MIT Pset6

## itertools.combinations code, available here:http://docs.python.org/library/itertools.html#itertools.combinations

def combinations(iterable, r):
    # combinations('ABCD', 2) --> AB AC AD BC BD CD
    # combinations(range(4), 3) --> 012 013 023 123
    pool = tuple(iterable)
    n = len(pool)
    if r > n:
        return
    indices = range(r)
    yield tuple(pool[i] for i in indices) ## seems like this is just adding the first combo -- why can't it be inside the for loop?
    while True:
        for i in reversed(range(r)):
            if indices[i] != i + n - r:
                break
        else: ## what's up with this syntax? shouldn't else be indented?
            return
        indices[i] += 1 ## is this still part of the for loop?
        for j in range(i+1, r):
            indices[j] = indices[j-1] + 1
        yield tuple(pool[i] for i in indices) ## why can't this go at the top of the for loop before everything else?

## my attempt

def getCombinations(sortSub, size):
    """returns list with all ordered combinations of len(size)
    see itertools in python documation for more info"""
    # Note: itertools returns a tuple. Tuple not returned here
    # b/c yield function is not used
    combinations = []
    # Create tuple holding all letters of sortSub
    pool = tuple(sortSub)

    n = len(pool)

    # do nothing if len(pool) is less than the length of how long you
    # want the return tuples
    if size > n:
        return

    indices = range(size)

    newCombo = ""
    while True: ## this is how you make an infinite loop?
        for i in reversed(range(size)):

            # append string to combinations
            for x in indices:
                newCombo += pool[x]
            combinations.append(newCombo)
            newCombo = ""

            # Debug
            print "Latest combination =",combinations[-1]
            print "i:",i,"n:",n,"size:",size,"i + n - size =",i+n-size,"indices[i] =",indices[i],"indices =",indices

            if indices[i] == i + n - size: #
                return combinations
            indices[i] += 1
            for j in range(i+1, size):
                indices[j] = indices[j-1] + 1