Conveniently, C provides us with a function, isdigit, in its ctype library, for determining whether a character is a decimal digit or not. It takes a character as an argument (not a pointer to a character!). Its return value is an int which tells us whether the given character was a digit or not. This return value follows the convention for all C functions that return a true/false answer. A return value of 0 signifies "false", whilst any other return value signifies "true". Such a value is called a boolean value.