Solving list comprehensions in python

Question

So I have been learning list comprehensions in python and don't understand the logic of how nested list comprehensions work in python. Consider the following snippets of codes :

####TRADITIONAL EXAMPLE-1###
L = []
for i in range(5):
   for j in range(4):
      L.append(j)

#output of l-> [0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]

####TRADITIONAL EXAMPLE-2###
K = []
for i in range(5):
   for j in range(4):
      K.append(i)

#output of k -> [0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4]

Okay, this all makes sense, but when I want to recreate these using list comprehensions the logic doesn't make sense to me. Now consider these examples :

###LIST COMPREHENSION EXAMPLE-3####
L1 = [j for j in range(4) for i in range(5)]
#Output of L1 -> [0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3]

###LIST COMPREHENSION EXAMPLE-4####
K1 = [i for j in range(4) for i in range(5)]
#Output of K1 -> [0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4]

###LIST COMPREHENSION EXAMPLE-5####
Q = [[j for j in range(4)] for i in range(5)]
#Output of Q -> [[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]

###LIST COMPREHENSION EXAMPLE-6####
Q1 = [[i for j in range(4)] for i in range(5)]
#output of Q1 -> [[0, 0, 0, 0], [1, 1, 1, 1], [2, 2, 2, 2], [3, 3, 3, 3], [4, 4, 4, 4]]

So I don't know why but I expect the output L and L1 (Examples 1 and 3) to be the same irrespective of using the traditional way or list comprehension way and the same goes for K and K1 (Examples 2 and 4) especially when I compare the output of Q and Q1 (Eg 5 & 6). Like compare Q1 and K1 (Example 4 and 6), they are almost following the same syntax but the output is way different.

I hope I am able to clarify myself about my confusion here based on the output each of these list comprehension gives compared to the traditional way of doing it. I tried searching online and following tutorials but they just state examples not explain the working or logic behind it. I just don't want to mug up the outputs without knowing the reason behind why the output is what it is. Thanks for the help :)

abhiarora · Accepted Answer · 2020-02-02T11:06:18.707

So I don't know why but I expect the output L and L1 (Examples 1 and 3) to be the same irrespective of using the traditional way or list comprehension way

You haven't used the list comprehension in the right way. Your inner and outer loop are swapped in your example for L1.

L1 = [j for i in range(5) for j in range(4)]

Outputs:

[0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]

See this answer to learn more on defining a List comprehension having two or more for Loops.

The best way to remember this is that the order of for loop inside the list comprehension is based on the order in which they appear in traditional loop approach. Outer most loop comes first, and then the inner loops subsequently.

Same goes for K and K1:

K1 = [i for i in range(5) for j in range(4)]

Output:

[0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4]

especially when I compare the output of Q and Q1 (Eg 5 & 6)

However, when you use list comprehension like the one in Q, you don't have nested for loops in these examples. It is called nested list comprehension. See this for information on nested loop comprehension.

Solving list comprehensions in python

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