Whenever we need to do something over and over again in a program, loops allow us to repeat those steps without having to type each one separately.

The for Loop

A for loop in Python iterates over a list of items, or repeats once for each item in a list -like the numbers 1 through 100, or 0 through 9. We want our loop to run four times - once for each circle - so we need to set up a list of four numbers.

The built-in function range() allows us to easily create lists of numbers. The simplest command to construct a range of n num- bers is range(n); this command will let us build a list of n numbers from 0 to n-1 (from zero to one less than our number n).

The while Loop

Instead of iterating over a predefined list of values, as the for loop does, a while loop can check for a condition or situation and decide whether to loop again or end the loop.

The condition is usually a Boolean expression, or true/false test. One everyday example of a while loop is eating and drinking. While you are hungry, you eat.

The Spiral

Spiral Goes Viral

Summary

At this point, you should be able to do the following:

  • Create your own for loops to repeat a set of instructions a cer- tain number of times.
  • Use the range() function to generate lists of values to control your for loops.
  • Create empty lists and add to lists using the append() function.
  • Create your own while loops to repeat while a condition is True or until the condition is False.
  • Explain how each type of loop works and how you code it in Python.
  • Give examples of situations in which you would use each type of loop.
  • Design and modify programs that use nested loops.

Challenges