In the first Python course, we learned how to execute the same code many times using the while loop. In this chapter, we will study another type of loop called the for loop. The for loop is a convenient way to operate on elements of a collection. For example, let's say that we want to calculate the sum of the elements in the following list:

[ 1, 2, 3 ]

Our plan is to create a variable, say, total, which initially contains 0. Then, for each element in the list, we will add the element to total.

Here is our implementation using the for loop:

Line 3 is the for statement, and line 4 is the for loop body.

Let's walk through the example program to see what it does.

1. In line 1, Python assigns the list [ 1, 2, 3 ] to variable numbers.
2. In line 2, Python assigns 0 to variable total.
3. In line 3, Python copies the first element in numbers, 1, to value and enters the for loop body.
4. In line 4, tab 1, Python adds value to total. total is now 1.
5. The for loop body has ended. Return to the for statement in line 3.
6. Python now copies the second element in numbers, 2, to value and enters the for loop body.
7. In line 4, tab 1, Python adds value to total. total is now 3.
8. Python returns to the for statement again and copies the third element in numbers, 3, to value.
9. In line 4, tab 1, Python adds value to total. total is now 6.
10. Python returns to the for statement again. Python copied all elements of the list, so Python exits the loop.

Whenever Python encounters the for statement, Python copies the next element in the numbers list to variable value. Initially, the next element is the first element in the list. When Python reaches the end of the list, Python exits the loop.

Here is the general format of the for statement:

for target in iterable:

• target must be a variable such as x or name.
• iterable must be an expression whose output is a collection of elements (such as a list). Here are some examples:
  • [ 1, 2, 3 ]
  • numbers

Example: Computing the length of a list

Python provides the len function to compute the length of a list, but we can also compute the length without using the len function. See if you can write the solution (hint: keep a counter as you loop through the list).

numbers = [1, 2, 3, 4]
sum = 0
for value in numbers:
    sum = sum + value 

numbers = [1, 2, 3, 4]
sum = 10
for value in numbers:
    sum = sum - value 

Let's now go over the properties that collectively describe the for loop in more detail.