Character and String functions

These functions are used for manipulating characters.
1. isalnum()
This functions carries out the test "Is the character alphanumeric?". An alphanumeric character is a letter of either lower or upper-case, or a digit. It has a prototype in the header file ctype.h. The argument to this function is a character represented as an integer.
The function isalnum() returns a non-zero value if the character is alphanumeric and zero if it is not so.
2. isdigit()
This function carries out the test "Is the character a digit?". It has a prototype in the header file ctype.h. The argument to this function is also characte represented as an integer.
The function isdigit() returns a non-zero value if the character is a digit and a zero if it is not so.
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