cppreference.com > Standard C String and Character > isdigit isdigit Syntax:
      #include <ctype.h>
      int isdigit( int ch );
The function isdigit() returns non-zero if its argument is a digit between 0 and 9. Otherwise, zero is returned.
       char c;
       scanf( "%c", &c );
       if( isdigit(c) )
         printf( "You entered the digit %c\n", c );
Related topics: isalnum isalpha iscntrl isgraph isprint ispunct isspace isxdigit