Incrementing a for loop, inside the loop

Question

Is it possible to increment a for loop inside of the loop in python 3?

for example:

for i in range(0, len(foo_list)):
    if foo_list[i] < bar
        i += 4

Where the loop counter i gets incremented by 4 if the condition holds true, else it will just increment by one (or whatever the step value is for the for loop)?

I know a while loop would be more applicable for an application like this, but it would be good to know if this (or something like this) in a for loop is possible.

Thanks!

Answer 1

You could use a while loop and increment i based on the condition:

while i < (len(foo_list)): 
    if foo_list[i] < bar: # if condition is True increment by 4
        i += 4
    else: 
        i += 1 # else just increment 1 by one and check next `foo_list[i]`

Using a for loop i will always return to the next value in the range:

foo_list = [1,2,3,4,5,6]
bar = 6
for i in range(len(foo_list)):
    print("range i ",i)
    if foo_list[i] < bar:
        i += 4
        print("if i",i)


('range i ', 0)
('if i', 4)
('range i ', 1)
('if i', 5)
('range i ', 2)
('if i', 6)
('range i ', 3)
('if i', 7)
('range i ', 4)
('if i', 8)
('range i ', 5)

Answer 2

In your example as written i will be reset at each new iteration of the loop (which may seem a little counterintuitive), as seen here:

foo_list = [1, 2, 3]

for i in range(len(foo_list)):
    print('Before increment:', i)
    i += 4
    print('After increment', i)

>>>
Before increment: 0
After increment 4
Before increment: 1
After increment 5
Before increment: 2
After increment 6

continue is the standard/safe way to skip to the next single iteration of a loop, but it would be far more awkward to chain continues together than to just use a while loop as others suggested.

Answer 3

a bit hackish...

>>> b = iter(range(10))
>>> for i in b:
...     print(i)
...     if i==5 : i = next(b)
... 
0
1
2
3
4
5
7
8
9
>>> 

Answer 4

while i < end:
  # do stuff
  # maybe i +=1
  # or maybe i += 4

I suppose you could do this in a for loop if you tried, but it's not advisable. The whole point of python's for loop is to look at items, not indices

Answer 5

It is possible to achieve this with a for loop using the send method of a generator, where the counter can be modified if the generator is sent a new value:

def jumpable_range(end):
    counter = 0
    while counter < end:
        jump = yield counter
        if jump is None:
            counter += 1
        else:
            counter = jump

foo_list = [1, 2, 3, 4, 5, 6, 7, 8]
bar = 6
iterable = jumpable_range(len(foo_list))
for i in iterable:
    if foo_list[i] < bar:
        i = iterable.send(i + 4)
    print(i)

This outputs:

4
5
6
7

Demo: https://repl.it/@blhsing/ImpossibleAcrobaticComputergraphics