Question

I would like to concatenate a list of strings into new strings grouped over values in a list. Here is an example of what I mean:

Input

key = ['1','2','2','3']
data = ['a','b','c','d']

Result

newkey = ['1','2','3']
newdata = ['a','b c','d']

I understand how to join text. But I don’t know how to iterate correctly over the values of the list to aggregate the strings that are common to the same key value.

Any help or suggestions appreciated. Thanks.

Answer 1

from collections import defaultdict

d = defaultdict(list)

for k, v in zip(key, data):
    d[k].append(v)

print [(k, ' '.join(v)) for k, v in d.items()]

Output:

[('1', 'a'), ('3', 'd'), ('2', 'b c')]

And how to get new lists:

newkey, newvalue = d.keys(), [' '.join(v) for v in d.values()]

And with saved order:

newkey, newvalue = zip(*[(k, ' '.join(d.pop(k))) for k in key if k in d])