user1332577 - 9 months ago 53

Python Question

What's the best way to go about adding the current element in an array to the next element in the array, then using that newly created element to add that to the next element of the array. For example, I have an array:

`s=[50, 1.2658, 1.2345, 1.2405, 1.2282, 1.2158, 100]`

I would like the end array to look like the following:

`new_s=[50, 51.2658, 52.5003, 53.7408, 54.969, 56.1848, 100]`

Thus leaving the max,min elements of the array unchanged. I started going this route:

`arr_length=len(s)`

new_s=[50]

for i, item in enumerate(s):

if i == 0:

new_s.append(new_s[i]+s[i+1])

elif 0<i<=(arr_length-2):

new_s.append(new_s[i]+s[i+1])

What am I doing wrong that isn't allowing me to keep the last item unchanged?

Currently I'm getting the following list:

`new_s=[50, 51.2658, 52.5003, 53.7408, 54.969, 56.1848, 156.1848]`

Answer

The beset way is using `numpy.cumsum()`

for all of your items except the last one then append the last one to the result of `cumsum()`

:

```
>>> import numpy as np
>>> s=[50, 1.2658, 1.2345, 1.2405, 1.2282, 1.2158, 100]
>>>
>>> np.append(np.cumsum(s[:-1]), s[-1])
array([ 50. , 51.2658, 52.5003, 53.7408, 54.969 , 56.1848,
100. ])
```

Or with python (3.X) use `itertools.accumulate()`

:

```
>>> import itertools as it
>>>
>>> list(it.accumulate(s[:-1])) + s[-1:]
[50, 51.2658, 52.500299999999996, 53.74079999999999, 54.968999999999994, 56.184799999999996, 100]
```