Is there a function that can convert upper case to lower case and vice-versa?
The toupper() and tolower() functions are declared in the standard header. Each of them takes a character and returns it uppercase or lowercase version, respectively. For example:
char low = 'a', up;
up = toupper(low); //up is now 'A'
You need to pass every character in the string to either function to make the string all uppercase or lowercase.