write numpy array to CSV with row indices and header

Question

I have a numpy array.

values = np.array([14, 21, 13, 56, 12])

I want to write values in one column of a CSV file, the row indices in another column, and a header. I found this function:

numpy.savetxt("foo.csv", values, header="Id,Values", delimiter=",")

I'm not sure how to add the indices (1, 2, 3, 4, 5). Also, my header turns out to be # Id,Values. I'm not sure where the # came from. This is what I get:

# Id,Values
14
21
13
56
12

I want something like this:

Id,Values
1,14
2,21
3,13
4,56
5,12

why are you writing two column names and using a delimiter for a single column? — Padraic Cunningham
– Padraic Cunningham, Commented Jul 23, 2015 at 21:51

NikT · Accepted Answer · 2016-07-31 18:26:04Z

5

There may be a better way but I don't think you can do it directly with numpy.savetxt:

import  numpy as np

arr = np.array([14 ,21, 13, 56, 12])
np.savetxt("foo.csv", np.dstack((np.arange(1, arr.size+1),arr))[0],"%d,%d",header="Id,Values")

*The # is the default behaviour as documented:

String that will be prepended to the header and footer strings, to mark them as comments. Default: ‘# ‘, as expected by e.g. numpy.loadtxt.*

You could use comments="" but it is there for a reason.

edited Jul 31, 2016 at 18:26

NikT

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answered Jul 23, 2015 at 22:20

Padraic Cunningham

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pcc426 · Accepted Answer · 2017-12-02 10:16:13Z

4

    import pandas as pd
dt = pd.DataFrame(data=a)
dt.to_csv('/test_csv.csv', mode='a', index=True)

you can use to_csv function in pandas by explicitly set index=True or default.

answered Dec 2, 2017 at 10:16

pcc426

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AMacK · Accepted Answer · 2015-07-23 22:21:32Z

2

You can do it with csv fairly easily, assuming you're ok with a 0-based index:

import numpy as np
import csv

a = np.array([14, 21, 13, 56, 12])

with open('out.csv', 'wb') as fh:
    writer = csv.writer(fh, delimiter=',')
    writer.writerow(['id','val'])
    writer.writerows(enumerate(a))

answered Jul 23, 2015 at 22:21

AMacK

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