# How to find all possible combinations in a list (and their sum) with Python

## Mini-learns with Python 3

While learning to code it’s easy to push through from exercise to exercise, chasing the feeling of accomplishment. I find that everything sinks in more, though, if I take a moment to flesh out my thought process from time to time.

I came across a problem on Hackerrank the other day that required me to find all possible sums of four out of five integers in a list. I’m making a note of what I did here, for future reference.

I used the combinations attribute from the itertools module.

### To import this into our Python REPL or code

``````from itertools import combinations
``````

### The function

``````def find_combos(arr):
combos = list(combinations(arr, 4))
print(combos)
``````

### Breaking this down

• `arr` — This will be the list we pass into the `find_combos` function. Ex: `[1, 2, 3, 4, 5]`
• `combos = list(combinations(arr, 4))` — Combinations takes two parameters. The first (`arr`) is what we are iterating over. The second (`4`), is how many of those things in `arr` (e.g — our list of numbers, strings, whatever) that we want to combine. All of the combinations are turned into a list using `list`, then set to a variable named `combos`.
• `combinations` — This is what we imported from itertools that does the dirty work behind the scenes. The code for this can be found in the Python documentation: https://docs.python.org/3/library/itertools.html#itertools.combinations
• `list` — This is a built-in function in Python. My layperson’s way of understanding this is that it turns all those combinations into a list that we can iterate over: https://docs.python.org/3/library/stdtypes.html#lists

### The output

Using this as our list:

``````arr = [1, 2, 3, 4, 5]
``````

Then passing it into our function:

``````find_combos(arr)
``````

The result is:

``````[(1, 2, 3, 4), (1, 2, 3, 5), (1, 2, 4, 5), (1, 3, 4, 5), (2, 3, 4, 5)]
``````

### Taking it further

The purpose of the exercise was to find the smallest and largest sum of these possible combinations. To do this, I fleshed out my code as follows:

``````def find_combos(arr):
combos = list(combinations(arr, 4))
combo_sums = []
for combo in combos:
combo_sums.append(sum(combo))
print(min(combo_sums), max(combo_sums))
``````
• `combo_sums = []` — This creates an empty list to dump all my sums into.
• The for loop iterates through each `combo` (or combination of four numbers that we saw in the above result: `[(1, 2, 3, 4), (1, 2, 3, 5)…` etc.), takes their `sum`, and appends the sum to the `combo_sums` list. The end result of the `combo_sums` list is:
``````[10, 11, 12, 13, 14]
``````
• `print(min(combo_sums), max(combo_sums))` — This prints the minimum number in `combo_sums` and the maximum number:
``````10 14
``````