python nested list comprehensions

Question

I am learning python and going through their tutorials. I understand list comprehensions and nested lists comprehensions. With the following code, though, I am trying to understand the order of events.

>>> matrix = [
...[1, 2, 3, 4],
...[5, 6, 7, 8],
...[9, 10, 11, 12],
... ]
>>> [[row[i] for row in matrix] for i in range(4)]
[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4,8,12]]

According to the nested list comprehension, is the first "i" and the second "i" the same variable and do they both increase at the same time? I guess I don't understand how the resulting big list goes from the first sublist [1, 5, 9] to the second sublist [2, 6, 10]

Answer 1

[[row[i] for row in matrix] for i in range(4)]

is equivalent to

my_list = []
for i in range(4):
    my_list_2 = []
    for row in matrix:
        my_list_2.append(row[i])
    my_list.append(my_list_2)

is the first "i" and the second "i" the same variable and do they both increase at the same time?

Of course, it is. If it was not the same i , the code would throw an error because one of the two would not be defined.

You may be interested in this question: Understanding nested list comprehension

Answer 2

I made a function in order to do it automatically (sorry for the example, i took it from someone) :

Let's say, you have this example :

# 2-D List 
matrix = [[1, 2, 3], [4, 5], [6, 7, 8, 9]] 
  
flatten_matrix = [] 
  
for sublist in matrix: 
    for val in sublist: 
        flatten_matrix.append(val) 

This is my function : (first, turn the example into a string that you will send to the function)

x = "for sublist in matrix:for val in sublist:flatten_matrix.append(val)"

then the function :

def ComprenhensionizeList(nested_for_loop_str):

    splitted_fors = nested_for_loop_str.split(':')
    lowest_val = splitted_fors[1].split(' ')[1]
    comprehensionizer = '[ '+ lowest_val+' '+splitted_fors[0]+' '+splitted_fors[1]+' ]'
    print(comprehensionizer)

and the output :

[ val for sublist in matrix for val in sublist ]