6.2. for Loops

If you were to instruct your pet turtle to draw a square in the sand, your commands would quickly become quite tedious. You must tell your pet to move then turn, move then turn, etc. four times. If you want your turtle to draw a hexagon, an octagon, or a polygon with 42 sides, it would be a nightmare to repeat all the commands!

Example

Turtle statements for drawing a square:

import turtle

pet = turtle.Turtle()

pet.forward(50)
pet.left(90)
pet.forward(50)
pet.left(90)
pet.forward(50)
pet.left(90)
pet.forward(50)
pet.left(90)

Note: If you have not practiced with turtles in class yet, check out the Python Turtles appendix.

Time to use iteration! The first Python iteration tool we will learn is the for loop.

6.2.1. for Loop Syntax

A for loop repeats a specific set of code statements a specific number of times. When a for loop begins, we must declare exactly how many times it will run.

Play around with the basic syntax of a for loop.

Try It

This program prints the integers 0 through 20, one number per line.

1. What do you notice about the numbers printed in the console vs. the value inside range()?

2. Try changing the value inside range(). What values throw an error?

3. What happens when you indent line 4 to match line 2?

Let’s break down the 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.

2. Line 2 begins the loop body. The loop body is ALWAYS indented. The indentation determines exactly what statements are “in the loop”. You saw this when you indented line 4 to match line 2. Originally, line 4 was outside of the loop and ran only once. By indenting line 4, it becomes part of the loop body and gets repeated multiple times.

3. The first unindented line after the for statement marks the end of the loop body.

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().

6.2.1.1. Line By Line

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

Example

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

print("Done!")

Console Output

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 picture to show the flow of execution of this for loop:

Flow of execution of a for loop

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

6.2.1.2. 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.

Note

Programmers like to count from 0!

For range(n), the loop variable will take each integer value from 0 up to BUT NOT INCLUDING n.

6.2.2. Check Your Understanding

Question

How does python know what lines are contained in the loop body?

1. The lines are indented by the same amount from the for statement.
2. There is always exactly one line in the loop body.
3. The loop body ends with an empty line.
4. The loop body ends at the next for statement.

Question

How many lines does the following code print?

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

1. 1
2. 9
3. 10
4. 12

Question

For the code above, what is the value of number the third time Python executes the loop?

1. 1
2. 2
3. 3
4. 4

Question

For the same code, what is the LAST line printed by the program?

1. I have 2 cookies. I'm going to eat one!
2. I have 3 cookies. I'm going to eat one!
3. I have 10 cookies. I'm going to eat one!
4. I have 12 cookies. I'm going to eat one!