[Tutor] Writing elements of an array to a file using CSV module

[Hanlie Pretorius]

Hi Evert,

Thanks for the reply. I've tried your suggestion and am still getting
an error (posted below code). Since my posting, I've also added the
code to convert the numpy array to a list. My complete script is:

-----
# -*- coding: utf-8 -*-

import pupynere
import os
import csv
import glob

def get_file_details(filename):
    sub1='3B42.'
    sub2='.nc'
    extract=filename.split(sub1)[-1].split(sub2)[0]
    elements=extract.split('.')
    ure={'0':'00:00:00',
         '3':'03:00:00',
         '6':'06:00:00',
         '9':'09:00:00',
         '12':'12:00:00',
         '15':'15:00:00',
         '18':'18:00:00',
         '21':'21:00:00'}
    date='20'+elements[0]
    time=ure[elements[1]]
    return[date,time]

def get_rain_data(filename):
    # trmm blocks covering study area
    rows=[87,86,85,86]
    cols=[833,833,833,834]

    # open file
    file = pupynere.netcdf_file(filename,'r')
    # read data
    precipitation=file.variables['precipitation']
    # close file    file.close()

    # read values for blocks of interest
    precip=precipitation[rows,cols]
    # convert numpy array to list
    precip=precip.tolist()
    return[precip]


# get list of .nc files in folder
os.chdir('F:\\Hanlie\\UCT\\M.Sc\\Data\\TRMM\\2000\\02_Februarie')
filepattern='*.nc'
nc_files=glob.glob(filepattern)

# write to CSV-file with spaces as delimiter
csv.register_dialect('spaces', delimiter=' ')
outfilename="trmm_c83a_feb2000.txt"
out_text=open(outfilename, 'wb')
writer = csv.writer(out_text,dialect='spaces')

for file in nc_files:
    file_details=get_file_details(file)
    data=get_rain_data(file)
    writer.writerow(('%s' % (file_details[0]),'%s' %
(file_details[1]),'%f' % float(data[0])))

out_text.close()
-----

I now get the error:
-----
Traceback (most recent call last):
  File "F:\Hanlie\UCT\M.Sc\Data\TRMM\lees_netcdf_per_trmm_blok_skryf_vir_pcraster.py",
line 76, in <module>
    writer.writerow(('%s' % (file_details[0]),'%s' %
(file_details[1]),'%f' % float(data[0])))
TypeError: float() argument must be a string or a number
-----

I tried the other suggestion on the list (posted by Alan Gould),
namely to replace the %f formatting string with %s in the
write.writerow statement:
-----
writer.writerow( ( file_details[0], file_details[1], str(data[0])  ) )
-----

This produces a file with lines such as:
-----
20000201 00:00:00 "[0.0, 0.0, 0.0, 0.0]"
-----

I seem to have a list within a list, I'm not sure why. I could write
the list elements to variables before I write to the file, but it
doesn't seem like a very elegant solution.

Can anyone see why I have such a nested data structure? Or is the
problem something completely different?

Thanks
Hanlie


2010/10/14, Evert Rol <evert.rol at gmail.com>:
>> I have a numpy array ('data') that is the result of reading a netCDF
>> file, and it typically looks like this:
>>
>> array([ 0.,  0.,  0.,  0.], dtype=float32)
>>
>>
>> I want to write this, after a date and time, to a CSV file, so the CSV
>> file would have the entry:
>>
>>    2000-02-01,09:00,0.0,0.0,0.0,0.0
>>
>>
>> The code I'm using to test it, is:
>>
>> # file_details[0] is the date and file_details[1] is the time
>> writer.writerow(('%s' % (file_details[0]),'%s' %
>> (file_details[1]),'%f' % (data[0])))
>>
>>
>> In my opinion, this should give an output line:
>>
>> 2000-02-01,09:00,0.0
>>
>>
>> Instead, I get an error
>>
>> TypeError: float argument required, not numpy.ndarray
>>
>>
>>
>> Can someone please explain to me why data[0] is giving a numpy array
>> rather than one element? I don't understand it, since I get:
>>
>>>>> data[0]
>> 0.0
>
> This means very little, since a string '0.0' will show the same output when
> printed:
>>>> print '0.0'
> 0.0
>
> The exception says 'TypeError', so try:
>
>>>> type(data[0])
> <type 'numpy.float32'>
>
> Which is indeed what you specified at the start using dtype=float32.
>
> The standard string formatting doesn't know what to do with a numpy.float32,
> so you'll need to convert it to a float:
>
>>>> '%f' % float(data[0])
> '0.000000'
>
> (use proper float formatting to get at '0.0', which is what you apparently
> want.)
>
>
>   Evert
>
>