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I'm trying to understand the following snippet of python code:

lst = [[c for c in range(r)] for r in range(3)] #line1
for x in lst:                                   #line2
    for y in x:                                 #line3
        if y < 2:                               #line4
            print('*', end='')                  #line5

I know what the functions like range(3) mean by its own, but I don't get the context. It's a bit complicated to see this nested piece of code. The first line with the 'lst' is most confusing. Why is the first line producing the following output:

[[], [0], [0, 1]]

and how does line2 and line3 works together? Thanks in advance for your answer. Every idea is welcome!

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  • just a nested list comprehension that could be written really way more simply [range(r) for r in range(3)] Commented Dec 11, 2019 at 20:56
  • this is a list comprehension (the first line). Try to add prints to understand better what's going on, or use this visualizer Commented Dec 11, 2019 at 20:58
  • It's not a real-world example of code. It's just an exam question, so I try to get a better understanding of it. Commented Dec 11, 2019 at 20:58
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    r is set to 0,1, and 2. For each r value it makes a list with the length of r, the values of range(r). It's best to test this in an interactive session. Try [c for c in range(3)], etc Commented Dec 11, 2019 at 20:59

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Re

"The first line with the 'lst' is most confusing.":

Wherever you see [ ...for...] you have what's called a "list comprehension." This is a way to build up a list based on a one-line-loop description of the elements. For example:

list1 = [letter for letter in 'abcd']

and

list2 = []
for letter in 'abcd':
    list2.append(letter)

yield identical lists list1 and list2

In your case, you have two sets of [] and two for statements, so you have a list comprehension inside a list comprehension: so the result is not just a list but a nested list.

Re

"and how does line2 and line3 works together?"

Line2 iterates through all the items in your list lst. But each of those items is also a list, because you have a nested list. So line3 iterates through each item in that inner list.

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2 Comments

Thank you very much! I got it :)
Awesome! Glad to hear it!
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In that nested list comprehension, r begins with a value of 0, so the inside list evaluates to [c for c in range(0)], which is []. When r is 1, it evaluates to [c for c in range(1)], which is [0]. When c is 2, you have [c for c in range(2)], which is [0,1]. When these are generated within the outer list comprehension, these are returned as list elements of the list. So you have a list of lists in lst.

The for loop then iterates over each of these lists in line 2. Line 3 then iterates over the integer elements within each list.

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lst = [[c for c in range(r)] for r in range(3)]

this is a nested list comprehension.

But here it could be simplified as we don't need a list of lists, just a list of range objects so we can iterate on them. So

lst = [range(r) for r in range(3)]

is way simpler.

And while we're at it, why creating a list comprehension at all? Just remove it, and use a classic loop

for r in range(3):
    for y in range(r):
        if y < 2:
            print('*', end='')
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0

The snippet creates a LIST of LISTS where size (and item's value actually) depends on the index in the first level list.

The snippet is the same as

result = []
for k in range(3):
    result.append([])
    for v in range(k):
        result[k].append(v)

print(result)
=>>>
[[], [0], [0, 1]]
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0

The first line is equivalent to

lst = [list(range(r)) for r in range(3)]

It is a list comprehension which generates a list containing list up to r-1. list(range(r)) generates [0, 1, 2, ..., r-1], and because the r variable gets the values 0, 1 and 2 in the list comprehension, it means it will generate a list with the lists [] (r=0), [0], (r=1) and [0, 1] (r=2).

Line 2 iterates over the lists of lst, and line 3 iterates over the values of each list. Therefore, these two lines jointly iterate over all the numbers in lst.

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