CODEWITHSUNDEEP
pythonpython-3.xfor-loop
Debu
Debu

Reputation: 83

How to force a for loop counter to skip iterations in Python3?

I recently ran into an issue where I was using a for loop somewhat similar to this:

for i in range(lineCount(fileToBeProcessed)):
    print(i)
    j = doSomeStuff() #returns number of lines in the file to skip
    i = i+j
    print(i)
    print('next_loop')

For a value of j={2,3,1} the output was:

1
3
next_loop
2
5
next_loop
.
.

My desired output:

1
3
next_loop
4
7
next_loop
.
.

Every time the next iteration started, the for loop counter i reset to the original cycle. My question is, is there a way to force the for loop to skip the iterations based on the return value j. I understand and was able to implement something similar with a while loop. However, I was curious as to how or why would Python not allow such manipulation?

Upvotes: 2

Views: 3704

Answers (3)

fahrenheit317
fahrenheit317

Reputation: 21

I've edited the code hope it may help

z =0
for i in range(lineCount(fileToBeProcessed)):
    if i <= z: #if i is a value that you don't want to be output, then skip loop to next one
        continue
    print(i)
    j = doSomeStuff()
    cnt += 1
    z = i+j #use a different variable from i since i the iterator value will not be updated
    print(z)
    print('next_loop')

Upvotes: 0

tobias_k
tobias_k

Reputation: 82929

Assuming that fileToBeProcessed is actually a file-like object, you can iterate directly over the file (i.e. over the lines in that file), or use enumerate(fileToBeProcessed) if you need the line numbers, and call next on that iterator to skip lines.

Like this (not tested):

iterator = enumerate(fileToBeProcessed) # or just iter = fileToBeProcessed
for i, line in iterator:
    print(i)
    j = doSomeStuff() #returns number of lines in the file to skip
    for _ in range(j):
        i, line = next(iterator) # advance iterator -> skip lines
    print(i)
    print('next_loop')

Upvotes: 1

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476813

It allows manipulations. But a for loop in Python works with a:

for <var> in <iterable>:
    # ...

So Python does not attaches a special meaning to range(n) as a for loop: a range(n) is an iterable that iterates from 0 to n (exclusive). At the end of each iteration the next element of the iterable. It furthermore means that once you constructed a range(n), if you alter n, it has no impact on the for loop. This in contrast with for instance Java, where n is evaluated each iteration again.

Therefore you can manipulate the variable, but after the end of the loop, it will be assigned the next value of the loop.

In order to manipulate the variable, you can use a while loop:

i = 0 # initialization
while i < lineCount(fileToBeProcessed): # while loop
    print(i)
    j = doSomeStuff() #returns number of lines in the file to skip
    i = i+j
    print(i)
    print('next_loop')
    i += 1 # increment of the for loop is explicit here

Usually a while loop is considered to be "less safe" since you have to do the increment yourself (for all code paths in the loop). Since it is something one tends to forget, it is easier to write an endless loop.

Upvotes: 3

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