For Loops

The for loop is the first Python tool for iteration that we will explore. A for loop is typically used for definite iteration. Definite iteration is the process of repeating a specific task with a specific data set. When a for loop begins it can usually be determined exactly how many times it will execute: once for each item in the data set.

For Loop Syntax

We have already seen the basic syntax of a for loop.

1
2
3
4

for num in range(51):
   print(num)

print("Not in the loop!")

This program prints the integers 0 through 50, one number per line. In the language of definite iteration, we say that the loop has a data set of 0-50, and its action is to print a value to the console.

Let’s break down this syntax piece by piece, so we can begin to understand how for loops are structured.

1. In line 1, num is called the loop variable. Each time the loop executes, num gets assigned a new value based on the number in the range. In this example, num will be assigned a value 51 times, starting with 0 and ending at 50. More on that later.
2. Line 2 begins the loop body. The loop body is ALWAYS indented. The indentation determines exactly what statements are “in the loop”. In this example, we are printing each value of num as we loop through the set range.
3. The first unindented line after the for statement marks the end of the loop body. In this example, line 4 is not part of the loop.
4. The loop body can contain any number of statements.
5. The number of times the loop body runs depends on the value in range().

Line By Line

Let’s modify the code just a little to follow the operation of a for loop.

1
2
3
4
5

for num in range(4):
   print(num)
   print("Hello" * num)

print("Done!")

0

1
Hello
2
HelloHello
3
HelloHelloHello
Done!

1. The first time Python executes the for statement in line 1, num is assigned a value of 0.
2. Next, Python checks if the value of num is less than the value inside range. Since num < 4 evaluates to True, the loop body executes.
3. Line 2 prints the current value of num.
4. Line 3 prints the string Hello zero times.
5. Python reaches the end of the loop body (the indented lines). At this point, it increases the value of num by 1 and then MOVES BACK TO THE for STATEMENT (line 1).
6. The new value of num (1) gets compared to the range value. Since num < 4 still returns True, the loop body executes again.
7. Lines 2 and 3 run with the new value of num, so we see 1 and Hello printed to the console.
8. Python again reaches the end of the loop body, increases the value of num and moves back up to the for statement.
9. This process continues until the value of num reaches the end of the specified range. Once the comparison num < 4 returns False, the loop ends. Since Python adds 1 after each iteration, this occurs when num is 4 (so 4 < 4 is False). At that point, the loop body will have run exactly 4 times, with num taking the values 0, 1, 2, and 3.
10. Once the loop finishes, Python proceeds to line 5 and prints Done! one time.

We can use a diagram to show the flow of execution of this for loop:

Notice that even though line 1 uses range(4), the value 4 is NOT included in the output. Why?

Begin Counting at 0

Iterating a certain number of times is a very common thing to do, and Python gives us the built-in range keyword to provide a set of values for the loop variable to use.

The sequence provided by range always starts with 0. If you ask for range(4), then you will get 4 values starting with 0. In other words, 0, 1, 2, and finally 3. Notice that 4 is not included since we started with 0. Likewise, range(10) provides 10 values, 0 through 9. Starting a count at 0 instead of at 1 is called zero-based indexing and is very common in computer programming.

Check Your Understanding

1
2

for number in range(10):
   print("I have", 12 - number, "cookies. I'm going to eat one!")