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isalpha()

Syntax:

#include <ctype.h>
int isalpha( int ch );

Description:
The function isalpha() returns non-zero if its argument is a letter of the alphabet. Otherwise, zero is returned.

Example:

char c;
scanf( "%c", &c );
if( isalpha(c) )
  printf( "You entered a letter of the alphabet\n" );