Eye of Hell Eye of Hell - 7 months ago 48
Python Question

Emulate a do-while loop in Python?

I need to emulate a do-while loop in a Python program. Unfortunately, the following straightforward code does not work:

l = [ 1, 2, 3 ]
i = l.__iter__()
s = None
while True :
if s :
print s
try :
s = i.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.

State machine:

s = ""
while True :
if state is STATE_CODE :
if "//" in s :
tokens.add( TOKEN_COMMENT, s.split( "//" )[1] )
state = STATE_COMMENT
else :
tokens.add( TOKEN_CODE, s )
if state is STATE_COMMENT :
if "//" in s :
tokens.append( TOKEN_COMMENT, s.split( "//" )[1] )
else
state = STATE_CODE
# Re-evaluate same line
continue
try :
s = i.next()
except StopIteration :
break

Tom Tom
Answer

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

Or:

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"

Update:

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( "//" )[1] )
        state = STATE_COMMENT
      else :
        tokens.add( TOKEN_CODE, s )
    if state is STATE_COMMENT :
      if "//" in s :
        tokens.append( TOKEN_COMMENT, s.split( "//" )[1] )
        break # get next s
      else
        state = STATE_CODE
        # re-evaluate same line
        # continues automatically