = total count = count average = total / 10): + x + 1 count print(average) ◎ We keep track of both the total sum and the count, then calculate the average at the end by dividing. 41 Aggregation: Maximum ◎ This program finds the maximum value from a list. ○ More about lists in a future lecture. maximum = 0 for x in [3, 1, 8, 4]: if x > maximum: maximum = x print(maximum) ◎ Summary variable: maximum. ◎ An if statement is used to conditionally update maximum whenever a larger value is encountered. 42 Count ◎ This program counts the number of values greater than 5 in a list. count = 0 for x in [4, 9, 5, 8, 9]: if x > 5: count = count + 1 print(count) ◎ Summary variable: count. ◎ The count is incremented for each item which meets the if statement condition. 43 Custom Aggregation ◎ With a little bit of thinking you can devise your own custom aggregations. ◎ You need to: 1. Define the sequence to aggregate. 2. Identify summary variable(s). 3. Identify the repeated operation(s) which builds up the summary variable(s). 4. Use the summary variable(s) to calculate the final result (not always necessary). 44 Custom Aggregation Task Definition Write a program which concatenates all single-digit numbers into one long string surrounded by square brackets and prints the result. 1. The sequence we are iterating over is 0, 1, …, 9. 2. The summary variable is the string being built. 3. The repeated operation is converting the item to a string and concatenating it with the summary variable. 4. The final result is the summary variable in square brackets. 45 Custom Aggregation s = '' for x in range(10): s = s + str(x) print('[‘ + s + ']’) #output >>> [0123456789] 1. The sequence we are iterating over is 0, 1, …, 9. 2. The summary variable is the string being built. 3. The repeated operation is converting the item to a string and concatenating it with the summary variable. 4. The final result is the summary variable in square brackets. 46 Custom Aggregation ◎ If you are still unsure about how this code works, think about what it looks like with the loop "unrolled". ◎ Recall that the str function converts a value to a string. ○ e.g. str(0) gives '0' s = '' for x in range(10): s = s + str(x) print('[‘ + s + ']') s = s = s = ... s = '' s + str(0) s + str(1) s + str(9) print('[‘ + s + ']') 47 Check Your Understanding Q. Which program correctly calculates the product of values between 1 and 10 (inclusive)? # Program A. prod = 1 for x in range(1, 11): prod = prod * x print(prod) # Program B. prod = 0 for x in range(1, 11): prod = prod * x print(prod) # Program C. prod = 1 for x in range(1, 11): x = prod * x print(prod) 48 Check Your Understanding Q. Which program correctly calculates the product of values between 1 and 10 (inclusive)? A. Program A. ◎ Program B uses an inappropriate initial value and will output 0. ◎ Program C does not update the summary variable. # Program A. prod = 1 for x in range(1, 11): prod = prod * x print(prod) # Program B. prod = 0 for x in range(1, 11): prod = prod * x print(prod) # Program C. prod = 1 for x in range(1, 11): x = prod * x print(prod) 49 Nested Loops 50 Nested Loops 51 Nested Loops 52 Summary 53 In This Lecture We... ◎ Iterated over ranges of numbers using for loops. ◎ Used selection and iteration to solve a common coding interview question. ◎ Performed aggregation using for loops. 54 Next Lecture We Will... ◎ Discover how functions allow us to use and write reusable chunks of code. 55 Thanks for your attention! The slides and lecture recording will be made available on LMS. The “Cordelia” presentation template by Jimena Catalina is licensed under CC BY 4.0. PPT Acknowledgement: Dr Aiden Nibali, CS&IT LTU. 56 Lecture 3.1 Comparisons and Boolean Logic We have covered in previous weeks (Week 1 and Week 2) ◎ ◎ ◎ ◎ ◎ ◎ Flowcharts How to install and run Python editor Program Development steps Input, Processing, and Output Variables, Statements, and Comments Expressions and Data Types Program Development steps ◎ A program can be used to solve complex problem ○ ○ Programs are a set of written actions in order to fulfil a need / solve a problem A programming language is the tool used to create a solution (Program) Design a flowchart Implement: Python print data Problem Task Definition if condition true print data Solution for x in range print x Call function ... 