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.g. i =5 somewhere in the code).

for with range doesn't give me the flexibility of an actual for loop.

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

Answer 2

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.

Answer 3

for i in range(n):

...is the Python equivalent of the C...

for (i = 0; i < n; i++){

Or well, you can use:

for i in range(a, n, s):

...which is equivalent to...

for (i = a; i < n; i+=s){

Answer 4

I provide the following entirely facetious solution by way of protest. Note that 'break' and 'continue' will not work. Also note that the loop body must not be indented.

class For:
    def __init__(self, **loop_vars):
        self.loop_vars = loop_vars
    def __call__(self, arg):
        if not hasattr(self, 'condition'):
            self.condition = arg
            return self
        if not hasattr(self, 'update'):
            self.update = arg
            return self
        while eval(self.condition, self.loop_vars, self.loop_vars):
            exec arg in self.loop_vars
            exec self.update in self.loop_vars


For(i = 1, j = 1)('i * j < 50')('i += 1; j += 1')('''
print i, j
''')

Answer 5

You can do the following, given an array a:

for i in range(len(a)):
  a[i] = i

That's the closest Python can get to C-style loops.

You can also give the range command more arguments; for example,

for i in range(2, len(a), 3)

will start at i = 2, and increment it by 3 as long as the result is less than len(a).

Answer 6

The Python for loop always has foreach semantics. You can, however, do this:

for i in xrange(10):
    print i

This is very much like a C for loop. xrange (or range, as it was renamed in Python 3) is a constructor for a Python object that iterates through a range of numbers. See the docs for more information.

Answer 7

For all the ones going "Why?", here's an example where a C-style loop would be helpful:

remaining_retries = 3
for i in range(num_runs):
  success = do_run()
  if not success and remaining_retries > 0:
    i = i - 1
    remaining_retries = remaning_retries - 1