Iterating each character in a string using Python

Question

In C++, I can iterate over an std::string like this:

std::string str = "Hello World!";

for (int i = 0; i < str.length(); ++i)
{
    std::cout << str[i] << std::endl;
}

How do I iterate over a string in Python?

Solution 1

As Johannes pointed out,

for c in "string":
    #do something with c

You can iterate pretty much anything in python using the for loop construct,

for example, open("file.txt") returns a file object (and opens the file), iterating over it iterates over lines in that file

with open(filename) as f:
    for line in f:
        # do something with line

If that seems like magic, well it kinda is, but the idea behind it is really simple.

There's a simple iterator protocol that can be applied to any kind of object to make the for loop work on it.

Simply implement an iterator that defines a next() method, and implement an __iter__ method on a class to make it iterable. (the __iter__ of course, should return an iterator object, that is, an object that defines next())

See official documentation

Solution 2

If you need access to the index as you iterate through the string, use enumerate():

>>> for i, c in enumerate('test'):
...     print i, c
... 
0 t
1 e
2 s
3 t

Solution 3

Even easier:

for c in "test":
    print c

Solution 4

Just to make a more comprehensive answer, the C way of iterating over a string can apply in Python, if you really wanna force a square peg into a round hole.

i = 0
while i < len(str):
    print str[i]
    i += 1

But then again, why do that when strings are inherently iterable?

for i in str:
    print i

Solution 5

Well you can also do something interesting like this and do your job by using for loop

#suppose you have variable name
name = "Mr.Suryaa"
for index in range ( len ( name ) ):
    print ( name[index] ) #just like c and c++ 

Answer is

M r . S u r y a a

However since range() create a list of the values which is sequence thus you can directly use the name

for e in name:
    print(e)

This also produces the same result and also looks better and works with any sequence like list, tuple, and dictionary.

We have used tow Built in Functions ( BIFs in Python Community )

1) range() - range() BIF is used to create indexes Example

for i in range ( 5 ) :
can produce 0 , 1 , 2 , 3 , 4

2) len() - len() BIF is used to find out the length of given string

View more solutions

Comments

[aiham]

Instead of your first while loop, you can do: for i in range(len(str)): print(str[i]) Which in my opinion is better than having to manage the counter on your own. Even better is marcog's answer using enumerate.

[Akseli Palén]

As a note, reversed iteration is archived with: for c in reversed("string")

[gkimsey]

This may be based on just having used C for so long, but I almost always end up using this C-ish method. For instance, I have a file with some 4-digit numbers scattered about, all of which start with 0. So I need to find a "0" and grab it and the next 3 characters, and move on without duplicating the number if there's another 0 following it. None of the "for c in str" or "for i,c in enumerate(str)" methods work because I need control of the index. I'm sure a regular expression would be much better, though.

[Mauro Vanetti]

I'm a newbie in Python. For some reason, this doesn't compile in my environment, and I had to put c in brackets to make it work: for c in "test": print (c) Why?

[Johannes Weiss]

@MauroVanetti that's almost certainly because you're using Python 3 and when I answered the question there was AFAIK only Python 2.

[izak]

for i in range(len(...)) is evil. In python 2.x, range() creates a list, so for a very long length you may end up allocating a very large block of memory. At the very least use xrange() in those cases. Also, repeated indexing of the same string is much slower than iterating directly over the string. If you need the index, use enumerate().

[winklerrr]

From which part of the documentation do you know that a string is a iterator type?

[shadow0359]

dir() a string..you with see iter attribute.

[Messa]

Pro tip: it starts from zero. If you need to start it from one: 1 t, 2 e, 3 s, 4 t use the parameter "start": for i, c in enumerate('test', start=1)

[Sam Mason]

note that this only applies to Python 2 which is hopefully a shrinking minority now