How to sum specific elements in an array

Question

I want to sum elements in an array. For example I have an array

[183948, 218520, 243141, 224539, 205322, 203855, 233281, 244830, 281245,
 280579, 235384, 183596, 106072,  88773,  63297,  38769,  28343]

I want to sum it in three different parts which are the first three elements, the next 10 elements and the rest. My only idea is to separate the array and use sum method. Is there better way to do it? Thanks in advance!

Slice the array and sum the slices. Since it seems your partitioning of the array is arbitrary, there's not going to be a truly elegant solution. Perhaps you can put your slice indices into a list and use a for loop to go through them — roganjosh
– roganjosh, Commented Jun 22, 2019 at 9:40
@roganjosh have you answered my question before? your name seems familiar? — Wolfeatspotatoes
– Wolfeatspotatoes, Commented Jun 22, 2019 at 10:37

bahij zeineh · Accepted Answer · 2019-06-22 11:43:36Z

1

try this:

arr=[183948, 218520, 243141, 224539, 205322, 203855, 233281, 244830, 281245,
 280579, 235384, 183596, 106072,  88773,  63297,  38769,  28343]

first=arr[0:3]
second=arr[3:13]
last=arr[13:]

print(sum(first))
print(sum(second))
print(sum(last))

the alternative more extensible version is as follows

arr=[183948, 218520, 243141, 224539, 205322, 203855, 233281, 244830, 281245,
 280579, 235384, 183596, 106072,  88773,  63297,  38769,  28343]

indices=[3,13]
results=[]
prev=0
for i in indices:
    results.append(sum(arr[prev:i]))
    prev=i
results.append(sum(arr[prev:]))

for res in results:
    print(res)

note: set prev = to the index you want to start from, in this case 0

edited Jun 22, 2019 at 11:43

answered Jun 22, 2019 at 9:45

bahij zeineh

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3 Comments

roganjosh Over a year ago

I think this is the only way, but a vast improvement would be having the indices in a list and iterating with a for loop. This is hard-coded.

bahij zeineh Over a year ago

that works too but i think for only 3 slices this is ok. if he had more slice points then that way would def be alot better tho

roganjosh Over a year ago

Ok, but would it not be better to post an extensible answer? What if someone else comes along that has 10 slices?

Paul Panzer · Accepted Answer · 2019-06-22 13:26:28Z

0

You can use the reduceat method of np.add:

data = [183948, 218520, 243141, 224539, 205322, 203855, 233281, 244830, 281245,
 280579, 235384, 183596, 106072,  88773,  63297,  38769,  28343]

sizes = 3, 10

np.add.reduceat(data, np.cumsum([0, *sizes]))
# array([ 645609, 2198703,  219182])

answered Jun 22, 2019 at 13:26

Paul Panzer

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1 Comment

cssstudent Over a year ago

What should I do if sizes is zero? e.g. sizes = 3, 0, 10 which returns array([ 645609, 224539, 2198703, 219182]), even though the desired result would be array([ 645609, 0, 2198703, 219182])

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