Lecture 5 - Functions [Python] PDF

INFR 1101u Introduction to Programming (Python) Lecture 5: Functions Samaneh Mazaheri Email: [email protected] 1 Topics • Introduction to Functions • Defining and Calling a Void Function • Local Variables • Passing Arguments to Functions • Global Variables and Global Constants •...

INFR 1101u Introduction to Programming (Python) Lecture 5: Functions Samaneh Mazaheri Email: [email protected] 1 Topics • Introduction to Functions • Defining and Calling a Void Function • Local Variables • Passing Arguments to Functions • Global Variables and Global Constants • Introduction to Value-Returning Functions • The math Module 2 Introduction to Functions • Function: group of statements within a program that perform a specific task. ▪ Usually, one task of a large program • Functions can be executed to perform overall program task ▪ Known as divide and conquer approach • Modularized program: program wherein each task within the program is in its own function. 3 Using functions to divide and conquer a large task 4 Modularizing a Program with Functions • The benefits of using functions or modularizing a program with functions include: ▪ Simpler code ▪ Code reuse • write the code once and call it multiple times ▪ Better testing and debugging • Can test and debug each function individually ▪ Faster development ▪ Easier facilitation of teamwork • Different team members can write different functions 5 Defining and Calling a Function • Functions are given names. • Function naming rules: • Cannot use key words as a function name • Cannot contain spaces • First character must be a letter or underscore • All other characters must be a letter, number or underscore • Uppercase and lowercase characters are distinct 6 Defining and Calling a Function • Function name should be descriptive of the task carried out by the function. ▪ Often includes a verb Function Header Block • Function header: first line of function – Includes keyword def and function name, followed by parentheses and colon • Block: set of statements that belong together as a group – Example: the statements included in a function 7 Defining and Calling a Function • Function definition/implementation: specifies what function does • Call a function to execute it • When a function is called: ➢Interpreter jumps to the function and executes statements in the block ➢Interpreter jumps back to part of program that called the function ▪Known as function return 8 Void Functions and Value-Returning Functions • A void function:  Simply executes the statements it contains and then terminates. • A value-returning function:  Executes the statements it contains, and then it returns a value back to the statement that called it. • The input, int, and float functions are examples of value-returning functions. 9 Indentation in Python • Each block must be indented ▪ Lines in block must begin with the same number of spaces •Use tabs or spaces to indent lines in a block, but not both as this can confuse the Python interpreter ▪ Blank lines that appear in a block are ignored 10 Exercise A. Write function hello() that: ▪ takes a name (i.e., a string) as input ▪ prints the following personalized welcome message ▪ Note that the function does not return anything hello(‘Sam’) Welcome Sam, to the world of Python! B. Write a Python function called calculate_rectangle_area that calculates the area of a rectangle, getting its length and width as parameters from user. The formula to calculate the area of a rectangle is: 𝐴𝑟𝑒𝑎 = 𝑙𝑒𝑛𝑔𝑡ℎ × 𝑤𝑖𝑑𝑡ℎ 11 Local Variables • Local variable: variable that is assigned a value inside a function. ▪ Belongs to the function in which it was created. • Only statements inside that function can access it, error will occur if another function tries to access the variable. • Scope: the part of a program in which a variable may be accessed. ▪ For local variable: function in which created. • Local variable cannot be accessed by statements inside its function which precede its creation. • Different functions may have local variables with the same name. ▪ Each function does not see the other function’s local variables, so no confusion 12 Passing Arguments to Functions • Argument: piece of data that is sent into a function ▪ Function can use argument in calculations ▪ When calling the function, the argument is placed in parentheses following the function name. The value variable is passed as an argument 13 Passing Multiple Arguments • Python allows writing a function that accepts multiple arguments ▪ Parameter list replaces single parameter • Parameter list items separated by comma • Arguments are passed by position to corresponding parameters ▪ First parameter receives value of first argument, second parameter receives value of second argument, etc. 14 Writing Value-Returning Functions • To write a value-returning function, you write a simple function and add one or more return statements ▪ Format: return expression •The value for expression will be returned to the part of the program that called the function ▪ The expression in the return statement can be a complex expression, such as a sum of two variables or the result of another value-returning function 15 Writing Value-Returning Functions 16 How to Use Value-Returning Functions • Value-returning function can be useful in specific situations ▪ Example: have function prompt user for input and return the user’s input ▪ Simplify mathematical expressions ▪ Complex calculations that need to be repeated throughout the program • Use the returned value ▪ Assign it to a variable or use as an argument in another function 17 Exercise 2 Write a function that returns smallest value of three arguments and provide a program to test it. def smallest(x, y, z) 18 The main Function • When defining and using functions in Python, it is good programming practice to place all statements into functions, and to specify one function as the starting point. • Any legal name can be used for the starting point, but we chose ‘𝑚𝑎𝑖𝑛’ since it is the required function name