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
```