How to Use find() in String in C++: Syntax and Examples

In C++, use the find() method of the std::string class to search for a substring or character. It returns the position index of the first match or std::string::npos if not found.

Syntax

The find() method is called on a std::string object and has several overloads. The most common form is:

• size_t find(const std::string& str, size_t pos = 0) const; - Finds the substring str starting from index pos.
• size_t find(char ch, size_t pos = 0) const; - Finds the character ch starting from index pos.

The method returns the index of the first occurrence or std::string::npos if not found.

cpp

size_t find(const std::string& str, size_t pos = 0) const;
size_t find(char ch, size_t pos = 0) const;

Example

This example shows how to find a substring and a character in a string. It prints the found positions or a message if not found.

cpp

#include <iostream>
#include <string>

int main() {
    std::string text = "Hello, world!";

    // Find substring "world"
    size_t pos1 = text.find("world");
    if (pos1 != std::string::npos) {
        std::cout << "Found 'world' at position: " << pos1 << '\n';
    } else {
        std::cout << "'world' not found" << '\n';
    }

    // Find character 'o'
    size_t pos2 = text.find('o');
    if (pos2 != std::string::npos) {
        std::cout << "Found 'o' at position: " << pos2 << '\n';
    } else {
        std::cout << "'o' not found" << '\n';
    }

    return 0;
}

Output

Found 'world' at position: 7
Found 'o' at position: 4

Common Pitfalls

Common mistakes when using find() include:

• Not checking if the result equals std::string::npos before using the position.
• Assuming find() returns -1 when not found (it returns std::string::npos, which is a large unsigned number).
• Using the found position without verifying it is valid, which can cause bugs.

Always compare the result with std::string::npos before using it.

cpp

#include <iostream>
#include <string>

int main() {
    std::string text = "example";

    // Wrong: assuming -1 means not found
    auto pos_wrong = text.find("test");
    if (pos_wrong == -1) { // This check is incorrect
        std::cout << "Not found (wrong check)" << '\n';
    }

    // Right: compare with std::string::npos
    size_t pos_right = text.find("test");
    if (pos_right == std::string::npos) {
        std::cout << "Not found (correct check)" << '\n';
    }

    return 0;
}

Output

Not found (correct check)

Quick Reference

Summary tips for using find() in C++ strings:

• Use find() to locate substrings or characters.
• Check if the result is std::string::npos to know if not found.
• Start searching from a specific position by passing the pos argument.
• find() returns the first occurrence index.

Key Takeaways

Use std::string::find() to search for substrings or characters in a string.

Always check if the returned position equals std::string::npos before using it.

The find() method returns the index of the first match or npos if not found.

You can specify the starting position for the search with an optional argument.

Do not assume find() returns -1; it returns std::string::npos, an unsigned value.