5.1 While loop

Learning objectives

By the end of this section you should be able to

• Explain the loop construct in Python.
• Use a while loop to implement repeating tasks.

While loop

A while loop is a code construct that runs a set of statements, known as the loop body, while a given condition, known as the loop expression, is true. At each iteration, once the loop statement is executed, the loop expression is evaluated again.

• If true, the loop body will execute at least one more time (also called looping or iterating one more time).
• If false, the loop's execution will terminate and the next statement after the loop body will execute.

    # Initializing the first two Fibonacci numbers
    f = 1
    g = 1
    print (f, end = ' ')

    # Running the loop while the last Fibonacci number is less than 20
    while g < 20:
      print(g, end = ' ')
      # Calculating the next Fibonacci number and updating the last two sequence numbers
      temp = f
      f = g
      g = temp + g
    

Counting with a while loop

A while loop can be used to count up or down. A counter variable can be used in the loop expression to determine the number of iterations executed. Ex: A programmer may want to print all even numbers between 1 and 20. The task can be done by using a counter initialized with 1. In each iteration, the counter's value is increased by one, and a condition can check whether the counter's value is an even number or not. The change in the counter's value in each iteration is called the step size. The step size can be any positive or negative value. If the step size is a positive number, the counter counts in ascending order, and if the step size is a negative number, the counter counts in descending order.

    # Initialization
    counter = 1

    # While loop condition
    while counter <= 10:
      if counter % 2 == 1:
        print(counter)
      # Counting up and increasing counter's value by 1 in each iteration
      counter += 1
    

    n = 4
    while n > 0:
      print(n)
      n = n - 1
    
    print("value of n after the loop is", n)
    

n = 4
while n > 0:
  print(n)
  # Modified line
  n = n + 1

print("value of n after the loop is", n)