How do I use a C-style for loop in Python?

Question

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.

Answer 1

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