String search

Learn to search in a string with .find().

The find method

To search for one string inside another you use the find method. The call s.find("needle") returns the index of the first occurrence of the needle. If it is not found, the method returns std::string::npos.

For example std::string s = "paris lyon marseille"; s.find("lyon") returns 6 because "lyon" starts at index 6. Indices count from zero and spaces count as characters too.

Checking with npos

When find cannot locate the searched string it returns std::string::npos, a special constant meaning "not found". To check whether a string contains another you use this pattern:

if (s.find("x") == std::string::npos) { /* not found */ }. This pattern is useful for banned word checks in user input or for looking up an extension in a file name.

Exercise Find a city

The sentence variable is set to "paris lyon marseille" and word is set to "lyon". Assign the result of sentence.find(word) to pos and print it with std::cout. The output must be 6.

Show example solution

#include <iostream>
#include <string>

int main() {
    std::string sentence = "paris lyon marseille";
    std::string word = "lyon";
    int pos = sentence.find(word);
    std::cout << pos << std::endl;
    return 0;
}

Test yourself

Write the find call

std::string s = "Welcome";
int pos = s____("co");

Show answer

std::string s = "Welcome";
int pos = s.find("co");

Match each find call to its result

s.find("Wel") on "Welcome"

0
s.find("co") on "Welcome"

3
s.find("xy") on "Welcome"

std::string::npos
s.find("om") on "Welcome"

4
What does find return when the needle is not in the string?
```
std::string s = "Welcome";
s.find("xy")
```
- -1
- 0
- std::string::npos
- ""
Show answer

std::string::npos — When not found, std::string::npos is returned.

Next lesson: Summary - Strings

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