Appending a list to a list of lists

Question

In the following code, I am trying to append a list called a to a list of lists b.

a = [5,4]
b = [[4],[3],[8]]
b[2].append(a)

Python outputs

[[4], [3], [8, [5, 4]]] 

However, I want the elements to be appended as integers not as a list so b should be [[4], [3], [8, 5, 4]] and then I want to merge the lists so b would be [4, 3, 8, 5, 4]. I want to be able to do this so I can use the sum function to find the sum of b's elements. Does anyone have suggestions regarding how this can be done?

Answer 1

First, you are trying to extend a list, not append to it. Specifically, you want to do

b[2].extend(a)

append() adds a single element to a list. extend() adds many elements to a list. extend() accepts any iterable object, not just lists. But it's most common to pass it a list.

Once you have your desired list-of-lists, eg

[[4], [3], [8, 5, 4]] 

then you need to concatenate those lists to get a flat list of ints. You can use sum() for that -- adding lists is not that different from adding ints.

b = sum(b, [])

The trick here is that you have to pass the initial (empty) value to sum() , otherwise it tries to add the lists in b as though they were numbers.

Finally, you can sum the flattened list as you intended:

sum(b)

Answer 2

My suggestion is to use the chain function taken from the itertools builtin package to achieve your goal in a more concise and pythonic way.

Just one line of code is needed:

sum(chain(a, tuple(chain.from_iterable(b))))

Answer 3

If I understood it correctly, you want a list full of integers -not a list with lists of integers-.
Anyways I'll try to give a solution for both of them.

for i in range(len(a)):
    b[-1].append(a[i])

Outputs: b = [[4], [3], [8, 5, 4]]

or maybe just every number in one list:

for i in range(len(a)):
    b.append([a[i]])

Outputs: b = [[4], [3], [8], [5], [4]]