the difference between zip and enumerat in for loop?

Question

The code is here, it deletes some element in a list. What is the difference of zip and enumerate in the loop condition? It seems that enumerate yield wrong result.

def delete(elem_l, del_e):
    n_del = 0
    for e, i in zip(elem_l, range(len(elem_l))):
    # for i, e in enumerate(elem_l):
        print "cycle:", i, "elem:", elem_l[i - n_del],  "len:", len(elem_l)
        if e == del_e:
            del elem_l[i - n_del]
            n_del += 1


elem_list = [1, 2, 3, 4, 3, 5, 5, 4, 3]
delete(elem_list, 3)

Answer 1

At issue here is your use of the zip() function. It produces a list of tuples up front. You created a copy of the original list with numbers added. From the documentation:

This function returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables.

Emphasis mine.

Thus, altering the elem_l in the loop does not affect the loop iteration count.

enumerate() on the other hand produces indices on demand, and as the elem_l list is deleted from, there are fewer elements to produce. You'd get the same outcome if you produced a copy of elem_l instead:

for i, e in enumerate(elem_l[:]):

where the [:] identity slice produces a new list, independent of the original elem_l list you then delete from in the loop.