Section 20.1 Repeating Steps & Range
Learning Objectives:
Often 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:
alex.forward(100)
alex.right(60)
alex.forward(100)
alex.right(60)
alex.forward(100)
alex.right(60)
alex.forward(100)
alex.right(60)
alex.forward(100)
alex.right(60)
alex.forward(100)
alex.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.
Subsection 20.1.1 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:
Checkpoint 20.1.1.
What is the list produced by range(6)
The list of numbers 0 to 6
Try running the program…
The list of numbers 1 to 6
Try running the program…
The list of numbers 1 to 5
Try running the program…
The list of numbers 0 to 5
Correct
For now, we don’t need to worry about exactly what values range produces. What is important 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. Each repetition is called an iteration and will repeat all of the code indented past the for . (To iterate is to go through items one at a time.)
That just leaves the variable side in the for statement to explain:
It is the name we will use for each item as we iterate through the values range produces. Right now we don’t really need to name those values - we don’t make use of them. But if we wanted to print out each of the values that range produces, we would use side to refer to “the value for the current iteration”. The program below does exactly that. In the loop, in addition to doing the turtle commands, it prints out side to tell us when it starts each side of the shape.
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.
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