Petr Petrov - 3 months ago 18

Python Question

I have a list:

`lst = [1, 2, 3, 5, 0, 0, 9, 45, 3, 0, 1, 7]`

And I need the sum of the elements between the

`0`

I tried

`lst1 = []`

summ = 0

for i, elem in enumerate(lst):

if elem != 0:

summ = summ + elem

else:

lst1.append(summ)

lst1.append(elem)

summ = 0

but it returns

`[11, 0, 0, 0, 57, 0]`

`[11, 0, 0, 57, 0, 8]`

Answer

Here's one way to this with `itertools.groupby`

and a *list comprehension*. The grouping is done by checking if an element is zero, and if not zero, all items in the group are summed:

```
from itertools import groupby
lst = [1, 2, 3, 5, 0, 0, 9, 45, 3, 0, 1, 7]
f = lambda x: x==0
result = [i for k, g in groupby(lst, f) for i in (g if k else (sum(g),))]
print(result)
# [11, 0, 0, 57, 0, 8]
```

And of course, if items in your list are only numbers (to avoid generalising and introducing ambuigities), the `lambda`

can be replaced with `bool`

:

```
result = [i for k, g in groupby(lst, bool) for i in ((sum(g),) if k else g)]
```