How Do You Traverse and Process a String?

01How do you visit each character?

Use an index to move through the string from the beginning to the end:

std::string s = "cat";

for (std::size_t i = 0; i < s.size(); i++) {
    std::cout << s[i] << ' ';
}

02You can also use a range-based for loop

for (char c : s) {
    std::cout << c << ' ';
}

The loop puts each character unit into c in turn.

03How do you count occurrences of a character?

int count = 0;
for (char c : s) {
    if (c == 'a') {
        count++;
    }
}

Each time the loop finds an 'a', it increases the counter by 1.

04How do you change a character in a string?

std::string s = "cat";
s[0] = 'b';

After the change, the string is "bat".

You can also traverse it using references:

for (char& c : s) {
    // You can change c directly
}

05How do you find a substring?

std::string s = "computer";
auto pos = s.find("put");

If the substring is found, pos is its first position. If it is not found, the result is std::string::npos.

06Why can you not traverse UTF-8 Chinese this way?

If the string contains UTF-8 Chinese or emoji:

std::string s = "你好";

Traversing with char usually reads one UTF-8 encoded byte at a time, not one complete Chinese character.

07Can you go out of bounds when traversing a string?

Yes. For a non-empty string, valid indexes are still:

Do not use s[s.size()] as an ordinary character position.

Use s.at(i) if you want runtime bounds checking.