How to make all lists in a list of lists the same length by adding to them

Question

I have a nested list which contains lists filled with strings. What I am trying to do is make each list in this nest the same length as the longest available list in that n

Answer 1

You can do this,

maxLen = max(map(len, arr))
[row.extend(['null']*(maxLen - len(row))) for row in arr]

Answer 2

The expression 'null' * (len(maxSS7) - len(row)) creates one potentially very long string.

Use

row.extend('null' for _ in xrange(maxSS7 - len(row)))

instead. The generator expression lets you avoid creating an extra list object just to extend row.

>>> ['null' * 2]
['nullnull']
>>> ['null' for _ in xrange(2)]
['null', 'null']

But the .extend call itself is never reached, as you if statement is testing the wrong thing; change it to:

 if len(row) < maxSS7:

maxSS7 is aready a number (the length of the longest list); asking that number for it's length is not what you were looking for.

Answer 3

The problem is with the line:

if row < len(maxSS7):

You're comparing the list row with the integer len(maxSS7). It will evaluate to False each time. Change it to:

maxLen = max(map(len, myList))
for row in myList:
    if len(row) < maxLen:
        row.extend(...)

Martijn Peters pointed another problem with your code in his answer.