The while loop

It allows you to repeat a set of actions until a statement is true. Eg:

n = 5
while n > 0:
    print(n)
    n = n - 1
print('Blastoff!')

This is executed as follows

  • Creates and sets n to 5.
  • Goes to the while statement and checks if n is greater than 0. Right now it’s true, so we move inside the loop.
    • Prints n, ie outputs 5.
    • Reduces n by 1, ie n is now 4
  • Goes back to the while statement
    • Checks if n is still greater than 0. It is, so we again move inside the while loop.
    • Prints 4.
    • Reduces n by 1.
  • Goes back to the while statement
    • …
    • …
  • This goes on until n = 0. When this happens, we exit the while loop and go to the statement after that which is print('Blastoff'). So the probram outputs Blastoff and ends.

Infinite loops

Don’t write infinite loops. Eg:

Lather, rinse, repeat

or

n = 10
while n < 20:
    print(n, end=' ')
    n = n - 1
print('Done!')

break statement

We still sometimes use infinite loops with the break statement for more complicated testing conditions. A simpler example is

while True:
    line = input('> ')
    if line == 'done':
        break
    print(line)
print('Done!')

# Code: http://www.py4e.com/code3/copytildone1.py

Here the loop runs repeatedly until it hits the break statement when the user inputs done.

continue statement

This one ends the current iteration and makes it move onto the next iteration of the loop. The loop still goes on. Eg:

while True:
    line = input('> ')
    if line[0] == '#':
        continue
    if line == 'done':
        break
    print(line)
print('Done!')

# Code: http://www.py4e.com/code3/copytildone2.py

An iteration ends if the the first letter of the input is #. A sample output is :

copytildone2

The for Loop

  • We use this when we want to loop inside a set.
  • The set could be a list of words , lines in a file or a list of numbers.

Eg:

friends = ['Joseph', 'Glenn', 'Sally']
for friend in friends:
    print('Happy New Year:', friend)
print('Done!')

for output

Some looping examples ➿

  • Counting the number of items

    count = 0
for itervar in [3, 41, 12, 9, 74, 15]:
  count = count + 1
print('Count: ', count)

  • Total of a set of numbers

    total = 0
for itervar in [3, 41, 12, 9, 74, 15]:
  total = total + itervar
print('Total: ', total)

  • Finding the largest value in a sequence

    largest = None
for itervar in [3, 41, 12, 9, 74, 15]:
    if largest is None or itervar > largest :
        largest = itervar
print('Largest:', largest)

  • Function to find the minimum

    def min(values):
  smallest = None
  for value in values:
      if smallest is None or value < smallest:
          smallest = value
  return smallest

Debugging sugestion

  • Debugging by bisection.
    • Break a problem in half and add a print statement to check.
    • If the mid-point check is incorrect, the problem must be in the first half of the program. If it is correct, the problem is in the second half.
    • Bisect the selection.
    • So on…

Exercises

My solutions

  1. Reads numbers until done; print total, count, and average:

    total = 0
count = 0
average = 0

while True:
    inp = input('Enter a number: ')
    if inp == 'done':
        break
    else:
        try:
            num = float(inp)
        except:
            print('Invalid input')
            continue
        total = total + num
        count = count + 1
        average = total / count

print('Total:', total, 'Count:', count, 'Average:',average)

  2. Prints max and min as well:

    max = None
min = None

while True:
    inp = input('Enter a number: ')
    if inp == 'done':
        break
    else:
        try:
            num = float(inp)
        except:
            print('Invalid input')
            continue
        if max is None or num > max:
            max = num
        if min is None or num < min:
            min = num

print('Max:', max, 'Min:', min)