Changing the value of range during iteration in Python

Question

>>> k = 8
>>> for i in range(k):
        print i
        k -= 3
        print k

Above the is the code which prints numbers from 0-7 if I use just print i in the for loop.

I want to understand the above code how it is working, and is there any way we can update the value of variable used in range(variable) so it iterates differently.

Also why it always iterates up to the initial k value, why the value doesn't updated.

I know it's a silly question, but all ideas and comments are welcome.

Aurora0001 · Accepted Answer · 2016-07-30 10:42:51Z

11

You can't change the range after it's been generated. In Python 2, range(k) will make a list of integers from 0 to k, like this: [0, 1, 2, 3, 4, 5, 6, 7]. Changing k after the list has been made will do nothing.

If you want to change the number to iterate to, you could use a while loop, like this:

k = 8
i = 0
while i < k:
    print i
    k -= 3
    i += 1

answered Jul 30, 2016 at 10:42

Aurora0001

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1 Comment

ShadowRanger Over a year ago

Adding on: In Python 3, range is a sequence itself, it doesn't make a list, but it's still based on the value of k at the time it was constructed, with subsequent modifications to k making no difference to the range object.

Martijn Pieters · Accepted Answer · 2016-07-30 10:42:14Z

5

The expression range(k) is evaluated just once, not on every iteration. You can't set k and expect the range(k) result to change, no. From the for statement documentation:

The expression list is evaluated once; it should yield an iterable object.

You can use a while loop instead:

i = 0
k = 8
while i < k:
    print i
    i += 1
    k -= 3

A while loop does re-evaluate the test each iteration. Referencing the while statement documentation:

This repeatedly tests the expression and, if it is true, executes the first suite

answered Jul 30, 2016 at 10:42

Martijn Pieters

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Comments

Aguy · Accepted Answer · 2016-07-30 10:59:39Z

1

If you do want to change k and affect the loop you need to make sure you are iterating over mutable object. For example:

k = list(range(8))
for i in k:
        print(i)
        k.pop()
        k.pop()
        k.pop()
        print(k)

Or alternatively:

k = list(range(8))
for i in k:
        print(i)
        k[:] = k[:-3]
        print(k)

Both will result with

0
[0, 1, 2, 3, 4]
1
[0, 1]

answered Jul 30, 2016 at 10:59

Aguy

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1 Comment

bhansa Over a year ago

thanks, I already tried this. This was the first thing after i figured out that we cannot change the value of range function again.

Unix · Accepted Answer · 2023-01-10 17:23:41Z

0

you could do it like this ,I think, but I dont know if this is what you want.

def to_infinity():
index = 0
while True:
    yield index
    index += 1

for i in to_infinity():
    print(i)

answered Jan 10, 2023 at 17:23

Unix

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