formatted string of series of numpy array elements

Question

It seems quit trivial to me but I'm still missing an efficient and "clean" way to insert a series of element belonging to numpy array (as aa[:,:]) in a formatted string to be printed/written. In fact the extended element-by-element specification syntaxes like:

formattedline= '%10.6f  %10.6f  %10.6f' % (aa[ii,0], aa[ii,1], aa[ii,2]) 
file1.write(formattedline+'\n')

are working.

But I have not found any other shorter solution, because:

formattedline= '%10.6f  %10.6f  %10.6f' % (float(aa[ii,:]))
file1.write(formattedline+'\n')

of course gives: TypeError: only length-1 arrays can be converted to Python scalars

or:

formattedline= '%10.6f  %10.6f  %10.6f' % (aa[ii,:]) 
file1.write(formattedline+'\n')

gives: TypeError: float argument required, not numpy.ndarray. I have tried with iterators but with no success.

Of course this is interesting when there are several elements to be printed.

So: how can I combine iteration over numpy array and string formatted fashion?

Andy Hayden · Accepted Answer · 2012-10-05 11:18:06Z

6

You could convert it to a tuple:

formattedline = '%10.6f  %10.6f  %10.6f' % ( tuple(aa[ii,:]) )

In a more general case you could use a join:

formattedline = ' '.join('%10.6f'%F for F in aa[ii,:] )

edited Oct 5, 2012 at 11:18

answered Oct 5, 2012 at 11:04

Andy Hayden

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3 Comments

gluuke Over a year ago

that is good and working! :) ... so the second solution is actually iterating over the numpy array, very nice.

gluuke Over a year ago

... but still, adding one string: formattedline= ' %4s %10.6f %10.6f %10.6f' % (string1, (tuple(aa[ii,:]))) gives TypeError: float argument required, not tuple and I don't understand why

Andy Hayden Over a year ago

@gluuke you need to add them: (string1,)+tuple(aa[ii,:]).

unutbu · Accepted Answer · 2012-10-05 11:38:28Z

2

If you are writing the entire array to a file, use np.savetxt:

np.savetxt(file1, aa, fmt = '%10.6f')

The fmt parameter can be a single format, or a sequence of formats, or a multi-format string like

'%10.6f  %5.6f  %d'

edited Oct 5, 2012 at 11:38

answered Oct 5, 2012 at 11:32

unutbu

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2 Comments

gluuke Over a year ago

... thanks! But what about if I'm not saving the whole array at the same time? ... So if I'm adding slices of array combined with some text?

unutbu Over a year ago

Then I think @hayden's suggestion is best. Under the hood, np.savetxt calls fh.write(asbytes(format % tuple(row) + newline)). (In Python2, asbytes = str.)

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