modifying iterator in for loop in python

Question

@ Padraic Cunningham Let me know if you want me to delete the
question.

I am new to python. I want to skip some iterator values based on some condition. This is easy in C but in python I am having a hard time.

So please help me in understanding why the code here loops 100 times instead of 10.

 for i in range(100):
    print i
    i = i +10

edit: I understand there is option to change step size of for loop. But I am interested in dynamically changing the iterator variable, like we can do in C. Okay, i get it, for loop is different in python than in C. Easy way to do is use the while loop, I did that in my code and it worked. Thank you community!

Asked By: physicist

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Source

Answer 1

To do this use a while loop. Changing the iterator in a for loop will not change the amount if times it iterates Instead you can do

i=0
while i < 100:
    print i
    i = i +10

Answered By: nathan.medz

Answer 2

The for loop is walking through the iterable range(100).

Modifying the current value does not affect what appears next in the iterable (and indeed, you could have any iterable; the next value might not be a number!).

Option 1 use a while loop:

i = 0
while i < 100:
    i += 4

Option 2, use the built in step size argument of range:

 for i in range(0,100,10):
       pass

This example may make it clearer why your method doesn’t make much sense:

for i in [1,2,3,4,5,'cat','fish']:
    i = i + i
    print i

This is entirely valid python code (string addition is defined); modifying the iterable would require something unintuitive.

See here for more information on how iterables work, and how to modify them dynamically

Answered By: en_Knight

Answer 3

If you try this code it should work.

for i in range(100)[::10]:
    print i

The [::10] works like string slicing. [first position to start at: position to stop at:number to steps to make in each loop]

I didn’t use the first two values so these are set to the default of first position and last position. I just told it to make steps of 10.

Answered By: foolish_snake

Answer 4

If you want to update an iterator, you can do something like that :

iterator= iter(range(100))
for i in iterator:
    print (i)
    for k in range(9): next(iterator)

But no practical interest !

Answered By: B. M.

modifying iterator in for loop in python

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