How to match two arrays

Question

I have two arrays

A = [a, b, c, d]

and

B = [a1, a2, b1, b2, b3, c1, c2, c3, d1, d2, d3, d4]

I want to match b

Answer 1

from collections import defaultdict

A = ["a", "b", "c", "d"]
B = ["a1", "a2", "b1", "b2", "b3", "c1", "c2", "c3", "d1", "d2", "d3", "d4"]
d = defaultdict(list)
for item in B:
    prefix = item[0]
    if prefix in A:
        d[prefix].append(item)

Answer 2

In pretty Python:

di = {}
for item in A:
    di[item] = filter(lambda v: v.startswith(item), B)

Answer 3

These solutions works fine both in python and IronPython.

Imperative solution:

A = ["a", "b", "c", "d"]
B = ["a1", "a2", "b1", "b2", "b3", "c1", "c2", "c3", "d1", "d2", "d3", "d4"]

results = []

for prefix in A:
    matches = []
    results.append((prefix, matches))
    for el in B:
        if el.startswith(prefix):
            matches.append(el)

for res in results:
    print res

---

Functional solution:

A = ["a", "b", "c", "d"]
B = ["a1", "a2", "b1", "b2", "b3", "c1", "c2", "c3", "d1", "d2", "d3", "d4"]

groups = [(x,[y for y in B if y.startswith(x)]) for x in A]
for group in groups:
    print group

---

RESULT:

('a', ['a1', 'a2'])
('b', ['b1', 'b2', 'b3'])
('c', ['c1', 'c2', 'c3'])
('d', ['d1', 'd2', 'd3', 'd4'])