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Making use of libraries in C:

If go to the CS50 Course reference site:

https://reference.cs50.net/

We can get information on all the libraries that we can use to simplify our programs.

For string related libraries, there is a library called "ctype.h", which means char types.

In this library, we can use like: 

isalpha(s[i]) // check if the char s[i] is an alphabetic char or not, return a boolean

islower(s[i]) // check if the char s[i] is lower case or not, return a boolean 

toupper(s[i]) // turn lower case char to upper case

Remember to #include <ctype.h>