used by other common languages. • Of course, we must have one statement in the program that calls the main function. • main function: called when the program starts ▪ Calls other functions when they are needed ▪ Defines the mainline logic of the program 19 Volume of a cube - Exercise Write a function that returns volume of a cube (side length will be sent to the function as an argument). Volume of a cube : V = a3 20 Syntax: The main Function 21 Returning Values • Returning Strings: ▪ We can write functions that return strings. • Returning Boolean Values: • Boolean function: returns either True or False ▪ Use to test a condition such as for decision and repetition structures • Common calculations, such as whether a number is even, can be easily repeated by calling a function ▪ Use to simplify complex input validation code 22 Function Comments • Whenever you write a function, you should comment its behavior • Remember, comments are for human readers, not compilers ## Computes the volume of a cube. # @param sideLength the length of a side of the cube # @return the volume of the cube # def cubeVolume(sideLength) : volume = sideLength ** 3 return volume Function comments explain the purpose of the function, the meaning of the parameter variables and the return value, as well as any special requirements 23 Exercise 3 Write a function that returns true, if three arguments are all the same, and provide a program to test it. def allTheSame(x, y, z) 24 Exercise 4 Write a function that returns true, if three arguments are all different, and provide a program to test it. def allDifferent(x, y, z) 25 Exercise 5 Write a function that returns true, if all three arguments are sorted, with the smallest one coming first and provide a program to test it. def sorted(x, y, z) 26 Quiz 9 27 Returning Multiple Values •In Python, a function can return multiple values ▪ Specified after the return statement separated by commas • Format: return expression1, expression2, etc. ▪ When you call such a function in an assignment statement, you need a separate variable on the left side of the = operator to receive each returned value. 28 Exercise • Write a function in Python that takes in two arguments (a and b) and returns three values (a + b, a - b, and a * b). 29 Global Variables • Global variable: created by assignment statement written outside all the functions. ▪ Can be accessed by any statement in the program file, including from within a function. ▪ If a function needs to assign a value to the global variable, the global variable must be redeclared within the function. • General syntax: global variable_name 30 Global Variables • Reasons to avoid using global variables: ▪ Global variables making debugging difficult •Many locations in the code could be causing a wrong variable value ▪ Functions that use global variables are usually dependent on those variables •Makes function hard to transfer to another program ▪ Global variables make a program hard to understand 31 Global Constants • Global constant: global name that references a value that cannot be changed ▪ Permissible to use global constants in a program ▪ To simulate global constant in Python, create global variable and do not re-declare it within functions 32 Exercise 6 Write a function that returns average of three arguments and provide a program to test it. def average(x, y, z) 33 Comments and Docstrings Python programs should be documented. • So, the developer who writes/maintains the code understands it • So, the user knows what the program does Comments def f(x): res = x**2 + 10 return res # compute result # and return it Docstring def f(x): 'returns x**2 + 10' res = x**2 + 10 # compute result return res # and return it 34 Standard Library Functions • Standard library: library of pre-written functions that comes with Python. ▪ Library functions perform tasks that programmers commonly need. • Viewed by programmers as a “black box”. • Some library functions built into Python interpreter. ▪ To use, just call the function. 35 Standard Library Functions • Modules: files that stores functions of the standard library. ▪ Help organize library functions, not built into the interpreter. ▪ Copied to computer when you install Python. • To call a function stored in a module, need to write an import statement. ▪ Written at the top of the program: ▪ Format: import module_name • Dot notation: notation for calling a function belonging to a module ▪ Format: module_name.function_name() 36 The math Module •math module: part of standard library that contains functions that are useful for performing mathematical calculations.  Typically accept one or more values as arguments, perform mathematical operation, and return the result.  Use of module requires an import math statement. 37 The math Module 38 The math Module  The math module defines variables pi and e, which are assigned the mathematical values for 𝜋 and 𝑒.  Can be used in equations that require these values, to get more accurate results.  Variables must also be called using the dot notation. 39 Exercise 7 • Using math module, write a function that calculates circle area. 40 Exercise 8 Write a function to count the number of the words in a sentence. You need to get the sentence from user. Period (dot) shows the end of the sentence. 41 Quiz 10 42 Summary • This chapter covered: ▪ The advantages of using functions ▪ The syntax for defining and calling a function ▪ Methods for designing a program to use functions ▪ Use of local variables and their scope ▪ Syntax and limitations of passing arguments to functions ▪ Global variables, global constants, and their advantages and disadvantages ▪ Value-returning functions ▪ Using library functions and the import statement 43 Feedback Let me know about your thoughts: It can be related to the course, the instructor (me), TAs, etc. https://form.jotform.com/222434547170048 You can share your thoughts anonymously! Any feedback, comments, suggestions, critics, etc. I appreciate your time! 44 Questions? Suggested Reading: ▪ Chapter 6 from textbook 45