Question

How to create a case-insensitive function for substring search in C++

Answer 1

#include <iostream>
#include <string>
#include <algorithm>

// Function to convert a string to lowercase
std::string toLower(const std::string& str) {
    std::string lowerStr = str;
    std::transform(lowerStr.begin(), lowerStr.end(), lowerStr.begin(),
                   [](unsigned char c) { return std::tolower(c); });
                   
    return lowerStr;
}

// Case-insensitive substring search function
const char* strstrcaseinsensitive(const char* haystack, const char* needle) {
    std::string haystackStr = haystack;
    std::string needleStr = needle;
    
    std::string lowerHaystack = toLower(haystackStr);
    std::string lowerNeedle = toLower(needleStr);
    
    size_t pos = lowerHaystack.find(lowerNeedle);
    if (pos != std::string::npos) {
        return haystack + pos;
    } 
    
    return nullptr;
}

int main() {
    const char* haystack = "C++ is a general-purpose programming language";
    const char* needle = "PROGRAMMING";
    
    const char* result = strstrcaseinsensitive(haystack, needle);
    if (result) {
        std::cout << "Found: " << result << std::endl;
    } else {
        std::cout << "Not Found" << std::endl;
    }
}


/*
run:

Found: programming language

*/

 

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