I need to emulate a do-while loop in a Python program. Unfortunately, the following straightforward code does not work:
list_of_ints = [ 1, 2, 3 ] iterator = list_of_ints.__iter__() element = None while True: if element: print element try: element = iterator.next() except StopIteration: break print "done"
Instead of "1,2,3,done", it prints the following output:
[stdout:]1 [stdout:]2 [stdout:]3 None["Traceback (most recent call last): ", " File "test_python.py", line 8, in <module> s = i.next() ", "StopIteration "]
What can I do in order to catch the "stop iteration" exception and break a while loop properly?
An example of why such a thing may be needed is shown below as pseudocode.
s = "" while True : if state is STATE_CODE : if "//" in s : tokens.add( TOKEN_COMMENT, s.split( "//" ) ) state = STATE_COMMENT else : tokens.add( TOKEN_CODE, s ) if state is STATE_COMMENT : if "//" in s : tokens.append( TOKEN_COMMENT, s.split( "//" ) ) else state = STATE_CODE # Re-evaluate same line continue try : s = i.next() except StopIteration : break
I am not sure what you are trying to do. You can implement a do-while loop like this:
while True: stuff() if fail_condition: break
stuff() while not fail_condition: stuff()
What are you doing trying to use a do while loop to print the stuff in the list? Why not just use:
for i in l: print i print "done"
So do you have a list of lines? And you want to keep iterating through it? How about:
for s in l: while True: stuff() # use a "break" instead of s = i.next()
Does that seem like something close to what you would want? With your code example, it would be:
for s in some_list: while True: if state is STATE_CODE: if "//" in s: tokens.add( TOKEN_COMMENT, s.split( "//" ) ) state = STATE_COMMENT else : tokens.add( TOKEN_CODE, s ) if state is STATE_COMMENT: if "//" in s: tokens.append( TOKEN_COMMENT, s.split( "//" ) ) break # get next s else: state = STATE_CODE # re-evaluate same line # continues automatically
Here"s a very simple way to emulate a do-while loop:
condition = True while condition: # loop body here condition = test_loop_condition() # end of loop
The key features of a do-while loop are that the loop body always executes at least once, and that the condition is evaluated at the bottom of the loop body. The control structure show here accomplishes both of these with no need for exceptions or break statements. It does introduce one extra Boolean variable.
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