Introduction to Loops in Python

Introduction to Loops

• A loop is a code structure that repeats a sequence of statements multiple times.
• Instead of duplicating code for repeated operations, loops allow writing the code once, and the computer repeats it as needed.
• This is achieved using repetition structures, also known as loops.

Disadvantages of Duplicating Code

• Makes the program larger and more complex.
• Increases the development time.
• If a correction is needed, it may need to be applied in several places across the program.

Categories of Loops

• Condition-controlled loops: These loops repeat as long as a specific condition is met.
  • In Python, the while statement is used for this type of loop.
• Counter-controlled loops: These loops repeat a specific number of times.
  • In Python, the for statement is used for this type of loop.

The while Loop

• A condition-controlled programming structure.
• A block of statements repeats until a given condition is no longer met.
• Examples of conditions:
  • Reaching the last student in a list.
  • Completing the checkout process from a menu.
  • Entering the correct password.
  • Receiving valid input.
• Each repetition of the block is called an iteration, which is one execution of the body of a loop

while Loop Mechanics

• while is a pretest loop, so it tests the condition before performing an iteration
• A condition is evaluated.
  • If True, the statement(s) are executed, and the condition is evaluated again.
  • If False, the loop terminates, and the program continues after the loop.
  • If the condition is initially False, the statement(s) in the loop body are never executed.

Infinite Loops

• The condition never becomes false, so it repeats until the program is interrupted
• This occurs if the programmer forgets to include stopping code in the loop

Counters

• A counter increments or decrements a variable each time a loop repeats.
• Counters control the execution of the loop, acting the "loop control variable".
• Counters are initialized before entering the loop.
• It may be incremented/decremented either inside the loop or in the loop test

Letting the User Control the Loop

• Allows the user to determines the number of loop repetitions.
• The user is prompted to enter input before the loop, which in turns controls the number of iterations.

Augmented Assignment Operators

• Shorthand operators used for updating variables
• Evaluates the right-hand side before the augmented assignment operation is done

Using while Loop for Input Validation

• Structures input data validation
• First, prompt for and read in the data.
• Use a while loop to test if the data is valid and only enter the loop if the data is invalid.
• Display an error message and prompt the user to re-enter the data inside the loop until valid data is entered.

Common Loop Errors

• A classic loop error involves indentation.
• If the increment is not indented, the loop will never end

Keeping a Running Total in a Loop

• Example: numbers from 1 to 10 is 55
• Sums numbers using a while loop

Averaging in a Loop

• An average combines counting and summing
• The division by an amount occurs when the loop is done

Sentinels

• A sentinel is a unique value in a sequence of values that marks the end.
• Examples include -999 or -1 for a test score, signaling the end of input.
• When the sentinel is reached, the loop terminates
• Sentinels are used when the number of values to be entered is unknown to the user and, by extension, the program

The for Loop

• for loops are count-controlled, iterating a specific number of times.
• for loops are pretest loops, executing zero or more times.
• They are designed to work with sequences of data items.

for Loop Mechanics

• The target variable "iterates" through the sequence (list).
• The block of code is executed once for each value in the sequence.
• The value in the sequence moves through all possible values

Using the range Function with the for Loop

• The range function simplifies writing for loops.
• It returns an iterable object with a sequence of values that can be iterated over.
• range characteristics:
  • One argument: the ending limit.
  • Two arguments: the starting value and ending limit.
  • Three arguments: the starting value, ending limit, and step value.
• The range function can generate a sequence in descending order.
  • Ensure the starting number is larger than the end limit, and the step value is negative.

Turtle Graphics : Using Loops to Draw Designs

• Loops with turtle code can allow you to create simple shapes and elaborate designs

Deciding Which Loop to Use

• While
  • For conditional loops
  • For pretest loops ( the loop body may not be executed at all)
  • For validating input
  • For repeating a menu
• For
  • For pretest loop (the loop body may not be executed at all
  • To test for initialization and update code
  • When a statement is useful with counters or if the precise number of repetitions is known

Nested Loops

• A nested loop is a loop inside the body of another loop

Breaking Out of a Loop

• The break statement ends the current loop and jumps to the statement immediately following the loop.
• When used in an inner loop, it terminates that loop only and returns to the outer loop.
• When used wisely, it can make code harder to understand because the flow is less controlled

The continue Statement

• The continue statement ends the current iteration and jumps to the top of the loop
• This skips a particular iteration for a given condition
• When used wisely, it can make code harder to understand because the flow is less controlled