Is it possible to prevent the for loop from proceeding to the next value in the list/iterator if a certain condition is satisfied?
lst = list('abcde')
for alphabet in lst:
if some_condition:
# somehow prevent the for loop from advancing so that in the
# next iteration, the value of alphabet remains the same as it is now
# do something
What you seem to want is a nested while loop. Only when the while loop exits will the for loop continue to the next value.
alphabet = "abcdefghijklmnopqrstuvwxyz"
for letter in alphabet:
while some_condition:
# do something
You can use break to exit the loop immediately, or continue to skip to the next iteration of the loop. See http://docs.python.org/tutorial/controlflow.html for more details.
Edit: Actually, upon closer inspection, what you're looking for is a rather odd case. Have you considered something like
lst = list('abcde')
specialcase = ""
for alphabet in lst:
if specialcase != "":
alphabet = specialcase
specialcase = ""
elif some_condition:
# somehow prevent the for loop from advancing so that in the
# next iteration, the value of alphabet remains the same as it is now
specialcase = alphabet
#do something
You'll have to modify this to suit your particular situation, but it should give you the idea.
alphabet at each iteration should be 'a','b','c','c','d','e' for example.alphabet in that iteration is set to specialcase and that value in lst is skipped. So, alphabet would take values 'a','b','c','c','e' (no 'd').
elif might need to be changed to an if, if you want to check that condition on every iteration regardless.Do the loop manually but be carefull not to be stuck
>>> i = 0
>>> j = 0
>>> abc = 'abcdefg'
>>> while i < len(abc):
... print abc[i]
... if abc[i] == 'd' and j == 0:
... print 'again'
... j = 1
... i -= 1
... i += 1
a
b
c
d
again
d
e
f
g
Another one using for but kind of a hack
>>> labc
4: ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> flag
5: True
>>> for i in labc:
... print i
... if i == 'd' and flag:
... flag = False
... labc[labc.index(i):labc.index(i)] = [i]
a
b
c
d
d
e
f
g
I don't think you can (or should) do this with a regular for list-iterator. You may want to look at this as a case of a work queue and use a while loop:
work_stack = list('abcde')
prev_item = ''
while work_stack:
item = work_stack.pop(0)
print item
if prev_item=='a' and item == 'b':
work_stack.insert(0,item)
prev_item = item
for letter in 'abcde':lstwhich in my case is a hodge-podge of many types!