Count occurrences in a list according to prefixes

Question

Let’s say I have a list

['rato', 'roeu', 'rolha', 'rainha', 'rei', 'russia']

and another list with prefixes

['ro', 'ra', 'r']

how do I count how many times each prefix is within the first list?

Answer 1

One-Liner functional abominable with sum and map:

sum(map(lambda x: 1 if x.startswith(tuple(pref)) else 0, words))

Abominable one-Liner with reduce:

reduce(lambda x, y: x + 1 if y.startswith(tuple(pref)) else x, words, 0)

:)

---

Update:

According to OP requirements, even more abominable one-Liner snow:

map(lambda p: reduce(lambda c, w: c + 1 if w.startswith(p) else c, words, 0), pref)

Less forced example:

def countPrefix(words, prefix):
    return len([1 for w in words if w.startswith(prefix)]) 

[countPrefix(words, p) for p in pref]   

Upshot:

[2, 2, 6]

Answer 2

I could do it this way:

>>> palavras = ['rato', 'roeu', 'rolha', 'rainha', 'rei', 'russia']
>>> prefixos = ['ro', 'ra', 'r']
>>> len(filter(None, [p if p.startswith(tuple(prefixos)) else None for p in palavras]))
6

*In this case to know how many words had one of the occurrences of the list.

Answer 3

Assuming that there is no "hierarchy" between prefixes (e.g., every word that begins with ro also begins with r), a simple and direct way is using the itertools.product. It will combine each element of the first list with each element of the second list. Then just filter those that the second prefix the first, and count:

>>> import itertools
>>> palavras = ['rato', 'roeu', 'rolha', 'rainha', 'rei', 'russia']
>>> prefixos = ['ro', 'ra', 'r']
>>> len([palavra for palavra,prefixo in itertools.product(palavras, prefixos) if palavra.startswith(prefixo)])
10

Note that the first 4 words have been counted twice (because they begin as much with ro/ra as to r). For a solution that only counts each word once, see for example Orion and Anthony Accioly (that of the Dherik does both, and also counts occurrences by prefix).

Answer 4

words = ['rato', 'roeu', 'rolha', 'rainha', 'rei', 'russia']
pref = ['ro', 'ra', 'r']

contTotal = 0
for p in pref:
    cont = 0
    for w in words:
        if w.startswith(p):
            cont+=1
    contTotal += cont
    print p + ' aparece ' + str(cont) + ' vezes nas palavras como prefixo'
print 'O numero total de vezes é ' + str(contTotal)