3 Python Input ◎ Take Input from User – direct or keyboard ◎ Take input from File – text, csv, binary, etc ◎ Take input from System – click, button, drop-down, mouse 4 Python Input ◎ Take Input from User – Direct >>> x = 10 # integer >>> y = 0.6 # float >>> z = “Hello” # string >>> x = 10 >>> y = 0.6 >>> z = x + y >>> x = “Hello” >>> y = “CSE4IP” >>> z = x + y # integer # float # string # string 5 Python Input ◎ Take Input from User – keyboard Python uses input() function to get input from the user - Keyboard Syntax: - input(“prompt”) - prompt : a string - a default message displayed before the input 6 Python Input ◎ Take Input from User – keyboard input(“prompt”) Example: take three different inputs from user. >>> x = input (“Enter a number: ”) Enter a number: 5 >>> y = input (“Enter a number: ”) Enter a number: 8.3 >>> z = input (“Enter a string: ”) Enter a string: Hello CSE4IP 7 Python Input Take Input from User – keyboard: input(prompt) Example: take three different inputs from user. >>> x = input (“Enter a number: ”) Enter a number: 5 >>> X ‘5’ >>> y = input (“Enter a number: ”) Enter a number: 8.3 >>> y ‘8.3’ >>> z = input (“Enter a string: ”) Enter a string: Hello CSE4IP >>> Z ‘Hello CSE4IP’ o o o o o We can see that all inputs are saved as string (‘’) We can use Explicit Type Conversion functions (covered in Lecture 2.2) to convert them into a proper data type. int() float() str() 8 Python Input Take Input from User – keyboard: input(prompt) 9 Python Output ◎ Display the output of a program to the standard output device - screen and console ◎ Save the output in a file – text, csv, binary, …, etc. 10 Python Output 11 Comparisons and Boolean Logic Topic 3 Intended Learning Outcomes ◎ By the end of week 3 you should be able to: ○ Write boolean expressions for questions with yes/no answers, and ○ Use selection control structures to specify different flows through a program. 13 Lecture Overview 1. Booleans 2. Comparison Operators 3. Logical Operators 14 1. Booleans 15 Review of Types ◎ We have encountered three data types so far: ○ str (string): A string of characters (i.e. text). ◉ e.g. ‘CSE4IP', "August". ○ int (integer): A whole number. ◉ e.g. 23, -1000 ○ float (floating point number): A number with a fractional part. ◉ e.g. 23.45, -5.0 16 How About Yes/No Values? ◎ How would we store the answer to a yes/no question? ○ e.g. Is the video paused? ◎ Could use strings: ○ is_paused = 'yes' ○ Quickly gets confusing. Does "YES" also mean yes? "Y"? "True"? ◎ Could use integers (0 means no, 1 means yes). ○ is_paused = 1 ○ Not very readable. 17 Introducing Booleans ◎ A boolean represents a binary value. ○ Yes/no, true/false, on/off. ◎ There are only two possible boolean values. ◎ Useful for representing answers to yes/no questions. ○ Did the user opt into the newsletter? ○ Is the car new? ○ Is the average test score greater than 50? 18 Visual Representations of Booleans 19 Booleans in Python ◎ In Python, booleans have the bool type. ◎ The two boolean literals in Python are True and False. ○ These are the only two values of type bool. >>> type(True) <class 'bool'> >>> type(False) <class 'bool'> ◎ For the previous example: ○ is_paused = True 20 Converting Values to Booleans ◎ Values of other types can be converted into booleans. ◎ A value that converts to True is said to be truthy. ◎ A value that converts to False is said to be falsy. 21 Truthy and Falsy: Numbers ◎ For numeric types: ○ Zero is falsy. ○ All other truthy. values are >>> bool(0) False >>> bool(-42) True >>> bool(0.00) False >>> bool(0.0001) True 22 Truthy and Falsy: Strings ◎ For strings: ○ The empty string is falsy. ○ All other values are truthy. ◎ The empty string is a string with no characters in it. >>> bool('hello') True >>> bool('') False >>> bool('False') True >>> bool(' ') True >>> bool("") False 23 Check Your Understanding Q. Is the string 'No' truthy or falsy? 24 Check Your Understanding Q. Is the string 'No' truthy or falsy? A. Truthy. The only falsy string is the empty string. Python does not attempt to read the contents of the string beyond checking if it is empty. 25 2. Comparison Operators 26 Boolean Expressions ◎ Recall that a numeric expression is an expression which evaluates to a number (e.g. 2 + 2). ◎ A boolean expression is an expression which evaluates to a boolean (i.e. True or False). ◎ A simple kind of boolean expression is comparing two values. For example: ○ Is one number greater than another? ○ Are two strings equal? 