Jackson wrote:
> I'm currently using a function pasted in below. This allows me to sum
> a column (index) in a list of lists.
>
> So if mylist = [[1, 2, 3], [1, 3, 4], [2, 3, 4], [2, 4, 5]]
> group_results(mylist,[0],1)
>
> Returns:
> [(1, 5), (2, 7)]
>
> What I would like to do is allow a tuple/list of index values, rather
> than a single index value to be summed up, so you could say
> group_results(mylist,[0],[1,2]) would return [(1, 5,7), (2, 7,9)] but
> I'm struggling to do so, any thoughts? Cheers
>
> from itertools import groupby as gb
> from operator import itemgetter as ig
>
> def group_results(table,keys,value):
> res = []
> nkey = ig(*keys)
> value = ig(value)
> for k, group in gb(sorted(table,key=ig(*keys)),nkey):
> res.append((k,sum(value(row) for row in group)))
> return res
You could write a version of sum() that can cope with tuples:
from itertools import groupby, imap
def itemgetter(keys, rowtype=tuple):
def getitem(value):
return rowtype(value[key] for key in keys)
return getitem
def sum_all(rows):
rows = iter(rows)
sigma = next(rows)
rowtype = type(sigma)
sigma = list(sigma)
for row in rows:
for i, x in enumerate(row):
sigma[i] += x
return rowtype(sigma)
def group_results(table, key, value):
get_key = itemgetter(key)
get_value = itemgetter(value)
table = sorted(table, key=get_key)
for keyvalue, group in groupby(table, get_key):
yield keyvalue + sum_all(imap(get_value, group))
but I'd probably use a dict-based approach:
def group_results(table, key, value):
get_key = itemgetter(key)
get_value = itemgetter(value)
grouped = {}
for row in table:
key = get_key(row)
value = get_value(row)
if key in grouped:
grouped[key] = tuple(a + b for a, b in zip(grouped[key], value))
else:
grouped[key] = value
return [k + v for k, v in sorted(grouped.iteritems())]
if __name__ == "__main__":
items = [(1, 2, 3), (1, 3, 4), (2, 3, 4), (2, 4, 5)]
print list(group_results(items, [0], [1, 2]))
Note that the function built with my version of itemgetter() will always
return a tuple.
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