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