Changing the iterator of the for loop inside the code block

Question

Suppose I have a code block like,

for i in range(15):
    print(i)
    i+=5

I expect the i value at each iteration should be i = 0,5,10, ....

Even though I am changing the iterator inside the code block of for loop, the value is not affecting the loop.

Can anyone explain the functionality happening inside?

[Changing the value of range during iteration in Python](https://stackoverflow.com/q/38672963). Or [Modify range variable in for loop (python)](https://stackoverflow.com/a/36051434) — 001, Dec 09 '21 at 20:53
Why would it? The range object doesn't know that the variable was changed, it can't return a next value that depends on it. — Barmar, Dec 09 '21 at 20:54
A new value is assigned to `i` by the `for` statement at the start of each iteration. You can monkey with `i` inside the loop, but it'll always get reset to the next value from the iterator. — Samwise, Dec 09 '21 at 20:54
To get your desired result: `for i in range(0, 15, 5): print(i)` — 001, Dec 09 '21 at 20:54
I think the OP wants the first 15 multiples of 5, not the multiples of 5 less than 15. — chepner, Dec 09 '21 at 20:56
If you thought that each iteration would add 1 to `i`, then shouldn't the result be `0, 6, 12, ...`? `i+=5` adds 5, then `for` would add 1 more to it. — Barmar, Dec 09 '21 at 21:08

Skyrider Feyrs · Answer 1 · 2021-12-10T03:30:47.523

0

Here, you're defining i as a number from 0 to 14 everytime it runs the new loop. I think that what you want is this:

i = 0
for _ in range(/*how many times you want i to print out*/):
    print(i)
    i += 5

edited Dec 10 '21 at 03:30

answered Dec 09 '21 at 20:54

Skyrider Feyrs

82
16

This will go past 15. – Barmar Dec 09 '21 at 20:55
I *know*. Thats on purpose because he wants it fifteen times – Skyrider Feyrs Dec 10 '21 at 03:26
It's not clear what he wants, or what he's thinking the logic is. – Barmar Dec 10 '21 at 03:29

score 0 · Answer 2 · answered Dec 09 '21 at 20:55

A for loop is like an assignment statement. i gets a new value assigned at the top of each iteration, no matter what you might do to i in the body of the loop.

The for loop is equivalent to

r = iter(range(15))
while True:
    try:
        i = next(r)
    except StopIteration:
        break
    print(i)
    i += 5

Adding 5 to i doesn't have any lasting effect, because i = next(r) will always be executed next.

score 0 · Answer 3 · answered Dec 09 '21 at 20:56

0

As the others have said, range emits the number and your assignment doesn't matter.

To get the desired result use something like

for i in range(0, 15, 5):
    print(i)

answered Dec 09 '21 at 20:56

oittaa

599
3
16

Changing the iterator of the for loop inside the code block

3 Answers3