# 8.1. Repeating Steps & Range¶

Learning Objectives:

• Use a for loop to repeat steps with turtles.

Often times we have to repeat steps in a process. If you are making a cake the recipe might say add an ingredient and then stir it 50 times or until the batter is smooth. The recipe would not say the word “stir” 50 times. It instead would instruct you to “stir 50 times” or “stir until smooth”.

When our programs need to repeat a step or series of steps, we want to use this same trick to make the programs easier to write and read. While writing turtle programs, we have seen many examples of steps repeated. Here is our recipe for a turtle alex to draw a hexagon:

 1alex.forward(100)
2alex.right(60)
3alex.forward(100)
4alex.right(60)
5alex.forward(100)
6alex.right(60)
7alex.forward(100)
8alex.right(60)
9alex.forward(100)
10alex.right(60)
11alex.forward(100)
12alex.right(60)


There are six repetitions of the forward and right commands. Although copy/paste can save us from having to type the lines six times, it is still not ideal. Remember that there is a general rule in programming: DRY - don’t repeat yourself. Repeating ourselves in this case makes for a long chunk of code that is awkward to read - you have to stop and think “OK, how many times is this repeating… ah, six… OK, this must be a hexagon.”

We would like to rewrite our code to tell alex to repeat the forward and right steps 6 times each instead of listing them six times. We can do so with a for loop:

A loop in a program is anything that causes lines of code to get executed multiple times in a row. A for loop is a specific kind of loop in Python that uses a list of values and a series of steps and repeats all of the steps for each item in the list.

In this case, the steps that are repeated are the forward and right commands that are indented after the for. Notice that the print, which is not indented, only happens one time, after we have repeated the forward and right commands.

## 8.1.1. Range¶

The list of items the for uses to determine how many times to repeat is created by the range function. This is a function built into Python that can be used to generate a list of numbers. This example shows exactly what range(6) produces:

For now, we don’t need to worry about exactly what values range produces. What is important for now is that range(6) makes a list that has 6 values. When that list is used in a for statement, it will cause the code to repeat 6 times. For each value in the list produced by range, the for loop will repeat the indented statements that come after it. Each of these repetition is called an iteration. To iterate is to go through items one at a time.

That just leaves the variable side in the for statement to explain:

for side in range(6):


It is the name we will use for each item as we iterate through them. For now we will not need to us the names of the items very often. We mostly care that range produces a certain number of values, not what those values are. But if we wanted to print out each of the values that range produces, we could modify the program to use side to name them. In this program, for each value in the list produced by range, we call the current item side, print it out, then do the turtle commands.

The body of a loop is the part that gets repeated. It is the lines of code after the for that are indented past the for itself. (Like how the body of a procedure is indented after def).

Notice that a blank line does not “end” the body of the for. Both print("side is currently", side) and the alex.forward and alex.right are part of the body and get repeated. The loop body ends at the first line that is indented to the same level as the for. In this case, the print("Done with program") is indented to the same level as for, so at that point the loop body is done and the line is not part of the loop. That last print will only happen one time.

Note

The code that is the body of a loop must be indented one step past the loop. The loop body ends at the first line that is indented the same (or fewer) spaces.