python: restarting a loop

Question

i have:

for i in range(2,n):
    if(something):
       do something
    else:
       do something else
       i = 2 **restart the loop

But that doesn't seem to work. Is there a way to restart that loop?

Thanks

Answer 1

You may want to consider using a different type of loop where that logic is applicable, because it is the most obvious answer.

perhaps a:

i=2
while i < n:
    if something:
       do something
       i += 1
    else: 
       do something else  
       i = 2 #restart the loop  

Answer 2

Changing the index variable i from within the loop is unlikely to do what you expect. You may need to use a while loop instead, and control the incrementing of the loop variable yourself. Each time around the for loop, i is reassigned with the next value from range() . So something like:

i = 2
while i < n:
    if(something):
        do something
    else:
        do something else
        i = 2 # restart the loop
        continue
    i += 1

In my example, the continue statement jumps back up to the top of the loop, skipping the i += 1 statement for that iteration. Otherwise, i is incremented as you would expect (same as the for loop).

Answer 3

Here is an example using a generator's send() method :

def restartable(seq):
    while True:
        for item in seq:
            restart = yield item
            if restart:
                break
        else:
            raise StopIteration

Example Usage:

x = [1, 2, 3, 4, 5]
total = 0
r = restartable(x)
for item in r:
    if item == 5 and total < 100:
        total += r.send(True) 
    else:
        total += item

Answer 4

Just wanted to post an alternative which might be more genearally usable. Most of the existing solutions use a loop index to avoid this. But you don't have to use an index - the key here is that unlike a for loop, where the loop variable is hidden, the loop variable is exposed.

You can do very similar things with iterators/generators:

x = [1,2,3,4,5,6]
xi = iter(x)
ival = xi.next()
while not exit_condition(ival):
    # Do some ival stuff
    if ival == 4:
        xi = iter(x)
    ival = xi.next()

It's not as clean, but still retains the ability to write to the loop iterator itself.

Usually , when you think you want to do this, your algorithm is wrong, and you should rewrite it more cleanly. Probably what you really want to do is use a generator/coroutine instead. But it is at least possible.

Answer 5

a = ['1', '2', '3']
ls = []
count = False

while ls != a :
    print(a[count])
    if a[count] != a[-1] :
        count = count + 1
    else :
        count = False

Restart while loop.