18. While loops

See: https://www.w3schools.com/python/python_while_loops.asp

While loops run a set of statements as long as a test condition is true.

The statements within the loop are indented.

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18.1. Events as conditions

While loops have a condition that is tested each time the loop runs.

The condition must be True for the loop to run.

If the condition is False, the loop is exited.

In the code below, the code within the while loop runs only while the x is greater than 10.

x = 10
while x > 0 :
    print(x)
    x = x - 1
x = 10

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18.2. Counters

While loops have a condition that is tested each time the loop runs.

The condition must be True for the loop to run.

If the condition is False, the loop is exited.

A counter can be used in the while loop condition.

The counter is set before the while loop.

The counter is incremented (increased by a constant amount) in a while loop.

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18.3. Counting up

In the example below, i is the counter.

i starts off at 0 and is increased by 1 in the while loop line: i += 1.

i += 1 is the same as i = i + 1

While the condition, i < 10, is True the while loop runs.

i += 1 causes i to increase from 0 to 9.

The while loop exits when i is 10 since i < 10 will be False when i = 10.

i = 0
while i < 10:
    print(i)
    i += 1

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18.4. Counting down

In the code below, i starts off at 5 and is decreased by 1 in the while loop line: i -= 1.

i -+= 1 is the same as i = i - 1

The test uses a > sign when counting down.

The loop below stops when i is no longer greater than 1, i.e. when it is 1.

i = 5
while i > 1:
    print(i)
    i -= 1

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18.5. Step size

The code below counts up from 0 to 10 in steps of 2.

i += 2 sets a step size of 2.

i = 0
while i < 11:
    print(i)
    i += 2

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Tasks

1. Write a while loop that counts up from 1 to 5, printing the numbers 1, 2, 3, 4, 5.

2. Write a while loop that counts up from 3 to 12 in steps of 3, printing the numbers 3, 6, 9, 12.

3. Write a while loop that counts down from 9 to 1, printing the numbers 9, 8, 7, 6, 5, 4, 3, 2, 1.

4. Write a while loop that counts down from 24 to 18 in steps of 2, printing the numbers 24, 22, 20, 18.

5. Write 2 while loops to print 0 to 8 going up in 2s then 9 down to 1 going down in 2s.

Q1

Write a while loop that counts up from 1 to 5, printing the numbers 1, 2, 3, 4, 5.

i = 1
while i < 6:
    print(i)
    i += 1

Q2

Write a while loop that counts up from 3 to 12 in steps of 3, printing the numbers 3, 6, 9, 12.

i = 3
while i < 13:
    print(i)
    i += 3

Q3

Write a while loop that counts down from 9 to 1, printing the numbers 9, 8, 7, 6, 5, 4, 3, 2, 1.

i = 9
while i > 0:
    print(i)
    i -= 1

Q4

Write a while loop that counts down from 24 to 18 in steps of 2, printing the numbers 24, 22, 20, 18.

i = 24
while i > 17:
    print(i)
    i -= 2

Q5

Write 2 while loops to print 0 to 8 going up in 2s then 9 down to 1 going down in 2s.

i = 0
while i < 9:
    print(i)
    i += 2
i = 9
while i > 0:
    print(i)
    i -= 2