cctype header in C++

tolower(char ch)

It converts the given character into lowercase and returns the ASCII code of the given 'ch'. 

•  If the 'ch' given is lower/upper case, it will return the ASCII code of the lowercase version of ch.
• If the 'ch' given is non-alphabet, it will return the ASCII code of the non-alphabet version of ch.

Reference: https://www.programiz.com/cpp-programming/library-function/cctype/tolower

isalnum(char ch)

It is the combination of isalpha(char ch) and isdigit(char ch). 

Click here for programs.