how to make all letters lowercase in c++ code example

Example 1: how to compare lower case character to uppercase cpp

for(int i=0;i<str.size();i++){
int c = str[i]; 
        if (islower(c))  
            str[i] = toupper(c);
}

Example 2: convert characters to lowercase c++

str[i] = tolower(str[i]);

Example 3: c++ string functions lowercase

transform(su.begin(), su.end(), su.begin(), ::toupper);
transform(sl.begin(), sl.end(), sl.begin(), ::tolower);