Comparing two words from two lists in python

Question

I would like to compare words that are in two different lists, so for example, I have:
['freeze','dog','difficult','answer'] and another list ['freaze','dot','dificult','anser']. I want to compare the words in this list and give marks for incorrect letters. So, +1 for being correct, and -1 for one letter wrong. To give some context, in a spelling test, the first list would be answers, and the second list would be the student's answers. How would i go about doing this?

Answer 1

Assuming the two lists are the same length and you have some function grade(a,b) where a,b are strings:

key = ['freeze','dog','difficult','answer']
ans = ['freaze','dot','dificult','anser']

pairs = zip(key, ans)
score = sum(grade(k,v) for (k,v) in pairs)

A possible grading function would be:

def grade(a,b):
    return 1 if a == b else -1

A grading function that punishes each wrong character and gives 1pt for a correct spelling (that sounds harsh...) might be:

def grade(a,b):
    score = sum(a == b for (a,b) in zip(a,b)) - max(len(a), len(b))
    return score if score else 1

If you want the Levenshtein distance, you would probably want your grade function to be a wrapper around the following, which was found on Wikibooks and appears to be reasonably efficient:

def levenshtein(seq1, seq2):
    oneago = None
    thisrow = range(1, len(seq2) + 1) + [0]
    for x in xrange(len(seq1)):
        twoago, oneago, thisrow = oneago, thisrow, [0] * len(seq2) + [x + 1]
        for y in xrange(len(seq2)):
            delcost = oneago[y] + 1
            addcost = thisrow[y - 1] + 1
            subcost = oneago[y - 1] + (seq1[x] != seq2[y])
            thisrow[y] = min(delcost, addcost, subcost)
    return thisrow[len(seq2) - 1]

You could also take a look at difflib to do more complicated stuff.