Loops

What if we want to repeat the same code multiple times? We can either:

1. Type out or copy all the code however many times we want it to run. This is very tedious and makes it difficult to fix mistakes.

2. Write the code for only one iteration and have Python repeat it. This is much faster and easier to spot or correct mistakes!

A for loop allows us to repeat a block of code a specific number of times or iterate over a sequence of items.

for i in range(10):
    # Loop body gets repeated
    print(i)
print("Done") # Outside of loop

Let’s break down the components of this statement:

• for is the keyword that signals the start of the loop.

• i represents the loop variable name, which you can change. Its value is updated each time through the loop.

• range(10) is the sequence that controls the loop.

• print(i) is the loop body: the code that gets run once for each iteration of the loop.

range function¶

range is a built-in function (like print) that produces a sequence of integers. It can be called with 1 argument, 2 arguments, or 3 arguments.

range(5)
range(2, 5)
range(1, 11, 2)

Practice: Range¶

Which range call will produce the numbers 10, 20, 30, 40, 50, 60, 70, 80, 90, 100?

range(100)
range(10, 100)
range(10, 110)
range(10, 100, 10)
range(10, 110, 10)

Practice: Rangefinding¶

Write code that uses a for loop to print:

-4
14
32
50
68
86

Accumulator pattern¶

We can accumulate (sum, multiply, etc.) values while going through a for loop. For example, to sum the numbers from 0 to 4:

total = 0
for i in range(5):
    total = total + i
print("The sum is:", total)

Practice: Totals¶

What is the output of the following loop?

total = 10
for number in range(1, total // 2):
    total = total - number
    print(total)

Practice: Squares¶

Write code that uses a for loop to print the numbers 1 through 10 squared:

1
4
9
16
25
36
49
64
81
100

As an extra challenge, modify your code so that it does not need to use the * multiplication operator. Do you notice any patterns between adjacent numbers?

Looping over lists and strings¶

If range produces a sequence of numbers, can we just specify the numbers ourselves?

A list allows us to specify a sequence of values manually using square brackets [].

for i in [10, 13, 7]:
    print(i)
print("Done")

It turns out we can iterate over a lot of things! We can iterate over lists of strings.

print("Some CSE 160 TAs:")
for name in ["Brianna", "Katie", "Suhas"]:
    print(name)

We can even iterate over the individual characters in a single string.

for letter in "Kevin":
    print(letter)