How to make index in "for" loop jump forward by N

Question

I want for loop to change its pass behavior

For example, make it jump every 3rd index to another 2 positions?

for i in range(n):
   if i % 3 == 0:
     print('found i div by 3, jump 2 positions forward')
     i = i + 2

This does not work.

It goes through every i in range from 0 to n.

What do you mean by this does not work? Are you getting any error or wrong output? — Sociopath, Jan 28 '20 at 08:07
@AkshayNevrekar it can't jump forward, it goes thru every i - 0 to n — ERJAN, Jan 28 '20 at 08:08
It seems to work okay, printed 4 times `found i div by 3, jump 2 positions forward` for range 10 — Haseeb Jehanzeb, Jan 28 '20 at 08:11
Does this answer your question? [Skip multiple iterations in loop](https://stackoverflow.com/questions/22295901/skip-multiple-iterations-in-loop) — Georgy, Jan 29 '20 at 11:08

score 5 · Accepted Answer · answered Jan 28 '20 at 08:08

5

You can use a while loop,

i = 0
while i < n:
   if i % 3 == 0:
     print('found i div by 3, jump 2 positions forward')
     i = i + 2
     continue
   i = i + 1

answered Jan 28 '20 at 08:08

Shubham Sharma

Masklinn · Answer 2 · 2020-01-28T09:02:10.770

if you just want a step other than 1, range has a parameter for that, just call range(0, 10, 3) and it'll iterate from 0 (inclusive) to 10 (exclusive) incrementing by 3 each time rather than 1
if you want to do something else and the skip condition is dynamic, don't use iterators, manage your loop manually (using a while and an explicit counter), that's what I'd recommend here
alternatively to (2) you can mess with the iterator itself, a for loop desugars to:
```
it = iter(range(n))
while True:
    try:
        i = next(it)
    except StopIteration:
        break
    ...
```
so if you get the it manually and keep a reference to it, you can call next on it explicitly to skip ahead:
```
it = iter(range(n))
for i in it:
    if i % 3 == 0:
        print('found i div by 3, jump 2 positions forward')
        next(it, None); next(it, None)
```
I would not recommend the second style, the first one already tells the reader upfront that you're doing something weird, and it does it in a straightforward manner.

score 3 · Answer 3 · answered Jan 28 '20 at 08:11

3

You can specify step-size in for loop if you want values divisible by 3

for i in range(0, n, 3):
    print(i)

Output:

0
3
6

answered Jan 28 '20 at 08:11

Sociopath

score 0 · Answer 4 · answered Jan 28 '20 at 08:52

0

We can also use the slice operator [::] to change the original range according to a condition

l= range(n)

for i in l:
    if i%3 == 0:
        print('found i div by 3, jump 2 positions forward')
        l = l[::2]

answered Jan 28 '20 at 08:52

Rain

4 Answers4