Looping

Introduction

Computer programs tend to focus on just a few kinds of things. One of them is making decisions, which in Python usually involves an if statement.

Another is repeating things, which usually involve loops.

In this chapter we will talk about for loops and while loops.

Objectives

Explain the purpose of a for loop and a while loop.
Write syntactically correct code using loops.
Use a for loop to process elements of a list.
Use range to count the number of times a loop runs.
Use while to repeat a task until something becomes true.
Use continue to skip one run of a loop.
Use break to stop the loop altogether.

Motivation

Suppose you wanted to print out a table of squares for your child’s math practice. We will do the first 5 to preserve our own sanity. The Python program below accomplishes this. Badly.

This results in the following output.

This code isn’t bad because it doesn’t function; it is bad because you, the programmer, repeated yourself over and over for the computer. This breaks Mattox’s Law of Computing.

Looking at this code, you see the only thing that changes is the value of the variable i. The code that makes use of i doesn’t change at all.

Looping with a for loop.

We can tell Python that we want to run the two lines repeatedly, but for different values of i. We do this with a for loop. Here is what it would look like:

for i in [1,2,3,4,5]:
    print(f"{i} * {i} = {i * i}")

Here is the syntax you should notice:

A for-loop starts with the keyword for.
Next comes a variable name, in this case i.
The in keyword separates the variable from the data, which comes next. In this case the data is the list [1,2,3,4,5].
The line ends with a colon. In python, this always indicates that a block of code follows.
The next line is indented: this is the line that gets repeated. This code is called the body of the for loop. It can be multiple lines long. Python knows when you are done when the code is no longer indented.

What Python actually does with this code is this:

It sets variable i to be 1, the first element in the data.
It then runs the print statement.
Once that is done, the for sets i to be the next element in the data, in this case 2.
It then runs the print statement (with i being 2).
… and so on until i has had all the values in the data list.

You should try this yourself to make sure it works. Here are some variations:

We can change the variable name to something else.

    for x in [1,2,3,4,5]:
        print(f"{x} * {x} = {x * x}")

We can iterate over different data. Maybe we only want odd numbers to 9 for our table:
```
    for i in [1,3,5,7,9]:
        print(f"{i} * {i} = {i * i}")
```

We can print lines in between each row. Notice now we have two lines in the body of the for loop. The last line is unindented, signalling to Python that the loop body code is over.

    for i in [1,2,3,4,5]:
        print(f"{i} * {i} = {i * i}")
        print(f"-------------------")
    print("Done.")

Here is what it will output:

    1 * 1 = 1
    -------------------
    2 * 2 = 4
    -------------------
    3 * 3 = 9
    -------------------
    4 * 4 = 16
    -------------------
    5 * 5 = 25
    -------------------
    Done.

Using range.

Typing out a list of sequential numbers is something we would need to do often. To automate this, Python provides a function called range(). It has three forms.

range(n) gives you a range from 0 to \(n-1\).
range(a,b) gives you a range from a to \(b-1\).
range(a,b,s) gives you a range from a to \(b-1\), but skips every \(s\) elements.

The output is a range object. For example,

print(range(10))

will result in

range(0, 10)

This can be used as the data part of a for loop, like this:

for i in range(5):
    print(f"{i} * {i} = {i * i}")
    print(f"-------------------")
print("Done.")

Here the result. The range function starts from zero by default, so we will have to adjust this in a minute.

0 * 0 = 0
-------------------
1 * 1 = 1
-------------------
2 * 2 = 4
-------------------
3 * 3 = 9
-------------------
4 * 4 = 16
-------------------
Done.

Why does it start from 0 and leave out the last element?