Looping from 1 to infinity in Python

Question

In C  I would do this   int i  for  i   0   i      if  thereIsAReasonToBreak i       break    How can I achieve something similar in Python

User · Answer

Simplest and best  i   0 while not there is reason to break i         some code here     i    1  It may be tempting to choose the closest analogy to the C code possible in Python  from itertools import count  for i in count        if thereIsAReasonToBreak i           break  But beware  modifying i will not affect the flow of the loop as it would in C   Therefore  using a while loop is actually a more appropriate choice for porting that C code to Python

User · Answer

If you re doing that in C  then your judgement there is as cloudy as it would be in Python  -   For a loop that exits on a simple condition check at the start of each iteration  it s more usual  and clearer  in my opinion  to just do that in the looping construct itself  In other words  something like  if you need i after loop end    int i   0  while    thereIsAReasonToBreak i            do something     i        or  if i can be scoped to just the loop    for  int i   0    thereIsAReasonToBreak i     i           do something       That would translate to the Python equivalent   i   0 while not there is a reason to break i         do something     i    1   Only if you need to exit in the middle of the loop somewhere  or if your condition is complex enough that it would render your looping statement far less readable  would you need to worry about breaking   When your potential exit is a simple one at the start of the loop  as it appears to be here   it s usually better to encode the exit into the loop itself

User · Answer

def infinity        i 0     while True          i  1         yield i   for i in infinity        if there is a reason to break i           break

User · Answer

Using itertools count  import itertools for i in itertools count start 1       if there is a reason to break i           break  In Python 2  range   and xrange   were limited to sys maxsize  In Python 3 range   can go much higher  though not to infinity  import sys for i in range sys maxsize  10      you could go even higher if you really want     if there is a reason to break i           break  So it s probably best to use count

User · Answer

Reiterating thg435 s comment   from itertools import takewhile  count  def thereIsAReasonToContinue i       return not thereIsAReasonToBreak i   for i in takewhile thereIsAReasonToContinue  count         pass   or something else   Or perhaps more concisely   from itertools import takewhile  count  for i in takewhile lambda x   not thereIsAReasonToBreak x   count         pass   or something else   takewhile imitates a  well-behaved  C for loop  you have a continuation condition  but you have a generator instead of an arbitrary expression  There are things you can do in a C for loop that are  badly behaved   such as modifying i in the loop body  It s possible to imitate those too using takewhile  if the generator is a closure over some local variable i that you then mess with  In a way  defining that closure makes it especially obvious that you re doing something potentially confusing with your control structure

User · Answer

def to infinity        index   0     while True          yield index         index    1  for i in to infinity        if i  gt  10          break

[python] Looping from 1 to infinity in Python

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