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isdigit is a function from the C standard library defined in the header file ctype.h, while std::isdigit is a function from the C++ standard library defined in the header file cctype. isdigit takes an integer argument and returns a non-zero value if it is a digit and zero if it is not, while std::isdigit takes a character argument and returns a bool value indicating whether it is a digit or not. In addition, std::isdigit is defined in the std namespace, while isdigit is not.
It is recommended to use std::isdigit in C++ programs as it conforms to C++ standard and is more type-safe, whereas isdigit may be more appropriate in C programs that use ctype.h.