Adding index to numpy arrays

Question

I have a numpy array as:

prob_rf = [[0.4, 0.4, 0.4], 
           [0.5, 0.5, 0.5], 
           [0.6, 0.6, 0.6]]

I want to add an index number to each of the inner arrays as:

prob_rf = [[1, 0.4, 0.4, 0.4],
           [2, 0.5, 0.5, 0.5],
           [3, 0.6, 0.6, 0.6]]

and then save this array into a csv file using numpy.savetxt.

I am currently doing this as:

    id = [i for i in xrange(1,len(prob)+1)]
    prob_rf = np.insert(prob_rf, 0, id, axis=1)
    np.savetxt("foo.csv", prob_rf, delimiter=",", fmt='%1.1f')

But this is giving the output as

[[1.0, 0.4, 0.4, 0.4], 
 [2.0, 0.5, 0.5, 0.5], 
 [3.0, 0.6, 0.6, 0.6]]

Could someone please tell me how do I get the output as

[[1, 0.4, 0.4, 0.4], 
 [2, 0.5, 0.5, 0.5], 
 [3, 0.6, 0.6, 0.6]]

Why are you telling numpy to format everything with fmt='%1.1f' if that's not what you want? — Carsten
– Carsten, Commented Mar 23, 2015 at 11:13
It's because I want them in the format [0.4, 0.4, 0.4] else np.savetext saves as [4.00E-01, 4.00E-01, 4.00E-01] which I don't want. — VeilEclipse
– VeilEclipse, Commented Mar 23, 2015 at 11:18

Carsten · Accepted Answer · 2015-03-23 11:22:05Z

7

Use a list with the fmt parameter to specify the formatting for each column:

fmt=['%d', '%1.1f', '%1.1f', '%1.1f']

Complete example:

import numpy as np
prob_rf = [[1, 0.4, 0.4, 0.4],
           [2, 0.5, 0.5, 0.5],
           [3, 0.6, 0.6, 0.6]]
np.savetxt("foo.csv", prob_rf, delimiter=",", fmt=['%d', '%1.1f', '%1.1f', '%1.1f'])

The resulting file:

1,0.4,0.4,0.4
2,0.5,0.5,0.5
3,0.6,0.6,0.6

answered Mar 23, 2015 at 11:22

Carsten

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