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Very new Python learner here - please be gentle. I'm having some problems picking up the concept of list comprehensions in Python so I'm trying to equate list comprehensions code in terms of for loops.

I was given a problem wherein I have 2 lists: One is a list of values and the second is a list of indices. The job was to use the list of indices to extract those columns from the list of values to create a shorter list. So for example if the lists are: 

my_table = ['1', '2', '3', '4']
           ['5', '6', '7', '8']
           ['-2', '-3', '-4', '-5']

col_indices = [0, 2]

We want:

Output = ['1', '3']
         ['5', '7']
         ['-2', '-4']

So I have seen this post on Stack Overflow before and I know a way to do this with one list comprehension is as follows:

answer = []
for row in my_table:
    reduced_row = [row[idx] for idx in col_indices] 
    answer.append(reduced_row)
return answer

To be honest I kind of get this but not completely. To better my understanding I tried to write some nested loops to get the same result but realized I don't have a good way to do this with my current level of comprehension.

Originally I tried the following:

col_list = []
for row in my_table:
    for idx in col_indices:
        new = [row[idx]]
        col_list.append(new)    
return col_list

But I know this is wrong because it just appends ever value to the first column. In short, I KNOW that list comprehension is the more efficient way to pull out columns from indices like this but this code was provided for me and I'm not sure I could have written it on my own with my current level of understanding. In an attempt to better parse the idea of list comprehension I'm trying to figure out a way this could be done only using loops but I have not yet figured out a way to do so. Can someone help me figure this out so hopefully I can get a better handle on how this code works?

chainhomelow
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  • You know your code above returns unexpected indent followed by `TypeError: list indices must be integers or slices, not tuple` – Xantium Jan 15 '18 at 20:21
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    There's an answer that translates the list-comprehension to a for-loop. Note, you could do the entire thing with a nested list comprehension: `[[row[i] for i in col_indices] for row in my_table]` – juanpa.arrivillaga Jan 15 '18 at 20:28
  • Thanks for the link and I apologize for the potential duplicate - most of the post I've seen were about how to make loops INTO list comprehension and I was working backwards so they weren't particularly relevant. @juanpa.arrivillaga I do think I conceptually understand your more compact list comprehension; thank you for it. Now it seems to be just a matter of practicing until I can recognize the nuances of getting the formatting I need. – chainhomelow Jan 15 '18 at 20:55

1 Answers1

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You need create a temporary list before the start of inner loop, then fill it:

for row in my_table:
    new = []
    for idx in col_indices:
        new.append(row[idx])
    col_list.append(new)

Basically, most of the list comprehensions could be translated to loop with this pattern.

georgexsh
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  • Thank you @georgexsh, I was able to step through this with a debugger and it was very helpful. I'm coming over from MATLAB and am still trying to wrap my head around not using indices or +=1 to create new cells/row. I appreciate your answer. – chainhomelow Jan 15 '18 at 20:51