Add current element in array + next element in array while iterating through array in Python

Question

What's the best way to add the first element in an array to the next element in the same array, then add the result to the next element of the array, and so on? 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 minimum and maximum 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])

Currently I get the following list:

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

What am I doing wrong that isn't leaving the last item unchanged?

@AndrewL. He does not want to change the last element

Vaibhav Bajaj
– Vaibhav Bajaj

2016-07-31 19:08:56 +00:00
Commented Jul 31, 2016 at 19:08 — Vaibhav Bajaj
– Vaibhav Bajaj, Commented Jul 31, 2016 at 19:08
Didn't see that, my mistake @VaibhavBajaj

Andrew Li
– Andrew Li

2016-07-31 19:10:31 +00:00
Commented Jul 31, 2016 at 19:10 — Andrew Li
– Andrew Li, Commented Jul 31, 2016 at 19:10

Kasravnd · Accepted Answer · 2016-07-31 19:13:45Z

1

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]

edited Jul 31, 2016 at 19:13

answered Jul 31, 2016 at 19:08

Kasravnd

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6 Comments

Vaibhav Bajaj Over a year ago

I am afraid this is not what he wants. He wants the last element to stay the same

Andrew Li Over a year ago

He isn't using numpy, so why the extra library?

Kasravnd Over a year ago

@AndrewL. Just updated the answer. I just tried to give the best answer.

Andrew Li Over a year ago

I'm just not sure why you need an external library to accomplish this. Sure, it's easier, but the OP doesn't mention the use of numPy

user1332577 Over a year ago

Works great! Thanks, it's also much cleaner than my loop.

|

Julien Spronck · Accepted Answer · 2016-07-31 19:12:14Z

1

You can use numpy.cumsum():

import numpy as np
np.append(np.cumsum(s[:-1]), s[-1])
# array([50., 51.2658, 52.5003, 53.7408, 54.969 , 56.1848, 100.])

answered Jul 31, 2016 at 19:12

Julien Spronck

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