For Statements

So far, if we had to iterate over something, we used while statements. As it turns out, Python actually has another type of iteration: for statements. Here’s an example:

>>> for i in range(0, 5):
...     print(i)
0
1
2
3
4

Let’s break down what happens here. The i in the for statement tells Python to create a new variable in the current environment and bind it to the first value in the range. What range does will be more clear after reading the Containers notes after this, but for now, just keep in mind that range(x,y) returns the values x, x+1, all the way to y-1 one by one.

What happens next is that we use the value of i, which is first set to 0, inside our loop in the print statement. Then, we go back to the for statement and i gets set to the next value – 1. This continues until i reaches 4. After this is printed, we return to the for statement, but the range statement refuses to return anything because it’s designed to not include the y index that’s passed in. If you’re wondering why, read the Containers notes after this.

We can generalize for statements like so (from the Containers lecture):

for <name> in <expression>:
    <suite>

Here, you evaluate the <expression>, which yields something you can iterate over (like a list, which you’ll see very shortly, I promise). For each element in this <expression>, you bind the element to the <name> and then execute the <suite>. Once you run out of elements to bind, you exit the loop.

This was a very brief overview of for statements. If you’re confused, please go on to the Containers notes. All of this will make much more sense after those, which are meant to go hand-in-hand with these.


Contributors: Vanshaj Singhania