Question

I’m trying to make histogram by python. I am starting with the following snippet:

def histogram(L):
    d = {}
    for x in L:
        if x in d:
            d[x] += 1
        else:
            d[x] = 1
    return d

I understand it’s using dictionary function to solve the problem.
But I’m just confused about the 4th line: if x in d:

d is to be constructed, there’s nothing in d yet, so how come if x in d?

Answer 1

The code inside of the for loop will be executed once for each element in L, with x being the value of the current element.

Lets look at the simple case where L is the list [3, 3]. The first time through the loop d will be empty, x will be 3, and 3 in d will be false, so d[3] will be set to 1. The next time through the loop x will be 3 again, and 3 in d will be true, so d[3] will be incremented by 1.