Hi,

I was trying to use loadtxt() today to read in some text data, and I had a problem when I specified a dtype that only contained as many elements as in columns in usecols.  The example below shows the problem:

import numpy as np
import StringIO
data = '''STID RELH TAIR
JOE 70.1 25.3
BOB 60.5 27.9
'''
f = StringIO.StringIO(data)
names = ['stid', 'temp']
dtypes = ['S4', 'f8']
arr = np.loadtxt(f, usecols=(0,2),dtype=zip(names,dtypes), skiprows=1)

With current 1.1 (and SVN head), this yields:

IndexError                                Traceback (most recent call last)

/home/rmay/<ipython console> in <module>()

/usr/lib64/python2.5/site-packages/numpy/lib/io.pyc in loadtxt(fname, dtype, comments, delimiter, converters, skiprows, usecols, unpack)
   309                             for j in xrange(len(vals))]
   310         if usecols is not None:
--> 311             row = [converterseq[j](vals[j]) for j in usecols]
   312         else:
   313             row = [converterseq[j](val) for j,val in enumerate(vals)]

IndexError: list index out of range
------------------------------------------

I've added a patch that checks for usecols, and if present, correctly creates the converters dictionary to map each specified column with converter for the corresponding field in the dtype. With the attached patch, this works fine:

>arr
array([('JOE', 25.300000000000001), ('BOB', 27.899999999999999)],
     dtype=[('stid', '|S4'), ('temp', '<f8')])

Comments?  Can I get this in for 1.1.1?