Operation #6: Repetition

Some procedures require even more than SIPO-plus-Selection; they require the ability to repeat a group of instructions, e.g.

• continue firing the boiler while the temperature is below the set point
• process records from the file until there are none left
• increase the brightness by 10% for every pixel in the image

In all these cases the same instructions are carried out multiple times (and sometimes on multiple items). The final control structure allows us to specify the repetition of commands. Python provides two repetition statements while, and for. We will consider while here to get a flavour of repetition. The format of the while statement is:

while test :
    statements to repeat

The effect of this construct is to continue executing the statements "inside" the while, as long as the test is true, e.g.

while the temperature is low :

    fire the boiler

Although the italicized portions are not Python statements I hope this gives you a sense of the construct. When the program reaches the while statement it evaluates the test. If the test is false it skips over the indented statements and continues with the next statement in the program (i.e. if the temperature is not low we do not fire the boiler). If it is true however it executes the statements, and then reevaluates the test (i.e. if the temperature is low we fire the boiler and then check to see if the temperature is still low). If the test is still true it executes the statements and reevaluates the test (i.e. if the temperature is still low, fire the boiler again). If the test is still true it executes the statements and reevaluates the test. This continues until the test is false. Thus the statements are done while the test is true.

Diagramatically this can be represented as,