Python make second array from first

Question

I'm working with python and I read out a column from a CSV file. I save the values in an array by grouping them. This array looks something like this:

[1, 5, 10, 15, 7, 3]

I want to create a second array where I take the number of the array and make the sum with the previous values. So in this case I would like the have the following output:

[1, 6, 16, 31, 38, 41]

My code is as follows:

import csv
import itertools

with open("c:/test", 'rb') as f:
    reader = csv.reader(f, delimiter=';')

    data1 = []
    data2 = []

    for column in reader:
        data1.append(column[2])

    results = data1
    results = [int(i) for i in results]
    results = [len(list(v)) for _, v in itertools.groupby(results)]
    print results

    data2.append(results[0])
    data2.append(results[0]+results[1])
    data2.append(results[0]+results[1]+results[2])
    print data2

So I can make the array by doing it manually, but this costs a lot of time and is probably not the best way to do it. So what is the best way to do something like this?

Possible duplicate of [How to find the cumulative sum of numbers in a list?](http://stackoverflow.com/questions/15889131/how-to-find-the-cumulative-sum-of-numbers-in-a-list) — mkrieger1, Apr 12 '17 at 11:13

score 3 · Answer 1 · answered Apr 12 '17 at 11:18

3

You are looking for the cumulative sum of a list. The easiest way is to let numpy do it.

>>> import numpy as np
>>> np.cumsum([1, 5, 10, 15, 7, 3])
array([ 1,  6, 16, 31, 38, 41])

answered Apr 12 '17 at 11:18

timgeb

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Stupid question, why there are 2 whitespaces before '6'? – dylan_fan Apr 12 '17 at 11:21
That's just how my REPL decided to print it. – timgeb Apr 12 '17 at 11:22
Thing is, mine also and it makes no sense hah – dylan_fan Apr 12 '17 at 11:23

dylan_fan · Accepted Answer · 2017-04-12T11:18:32.200

1

a = [1, 5, 10, 15, 7, 3]
b = [a[0]]
for i in range(1, len(a)):
    b.append(b[-1]+ a[i])

a is your column from .csv. b is a list with already one value in it, which is first item of a. Then we loop through a starting from it's second item and we add the consequent values from it to last item of b and append it to b.

edited Apr 12 '17 at 11:18

answered Apr 12 '17 at 11:12

dylan_fan

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score 0 · Answer 3 · answered Apr 13 '17 at 07:45

Using your code objects, what you look for would be something like:

from __future__ import print_function

import csv
import itertools

"""
with open("c:/test", 'rb') as f:
    reader = csv.reader(f, delimiter=';')
    for column in reader:
        data1.append(column[2])
"""

data1 = [1, 5, 10, 15, 7, 3]

results = [data1[0]]

for i in range(1, len(data1)):
    results.append(results[i-1] + data1[i])

print(data1, results)

Python make second array from first

3 Answers3