27 Comparison Operators ◎ Two values can be compared using a comparison operator. ◎ The result of a comparison will be True or False, depending on whether the condition is satisfied. ◎ Note that Python uses double equals (==) to check for equality. PYTHON MATHS CONDITION x == y x=y x is equal to y x != y x≠y x is not equal to y x > y x>y x is greater than y x < y x<y x is less than y x >= y x≥y x is greater than or equal to y x <= y x≤y x is less than or equal to y 28 Numeric Comparisons ◎ Numeric comparisons work as you would expect from mathematics. >>> x = 5 >>> x > 2 True >>> x > 5 False >>> x >= 5 True >>> y = 7.5 >>> x < y True >>> -50 > y False 29 String Comparisons ◎ String comparisons are based on lexicographical ordering. ○ This is similar to dictionary ordering. >>> word = 'apple’ ◎ However, all uppercase letters come before lowercase letters. >>> word < 'apple pie' True >>> word > 'banana' False >>> word == 'apple' True >>> word < 'Zucchini' False 30 Comparisons Across Types Beware! Avoid comparing values of different types (e.g. a number and a string)---the results are probably not what you expect. >>> 5 == '5' False >>> 0 < '5' Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: '<' not supported between instances of 'int' and 'str' 31 Check Your Understanding Q. What is the Python boolean expression for "twice the value of x is not equal to 100"? 32 Check Your Understanding Q. What is the Python boolean expression for "twice the value of x is not equal to 100"? A. x * 2 != 100 Or, equivalently: ◎ 2 * x != 100 ◎ 100 != x * 2 ◎ 100 != 2 * x 33 3. Logical Operators 34 Logical Operators ◎ Logical operators can be used to combine and modify boolean expressions. ◎ Python has three logical operators: and, or, not. ○ These generally behave as you would expect from the meaning of the words. 35 The and Operator (Logical And) ◎ True if both expressions are truthy. ◎ False if at least one expression is falsy. P Q P and Q True True True True False False False True False False False False >>> x = 5 >>> y = 7.5 >>> x < y and x == 5 True >>> y == 5 and x == 5 False >>> x == 0 and y <= 0 False 36 The or Operator (Logical Or) ◎ True if at least one expression is truthy. ◎ False if both expressions are falsy. >>> x = 5 >>> y = 7.5 >>> x < y or x == 5 True P Q P or Q True True True >>> y == 5 or x == 5 True True False True >>> x == 0 or y <= 0 False True True False False False False 37 The not Operator (Logical Not) ◎ Negates the expression. ◎ True if the expression is falsy. ◎ False if the expression is truthy. P not P True False False True >>> x = 5 >>> y = 7.5 >>> not x < y False >>> not y == 5 True 38 Combining Boolean Expressions ◎ Logical operators can be used to build complex boolean expressions. ◎ Other kinds of operators can be included in the expression. ◎ For example, "either x is greater than 10 and even, or y is less than 0" can be expressed as: (x > 10 and x % 2 == 0) or y < 0 39 Extended Operator Precedence OPERATOR () NAME Higher Parentheses ** Exponentiation *, /, %, // Multiplication, etc. +, - Addition, etc. ==, !=, >, <, >=, <= Comparison not Logical "not" and Logical "and" or Lower Logical "or" 40 Check Your Understanding Q. What does the following expression evaluate to? 2 * 7 > 5 and not 27 < 8 OPERATOR () NAME Higher Parentheses ** Exponentiation *, /, %, // Multiplication, etc. +, - Addition, etc. ==, !=, >, <, >=, <= Comparison not Logical "not" and Logical "and" or Lower Logical "or" 41 Check Your Understanding Q. What does the following expression evaluate to? 2 * 7 > 5 and not 27 < 8 A. True. 1. 2 * 7 > 5 and not 27 < 8 2. 14 > 5 and not 27 < 8 3. True and not 27 < 8 4. True and not False 5. True and True 6. True OPERATOR () NAME Higher Parentheses ** Exponentiation *, /, %, // Multiplication, etc. +, - Addition, etc. ==, !=, >, <, >=, <= Comparison not Logical "not" and Logical "and" or Lower Logical "or" 42 Summary 43 In This Lecture We... ◎ Discovered the boolean data type. ◎ Compared values using comparison operators. ◎ Used logical operators to construct more complex boolean expressions. 44 Next Lecture We Will... ◎ Use boolean expressions to control the flow of program execution. 45 Thanks for your attention! The slides and lecture recording will be made available on LMS. The “Cordelia” presentation template by Jimena Catalina is licensed under CC BY 4.0. PPT Acknowledgement: Dr Aiden Nibali, CS&IT LTU. 46