Finding empty columns. Is there a faster way?

Jon Clements joncle at googlemail.com
Thu Apr 21 22:32:47 CEST 2011


On Apr 21, 5:40 pm, nn <prueba... at latinmail.com> wrote:
> time head -1000000 myfile  >/dev/null
>
> real    0m4.57s
> user    0m3.81s
> sys     0m0.74s
>
> time ./repnullsalt.py '|' myfile
> 0 1 Null columns:
> 11, 20, 21, 22, 23, 24, 25, 26, 27, 30, 31, 33, 45, 50, 68
>
> real    1m28.94s
> user    1m28.11s
> sys     0m0.72s
>
> import sys
> def main():
>     with open(sys.argv[2],'rb') as inf:
>         limit = sys.argv[3] if len(sys.argv)>3 else 1
>         dlm = sys.argv[1].encode('latin1')
>         nulls = [x==b'' for x in next(inf)[:-1].split(dlm)]
>         enum = enumerate
>         split = bytes.split
>         out = sys.stdout
>         prn = print
>         for j, r in enum(inf):
>             if j%1000000==0:
>                 prn(j//1000000,end=' ')
>                 out.flush()
>                 if j//1000000>=limit:
>                     break
>             for i, cur in enum(split(r[:-1],dlm)):
>                 nulls[i] |= cur==b''
>     print('Null columns:')
>     print(', '.join(str(i+1) for i,val in enumerate(nulls) if val))
>
> if not (len(sys.argv)>2):
>     sys.exit("Usage: "+sys.argv[0]+
>          " <delimiter> <filename> <limit>")
>
> main()


What's with the aliasing enumerate and print??? And on heavy disk IO I
can hardly see that name lookups are going to be any problem at all?
And why the time stats with /dev/null ???


I'd probably go for something like:

import csv

with open('somefile') as fin:
    nulls = set()
    for row in csv.reader(fin, delimiter='|'):
        nulls.update(idx for idx,val in enumerate(row, start=1) if not
val)
    print 'nulls =', sorted(nulls)

hth
Jon



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