writing arrays to binary file

[Travis E. Oliphant]

Sheldon wrote:
> Hi everyone,
> 
> I have a short program the writes the output data to an acsii file:
> 
> import sys
> import os
> import string
> import gisdata
> from Numeric import *
> 
> def latlon(area_id, i, j):
>     lon_file = "lon_%s.dat" % area_id
>     flon        = open(lon_file, 'wa')
>     lat_file   = "lat_%s.dat" % area_id
>     flat          = open(lat_file, 'wa')
>     for x in range(i):
>         for y in range(j):
>             flat.write("%f\n" % gisdata.xy2lonlat(area_id,x,y)[1])
>             flon.write("%f\n" % gisdata.xy2lonlat(area_id,x,y)[0])
>     flat.close()
>     flon.close()
> #------------------------------------------------------------
> if __name__ == '__main__':
>     tile_info ={"ibla_35n13e":[1215,1215],
>                        "ibla_46n16e":[1215,1215],
>                        "ibla_57n40w":[1215,1215],
>                        }
>     for t in tile_info.keys():
>         xsize = tile_info[t][0]
>         ysize = tile_info[t][1]
>         result = latlon_(t, xsize, ysize)
> Now this works but the output is in ascii form. Whenever I try to write
> it binary form by creating the 2D array and then opening the file with
> open("filename", 'wb'), I lose the decimals that are vital. The input
> data is float with about 4 decimal places.

You need to use the tostring() method and write that out in binary form.

flon = open(lon_file,'wb')
flon.write(your2darray.tostring())

of course you will need to manage byte-order issues if you will be 
transferring this file to a different kind of processor.


Alternatively, with new NumPy there is a tofile method that you can use 
to write in either ascii or binary form.


-Travis Oliphant