This guide covers the C++ Standard Template Library (STL) string type, which encapsulates common string operations for ease of use. Too use it, include the <string> header.

String Definition

Define a string variable similarly to other variables:

std::string text;

String Initialization

Initialize by direct assignment:

std::string text = "sample";

String Input and Output

Output

Use cout directly:

#include <iostream>
#include <string>
int main() {
    std::string data = "output";
    std::cout << data;
    return 0;
}

Input

Use cin directly:

#include <iostream>
#include <string>
int main() {
    std::string input;
    std::cin >> input;
    std::cout << input;
    return 0;
}

String Access

Access characters using endices, similar to arrays:

#include <iostream>
#include <string>
int main() {
    std::string str;
    std::cin >> str;
    std::cout << str[2];
    return 0;
}

String Copy

Copy strings with direct assignment:

#include <iostream>
#include <string>
int main() {
    std::string source, destination;
    std::cin >> source;
    destination = source;
    std::cout << "Source: " << source << "\nDestination: " << destination;
    return 0;
}

String Concatenation

Combine strings using the + operator or append() function.

Direct Addition

#include <iostream>
#include <string>
int main() {
    std::string first, second, combined;
    std::cin >> first >> second;
    combined = first + second;
    std::cout << "First: " << first << "\nSecond: " << second << "\nCombined: " << combined;
    return 0;
}

append(const char* str, int n)

The append() function can take one or two parameters:

#include <iostream>
#include <string>
int main() {
    std::string base = "prefix ";
    std::cout << "Before: " << base << '\n';
    base.append("suffix", 3); // Appends first 3 characters
    std::cout << "After partial append: " << base << '\n';
    base = "prefix ";
    base.append("suffix"); // Appends entire string
    std::cout << "After full append: " << base;
    return 0;
}

String Comparison

Compare strings using relational operators (==, !=, <, <=, >, >=), based on ASCII values from the first character onward. Returns 1 for true, 0 for false.

#include <iostream>
#include <string>
int main() {
    std::string str1 = "abc", str2 = "abc", str3 = "acb";
    std::cout << "str1 == str2: " << (str1 == str2) << '\n';
    std::cout << "str1 <= str3: " << (str1 <= str3) << '\n';
    std::cout << "str1 >= str3: " << (str1 >= str3);
    return 0;
}

String Length

Get length with length() or size(), which return the actual length without null terminators.

#include <iostream>
#include <string>
int main() {
    std::string example = "length";
    std::cout << "String: " << example << '\n';
    std::cout << "Length: " << example.length() << '\n';
    std::cout << "Size: " << example.size();
    return 0;
}

String Search with find(string str)

Find the first occurrence of a substring using find(). Returns the index if found, otherwise std::string::npos (typically -1).

#include <iostream>
#include <string>
int main() {
    std::string mainStr = "searchable", subStr = "ch";
    std::cout << "Find 'ch' in 'searchable': " << mainStr.find(subStr) << '\n';
    std::cout << "Find 'xy' in 'searchable': " << mainStr.find("xy");
    return 0;
}

Substring Extraction with substr(int a, int b)

Extract substrings using substr(). Parameters: start index and optional length.

#include <iostream>
#include <string>
int main() {
    std::string original = "extract";
    std::cout << "Original: " << original << '\n';
    std::cout << "Substring from index 2, length 3: " << original.substr(2, 3) << '\n';
    std::cout << "Substring from index 4 to end: " << original.substr(4);
    return 0;
}

Case Conversion

Convert to Lowercase with tolower(char a)

Convert individual characters; cast result to char.

#include <iostream>
#include <string>
#include <cctype>
int main() {
    std::string upper = "UPPER";
    std::cout << "Original: " << upper << '\n';
    std::cout << "Lowercase: ";
    for (char ch : upper) {
        std::cout << static_cast<char>(std::tolower(ch));
    }
    return 0;
}

Convert to Uppercase with toupper(char a)

Convert individual characters; cast result to char.

#include <iostream>
#include <string>
#include <cctype>
int main() {
    std::string lower = "lower";
    std::cout << "Original: " << lower << '\n';
    std::cout << "Uppercase: ";
    for (char ch : lower) {
        std::cout << static_cast<char>(std::toupper(ch));
    }
    return 0;
}

ASCII Notes

• Uppercase letters are 32 less than lowercase letters in ASCII.
• Character digits differ from integer digits by 48.

#include <iostream>
int main() {
    std::cout << "A: " << static_cast<int>('A') << '\n';
    std::cout << "Z: " << static_cast<int>('Z') << '\n';
    std::cout << "a: " << static_cast<int>('a') << '\n';
    std::cout << "z: " << static_cast<int>('z') << '\n';
    std::cout << "Difference a - A: " << 'a' - 'A' << '\n';
    std::cout << "0: " << static_cast<int>('0') << '\n';
    std::cout << "'1' - 48: " << '1' - 48;
    return 0;
}