Adding Specific Value to Nested Lists

Question

I'm having a comprehension issue on a specific problem. I have a preexisting nested list, and I want to match and append one value from a different list to the end of each nested list. A quick example of what I've tried, but where I'm stuck:

 initial_values = [["First", 1], ["Second", 2], ["Third", 3], ["Fourth", 4]]
other_values = [1,2,3,4]
for sublist in initial_values:
 for i in other_values:
 sublist.append(i)
print initial_values

This returns [['First', 1, 1, 2, 3, 4], ['Second', 2, 1, 2, 3, 4], ['Third', 3, 1, 2, 3, 4], ['Fourth', 4, 1, 2, 3, 4]]

I want it it to ideally return [['First', 1, 1], ['Second', 2, 2], ['Third', 3, 3], ['Fourth', 4, 4]]

Answer 1

You can use zip to match elements of the same index from different lists. From there on, you're a simple list concatenation away:

[a + [b] for a,b in zip(initial_values, other_values)]

Answer 2

Your double for-loop takes each sublist in turn (the outer for-loop) and appends every element of other_values to it (the inner for-loop). What you want instead is to add each element of other_values to the corresponding sublist (i.e. the sublist at the same position/index). Therefore, what you need is only one for-loop:

initial_values = [["First", 1], ["Second", 2], ["Third", 3], ["Fourth", 4]]
other_values = [1,2,3,4]
for i in range(len(initial_values)): # get all the valid indices of `initial_values`
 initial_values[i].append(other_values[i])

Here's are some simpler ways to do it:

for i,subl in enumerate(initial_values):
 subl.append(other_values[i]))

Or

for subl, val in zip(initial_values, other_values):
 subl.append(val)

• Jean-François Fabre: the solution with enumerate seems to be the most efficient one. No need to bother zip for this and create a lot of temporary objects.
• inspectorG4dget: @Jean-FrançoisFabre: in python3, zip doesn't create temp objects - it returns an iterator with pointers to the original objects
• Jean-François Fabre: I know that, but the tuples inside still have to be created.
• inspectorG4dget: @Jean-FrançoisFabre: it's still O(1) space, since it's an iterator instead of a full list. Technically enumerate and zip have the same space complexity, though the time complexity may be different due to memory caching
• Jean-François Fabre: I was talking about memory allocation avoided by the use of enumerate of course.

Answer 3

It seems like you want to go through both lists with a single iteration. You could achieve that using zip:

for sublist, i in zip(initial_values, other_values):
 sublist.append(i)

Answer 4

If you want to use for loop you can try

initial_values = [["First", 1], ["Second", 2], ["Third", 3], ["Fourth", 4]]
other_values = [1,2,3,4]
for i in range(0,len(other_values)):
 initial_values[i].append(other_values[i])
print initial_values

output:
[['First', 1, 1], ['Second', 2, 2], ['Third', 3, 3], ['Fourth', 4, 4]]

Answer 5

Built-in function zip will match first item in first list with first item in second list etc.

initial_values = [["First", 1], ["Second", 2], ["Third", 3], ["Fourth", 4]]
other_values = [1,2,3,4]
for initials, other in zip(initial_values, other_values):
 initials.append(other)

Answer 6

One of the alternative to achieve it using map() as:

>>> map(lambda x: x[0] + [x[1]], zip(initial_values, other_values))
[['First', 1, 1], ['Second', 2, 2], ['Third', 3, 3], ['Fourth', 4, 4]]