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Learn Data Structures and Algorithms in C++

Other Common String Operations

Explore essential string operations in C++ including substring searching, lexicographic comparison, case conversion, and trimming. Understand how to use std::string methods like find, rfind, and the C++20 starts_with and ends_with. This lesson prepares you to handle common string manipulation tasks efficiently, improving your ability to solve string algorithm problems.

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Knowing the basic string operations such as finding the length, traversal, indexing, concatenation, and slicing is an important start. A problem may ask whether two strings are equal, whether one string starts with another, whether a smaller string appears inside a larger one, or how to transform a string for case-insensitive processing. These additional operations will better prepare you for a wider range of string algorithms.

Searching within a string

Searching refers to locating a character or a substring within a larger string. The std::string class provides the find() member function, which returns the starting index of the first occurrence. If the substring is not found, find() returns the special constant std::string::npos.

C++ 17
#include <iostream>
#include <string>
int main() {
    std::string s = "hello world";
    std::cout << "string: \"" << s << "\"" << std::endl;
    
    // Find the starting index of "world"
    size_t pos = s.find("world");
    if (pos != std::string::npos) {
        std::cout << "\n\"world\" found at index: " << pos << std::endl;
    } else {
        std::cout << "\n\"world\" not found" << std::endl;
    }
    // Searching for a substring that does not exist
    pos = s.find("xyz");
    if (pos != std::string::npos) {
        std::cout << "\n\"xyz\" found at index: " << pos << std::endl;
    } else {
        std::cout << "\n\"xyz\" not found" << std::endl;
    }
    return 0;
}

The find() method scans the string from left to right and returns the index of the first match. Always check the return value against std::string::npos before using the index.

Finding from the end and finding any of a set

C++ provides several additional search variants:

  • rfind(str) searches from right to left, returning the index of the last occurrence.

  • find_first_of(str) returns the index of the first character that matches any character in the given set.

  • find_last_of(str) returns the index of the last character that matches any character in the given set.

C++ 17
#include <iostream>
#include <string>
int main() {
    std::string s = "hello world";
    std::cout << "string: \"" << s << "\"" << std::endl;
    // Find the last occurrence of 'l'
    std::cout << "\nLast 'l' at index: " << s.rfind("l") << std::endl;
    // Find the first vowel
    size_t firstVowel = s.find_first_of("aeiouAEIOU");
    std::cout << "\nFirst vowel at index: " << firstVowel << std::endl;
    return 0;
}

Checking for existence without needing the

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