Setting the fmt option in numpy.savetxt

Knowing that np.savetxt only works for 1D or 2D arrays, the general idea is:

• when fmt is a single formatting string it applies to all elements in the
  array (1D or 2D input array)
• when fmt is a sequence of formatting strings, it applies to each column of the 2D input array

I’m presenting here some examples using the following input array:

import numpy as np

a = np.array([[11, 12, 13, 14],
              [21, 22, 23, 24],
              [31, 32, 33, 34]])

1) Setting floating point precision: np.savetxt('tmp.txt', a, fmt="%1.3f")

11.000 12.000 13.000 14.000
21.000 22.000 23.000 24.000
31.000 32.000 33.000 34.000

2) Adding characters to right-justify.

With spaces: np.savetxt('tmp.txt', a, fmt="% 4d")

  11   12   13   14
  21   22   23   24
  31   32   33   34

With zeros: np.savetxt('tmp.txt', a, fmt="%04d")

0011 0012 0013 0014
0021 0022 0023 0024
0031 0032 0033 0034

3) Adding characters to left-justify (use of “-“).

With spaces: np.savetxt('tmp.txt', a, fmt="%-4d")

11   12   13   14  
21   22   23   24  
31   32   33   34  

4) When fmt is a sequence of formatting strings, each row of a 2D input array is processed according to fmt:

fmt as a sequence in a single formatting string

fmt="%1.1f + %1.1f / (%1.1f * %1.1f)"
np.savetxt('tmp.txt', a, fmt=fmt)

11.0 + 12.0 / (13.0 * 14.0)
21.0 + 22.0 / (23.0 * 24.0)
31.0 + 32.0 / (33.0 * 34.0)

fmt as an iterator of formatting strings:

fmt="%d", '%1.1f', '%1.9f', '%1.9f'
np.savetxt('tmp.txt', a, fmt=fmt)

11 12.0 13.000000000 14.000000000
21 22.0 23.000000000 24.000000000
31 32.0 33.000000000 34.000000000