memory usage multi value hash

[Peter Otten]

christian wrote:

> Hello,
> 
> i'm not very experienced in python. Is there a way doing below more
> memory efficient and maybe faster.
> I import a  2-column file and  then concat for every unique value in
> the first column ( key) the value from the second
> columns.
> 
> So The ouptut is something like that.
> A,1,2,3
> B,3,4
> C,9,10,11,12,90,34,322,21
> 
> 
> Thanks for advance & regards,
> Christian
> 
> 
> import csv
> import random
> import sys
> from itertools import groupby
> from operator import itemgetter
> 
> f=csv.reader(open(sys.argv[1]),delimiter=';')
> z=[[i[0],i[1]] for i in f]
> z.sort(key=itemgetter(0))
> mydict = dict((k,','.join(map(itemgetter(1), it)))
>            for k, it in groupby(z, itemgetter(0)))
> del(z)
> 
> f = open(sys.argv[2], 'w')
> for k,v in mydict.iteritems():
>     f.write(v + "\n")
> 
> f.close()

I don't expect that it matters much, but you don't need to sort your data if 
you use a dictionary anyway:

import csv
import sys

infile, outfile = sys.argv[1:]

d = {}
with open(infile, "rb") as instream:
    for key, value in csv.reader(instream, delimiter=';'):
        d.setdefault(key, [key]).append(value)

with open(outfile, "wb") as outstream:
    csv.writer(outstream).writerows(d.itervalues())