Processing Strings

One use of the for loop is to process strings. The for loop, and the asymmetric bounds pattern are ideal, because the subscripts used by strings and arrays all begin at 0, and the last element is always found at size() – 1.

The canonical classic for loop to process every character in a string, should look something like this: 

for (size_t i = 0, len = str.size(); i < len; ++i)
{
    char c = str.at(i);   // now, process c in some way
}

Note that the string::size() member function returns an unsigned type. If you are not careful, that can lead to unexpected results like this:

for (size_t len = str.size() - 1; i >= 0; --i) ...

This loop intended to count down from the last character to the first at index 0. However, if str is an empty string, then i starts at the largest possible unsigned number, instead of -1, since unsigned numbers "wrap around". Even worse, because i is an unsigned type, the condition i >= 0 can never be false, so you can never exit the loop.

Here is a loop that is written correctly: 

for (size_t i = str.size(); i > 0; --i)
{
    char c = str.at(i - 1);
}

Alternatively, you can store str.size() in an int variable, as long as you: