Python Lesson #4: Loops

Loops let us repeat things! Loops let us repeat things!

For Loops

For loops repeat code a specified number of times. They are formatted like this:

for (counter variable) in (sequence):
    (do something)

The counter variable can be named anything, naming it i is quite common. The sequence can be a list, tuple, dictionary (we will cover these later), set (these aren’t that useful, search them up if you’d like), or string.

To repeat code a specific number of times, range() can be used as the sequence. range() returns a sequence of numbers, starting from 0 (by default), goes up by 1 (by default), and stops at a specified number. The following example prints "Hello!" three times because Python runs the code for 0, 1, and 2. It stops counting at 3, our specified number.

for i in range(3):

More commonly, for loops will be used to go through items in a list and do something for each element in sequence. A real-life analogy would be knocking on each watermelon at a grocery store to hear if they are hollow, and then taking out the hollow-sounding ones to purchase. In code, it would look something like this:

basket = ["hollow", "not_hollow", "hollow"]
for watermelon in basket:
    if watermelon == "hollow":
        print("A hollow watermelon was taken out!")

For loops can be used on strings like this, taking each letter as a unit to iterate over. Think of a string as a list of characters!

for letter in "word":

While loops

While loops repeat as long as a specified condition evaluates to True.

i = 1
while i < 4:
    i += 1

Since i is initially 1, which is less than 4, Python prints 1 and then adds 1 to i (to become 2). i += 1 is just a cleaner way of writing i = i + 1. All this repeats this until i becomes 4, and since i < 4 now evaluates to False, the loop stops.

Important notes

Loops can be dangerous because they have the potential to run indefinitely. It can overwhelm your computer if there’s no end to a loop with intensive code. If you ever make a mistake, you can hit Ctrl+C to interrupt the code in the terminal/shell. An example:

while True:
    print("Hit Ctrl+C")

while True is one way to make code constantly run. Once you know what you’re doing, you may want a piece of code to run indefinitely. However, we don’t want this in our case, so once the code is run we need to hit Ctrl+C to stop it.

The keyword break can also be used in code. Once the line is run, it will break the loop.

while True:
    print("Hit ctrl+C")

This will only print one line.

Try it!

Create a script that prints out the numbers 1 to 10 on separate lines when run. Try making it using both for and while loops (separately).

The result should look like:



With a for loop:

for i in range(10):
    print(i + 1)

Remember, range() returns a sequence of numbers. For every unit in the sequence, the number (add 1 because range() counts from 0) is printed.

With a while loop:

count = 1
while count <= 10:
    count += 1

A variable called count is initially created. Since it is less than or equal to 10, the number is printed. The count is increased by one, and the loop continues until it is greater than 10.