While loops

A while loop continuously loops until a conditional check is no longer True.

counter = 0
while counter < 5:
    print(counter)
    counter += 1

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In this example the loop checks if the counter variable is less than 5. So long as that check returns a boolean True then the code inside the loop will run.

Be careful with while loops to avoid infinite looping. In the code below the programmer forgot the line that changes the value of counter. As such it always stays at 0 which will never be equal or greater than 5.

counter = 0
while counter < 5:
    print(counter)

Break

Just as before with the for loop, the break keyword will stop the loop and go to the first line of code after it.

counter = 0
value_to_check = 3

while counter < 5:
    print(counter)
    if value_to_check == counter:
        break
    counter += 1

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1
2
3

Continue

Same as with the for loop but remember when the continue line is encountered, the program jumps back to the start of the loop section. Because of this it is important to remember to change the value used in the conditional check before the continue line if using a While loop. Otherwise the program can get stuck in an infinite loop.

counter = 0
value_to_check = 3
while counter < 5:
    counter += 1
    if value_to_check == counter:
        continue
    print(counter)

counter = 0
value_to_check = 3
while counter < 5:
    if value_to_check == counter:
        continue
    counter += 1
    print(counter)

In the bad example, when the counter is equal to 3 the continue line runs. So it skips to the start of the loop, but the counter is still 3. It’s stuck in an infinite loop.

Design

So how do represent the above code. Pretty much the same as a for loop. Just change the description in the decision box. But notice the lines where we add a number to our counter. That is neither a decision, initialisation or an input/output. We need a new shape for the flowchart.

The process box can be used to represent one or more lines covering anything from simple arithmetic to function calls. If another shape doesn’t cover what you need to do, then use a process box.

Next: Challenge 4