For loop variable in Python

Question

Can anyone explain how x is taking integer values. We are directly using x in for loop "for x in a" How is the compiler going to recognize it that x represents the strings inside the list?

>>> # Measure some strings:
... a = ['cat', 'window', 'defenestrate']
>>> for x in a:
... print x, len(x)
...
cat 3
window 6
defenestrate 12

And the problem is? You loop through an array of strings so each item in the loop is a string. — Eric
– Eric, Commented Nov 22, 2012 at 11:21

Tim Pietzcker · Accepted Answer · 2012-11-22 11:21:23Z

5

for works differently in Python than it works in languages like C. Instead of counting up/down a numerical value and checking an end condition as you usually would in C:

for (i=0; i<=max; i++) do_something();

it iterates over all the elements in the container (whose name is referenced after the in):

for item in iterable:
    do_something(item)

Its precise behavior depends on the type of container (iterable) used; in a list or tuple, it will start at the first element, then move through the list/tuple one item at a time until the final element has been reached. Each of the elements will be then referenced by a name (item in this example) so that it can be operated on in the body of the loop.

In a dictionary, for would iterate through the dictionary's keys (in an unspecified order), so item would contain a key of the dictionary.

In a string, it iterates through the letters of the string, one by one. Etc.

answered Nov 22, 2012 at 11:21

Tim Pietzcker

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3 Comments

Tim Over a year ago

"move through the list in ascending order" is a bit misleading.

Ben Allison Over a year ago

It may or may not help to note that this is what's often explicitly termed a 'foreach' loop

dbaumann Over a year ago

What you're actually saying is that python doesn't have a for loop. Instead it simply renames 'foreach' to 'for'. A true for loop permits the index of iteration to be changed within the loop, which can be confusing to beginners but also useful for some algorithms.

NPE · Accepted Answer · 2012-11-22 11:24:19Z

2

how X is taking integer values

It's not. It is taking the successive values contained in the sequence you're iterating over, which is this case are strings.

The elements don't have to be of a particular type, nor do they even have to be of the same type.

answered Nov 22, 2012 at 11:24

NPE

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Comments

glglgl · Accepted Answer · 2012-11-22 11:56:33Z

To be a little bit more technical, the for loop iterates over an iterator provided by an iterable.

An iterable can be a list, a string, a tuple etc., an iterator is an object used for iteration.

With for i in a:, you request for an iterator for a, exactly the way you do with iter(a).

This happens

either by calling the_iterator = a.__iter__()
or, if that doesn't work, but a has a __getitem__(), by calling this with successively growing indexes until IndexError is raised.

This interator is then called .next() (.__next__() in Python 3) on each loop run until the iterator is exhausted and raises StopIteration.

So, in other words:

for x in a: print x, len(x)

internally works like

a_iter = iter(a)
while True:
    try: x = next(a)
    except StopIteration: break
    print x, len(x)

but looks much nicer.

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