a. Character functions:
1. isalnum(): It is used to check whether a character is alphabet or digit. It returns a non zero value if it is an alphabet or digit otherwise it returns zero.
2. isalpha(): It is used to check whether a character is alphabet or not. It returns a non zero value if it is an alphabet otherwise it returns zero.
3. isdigit(): It is used to check whether a character is digit or not. It returns a non zero value if it is digit otherwise it returns zero.
4. islower(): It is used to check whether a character is lower case alphabet or not. It returns a non zero value if it is a lower case alphabet otherwise it returns zero.
5. isupper(): It is used to check whether a character is upper case alphabet or not. It returns a non zero value if it is an upper case alphabet otherwise it returns zero.
6. tolower(): It is used to convert the alphabet into lowercase.
7. toupper(): It is used to convert the alphabet into upper case.
b. String functions:
1. strcpy(): This function is used to copy one string into another.
2. strcat(): This function is used to concatenate(join) second string into first string.
3. strlen(): This function is used to find the length of a string.
4. strcmp(): This function is used to compare 2 strings. If the first string is less than second string then it returns a negative value. If the first string is equal to the second string then it returns a value zero and if the first string is greater than the second string then it returns a positive value.
