How do I use a C-style for loop in Python?
I want to use the traditional C-style for loop in Python. I want to loop through characters of a string, but also know what it is, and be able to jump through characters (e.g. i =
5 somewhere in the code).
for
with range
doesn't give me the flexibility of an actual for loop.
In C
:
for(int i=0; i<9; i+=2)
{
dosomething(i);
}
In python3
:
for i in range(0, 9, 2):
dosomething(i)
You just express the same idea in different languages.
The simple answer is that there is no simple, precise equivalent of C's for
statement in Python. Other answers covered using a Python for
statement with a range. If you want to be able to modify the loop variable in the loop (and have it affect subsequent iterations), you have to use a while
loop:
i = 0
while i < 7:
if someCondition(i):
i = 5
i += 1
But in that loop, a continue
statement will not have the same effect that a continue
statement would have in a C for
loop. If you want continue
to work the way it does in C, you have to throw in a try
/finally
statement:
i = 0
while i < 7:
try:
if someCondition(i):
i = 5
elif otherCondition(i):
continue
print 'i = %d' % i
finally:
i += 1
As you can see, this is pretty ugly. You should look for a more Pythonic way to write your loop.
UPDATE
This just occurred to me... there is a complicated answer that lets you use a normal Python for
loop like a C-style loop, and allows updating the loop variable, by writing a custom iterator. I wouldn't recommend this solution for any real programs, but it's a fun exercise.
Example “C-style” for loop:
for i in forrange(10):
print(i)
if i == 3:
i.update(7)
Output:
0
1
2
3
8
9
The trick is forrange
uses a subclass of int
that adds an update
method. Implementation of forrange
:
class forrange:
def __init__(self, startOrStop, stop=None, step=1):
if step == 0:
raise ValueError('forrange step argument must not be zero')
if not isinstance(startOrStop, int):
raise TypeError('forrange startOrStop argument must be an int')
if stop is not None and not isinstance(stop, int):
raise TypeError('forrange stop argument must be an int')
if stop is None:
self.start = 0
self.stop = startOrStop
self.step = step
else:
self.start = startOrStop
self.stop = stop
self.step = step
def __iter__(self):
return self.foriterator(self.start, self.stop, self.step)
class foriterator:
def __init__(self, start, stop, step):
self.currentValue = None
self.nextValue = start
self.stop = stop
self.step = step
def __iter__(self): return self
def next(self):
if self.step > 0 and self.nextValue >= self.stop:
raise StopIteration
if self.step < 0 and self.nextValue <= self.stop:
raise StopIteration
self.currentValue = forrange.forvalue(self.nextValue, self)
self.nextValue += self.step
return self.currentValue
class forvalue(int):
def __new__(cls, value, iterator):
value = super(forrange.forvalue, cls).__new__(cls, value)
value.iterator = iterator
return value
def update(self, value):
if not isinstance(self, int):
raise TypeError('forvalue.update value must be an int')
if self == self.iterator.currentValue:
self.iterator.nextValue = value + self.iterator.step