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? — mkrieger1
– mkrieger1, Commented Apr 12, 2017 at 11:13

timgeb · Accepted Answer · 2017-04-12 11:18:15Z

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, 2017 at 11:18

timgeb

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3 Comments

dylan_fan Over a year ago

Stupid question, why there are 2 whitespaces before '6'?

timgeb Over a year ago

That's just how my REPL decided to print it.

dylan_fan Over a year ago

Thing is, mine also and it makes no sense hah

dylan_fan · Accepted Answer · 2017-04-12 11:18:32Z

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, 2017 at 11:18

answered Apr 12, 2017 at 11:12

dylan_fan

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Comments

boardrider · Accepted Answer · 2017-04-13 07:45:04Z

0

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)

answered Apr 13, 2017 at 7:45

boardrider

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