How to end loop one iteration earlier?

Question

I often need to end loop one iteration earlier, how do I do it elegantly? Breaking out of nested loops looks messy to me. For now I workaround it with return, but when later on I want to make a class, having return in the middle of constructor looses sense.

def forward(neurons):
for layerId, layer in enumerate(neurons):
    if layerId == neurons.__len__() - 1:
        return
    for idx, i in enumerate(neurons[layerId]):
        for idx2, n in enumerate(neurons[layerId+1]):
            neurons[layerId+1][idx2] += i * sigmoid(weights[layerId][idx][idx2])

*having return in the middle of constructor looses sense* You could still use a function and call it from the contructor. — sloth, Oct 19 '15 at 10:22
Note that you should use `len(neurons)` rather than `neurons.__len__()`. What's the problem with `break`? — jonrsharpe, Oct 19 '15 at 10:22
Your example is not a case of breaking out of nested loops. At the point you seem to be wishing to break from the loops are not nested (yet). Then what's the problem with using `break`? — skyking, Oct 19 '15 at 10:26
Sorry; my mistake; break applied on lowest depth works ok. I don't delete question though, since i generally want to know how to get out of nested loop with enumerate earlier. — Huxwell, Oct 19 '15 at 11:25

score 0 · Answer 1 · answered Oct 19 '15 at 10:22

0

You can also just use break:

for i in range(1,100):
  if i==10:
    break

That will kill the loop as soon as a condition is matched.

answered Oct 19 '15 at 10:22

Henry

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12
28

But surely you can just apply this condition on the `if layerId == neurons.__len__() -1` condition. – Henry Oct 19 '15 at 10:23

score 0 · Accepted Answer · answered Oct 19 '15 at 10:30

One generic solution would be to create a generator from the iterator (that will yield all but the last element the iterator yields):

def but_last(p):
    first = True
    for x in p:
        if not first:
            yield last
        first = False
        last = x

for layerId, layer in but_last(enumerate(neurons)):
    do_your_stuff(layerId, layer)

score 0 · Answer 3 · answered Oct 19 '15 at 10:58

I would recommend you to implement your neurons object as a class with method __iter__ to make the object iterable according to the behavior you like:

class MyClass(object):
    def __iter__(self):
        yield 2
        yield 5
        yield 10


obj = MyClass()

for i in obj:
    print i

How to end loop one iteration earlier?

3 Answers3