Try this function :)

string toLowerCase(string str) {
    int str_len = str.length();
    string final_str = "";
    for(int i=0; i<str_len; i++) {
        char character = str[i];
        if(character>=65 && character<=92) {
            final_str += (character+32);
        } else {
            final_str += character;
        }
    }
    return final_str;
}

Try this function :)

string toLowerCase(string str) { 

    int str_len = str.length(); 

    string final_str = ""; 

    for(int i=0; i<str_len; i++) { 

        char character = str[i]; 

        if(character>=65 && character<=92) { 

            final_str += (character+32); 

        } else { 

            final_str += character; 

        } 

    } 

    return final_str; 

}