How to conditionally skip number of iteration steps in a for loop in python?

Question

We have a list item_list,

item_list = [\"a\", \"b\", \"XYZ\", \"c\", \"d\", \"e\", \"f\", \"g\"]

We iterate over its items with a

Answer 1

list_iter = iter(item_list)

for item in list_iter:
    if item == "XYZ":
        do_something()
        for _ in range(3):   # skip next 3 items
            next(list_iter, None)

# etc.

Basically, rather than iterating over the list directly, you create an abstraction for it called an iterator and iterate over that. You can tell the iterator to advance to the next item by calling next(...) which we do three times to skip the next three items. The next time through the loop, it picks up at the next item after that.

Answer 2

Use an iterator:

$ cat t.py 
item_list = ["a", "b", "XYZ", "c", "d", "e", "f", "g"]

it = iter(item_list)
for item in it:
    if item == "XYZ":
       print item
       for _ in range(3):
           next(it, None)
    else:
        print item

This gives:

$ python t.py 
a
b
XYZ
f
g

Answer 3

Since nobody has mentioned a while loop, I will:

item_list = ["a", "b", "XYZ", "c", "d", "e", "f", "g"]
i = 0
while i < len(item_list):
    item = item_list[i]
    if item == "XYZ":
        do_something()
        i += 3
    else:
        do_something_else()
    i += 1

Answer 4

I would split the processing it two parts for sake of readability

>>> def foo(item_list,key = "XYZ", skip = 3):
    from itertools import takewhile, islice
    def do_something():
        return "do_something()"
    def do_something_else():
        return "do_something_else()"
    it = iter(item_list)
    for items in takewhile(lambda e: e != key, it):
        print items, do_something_else()
    print do_something()
    it = islice(it,skip, None)
    for items in it:
        print items, do_something_else()


>>> foo(item_list)
a do_something_else()
b do_something_else()
do_something()
f do_something_else()
g do_something_else()

Answer 5

I'd store a counter that handles skipping items.

def skipper(item_list):
    skip_count = 0
    for item in item_list:
        if item == "XYZ":
            skip_count = 3
        else:
            if skip_count:
                skip_count -= 1
            else:
                # do_something()
                print item,

Example:

In [23]: item_list
Out[23]: ['a', 'b', 'XYZ', 'c', 'd', 'e', 'f', 'g']

In [24]: skipper(item_list)
a b f g

Answer 6

I'd probably write it out like this, personally:

xyz_i = item_list.index('XYZ')

do_something('XYZ') #or do_something(item_list[xyz_i]) but.. just save yourself the list lookup

for x in item_list[xyz_i+4:]:
    do_something_else(x)