is their a better way of checking for alpha?

**Kinasz** · 07-16-2004

Is this a proper way to test if a char is an alpha or am I dreaming. If it even works I feel like I might be doing something the hard way when there is something easier.

Code:

 #define LOWEST_ALPHA 0x41
 #define HIGHEST_ALPHA 0x5A
 #define LOWEST_CAPS_ALPHA 0x61
 #define HIGHEST_CAPS_ALPHA 0x7A
 
 /* 0000 0000 0000 0000 0000 0000 0111 1111 */
 #define NON_ASCII_BIT_MASK 0x7F
 
 /* 1000 0000 0000 0000 0000 0000 0000 0000 */
 #define ASCII_SIGNED_BIT 0x80000000
 
 /* Clear non ascii bits then set the signed bit */
 #define MAKE_SIGNED_ASCII( ch ) \
 	( ((ch) & NON_ASCII_BIT_MASK ) | ASCII_SIGNED_BIT )
 
 
 #define IN_ASCII_ALPHA_RANGE( ch ) \
 (( ch >= LOWEST_ALPHA && ch <= HIGHEST_ALPHA ) || \
 ( ch >= LOWEST_CAPS_ALPHA && ch <= HIGHEST_CAPS_ALPHA ))
 
 int isalpha( char ch )
 {
 	int result;
 
 	/* Clear non ascii bits then set the signed bit */
 	if ( IN_ASCII_ALPHA_RANGE( MAKE_SIGNED_ASCII( ch ) ){
 		return true;
 	} else {
 		return false;
 	}
 }

NB: I know of the BSD isalpha() function but my code has to compile on windoze as well.

**IceBall** · 07-16-2004

isn't isalpha() a standard function -> i guess ctype.h

**Kinasz** · 07-16-2004

i thought it was *nix only?

**Brain Cell** · 07-16-2004

isaplha DOES compiler on windows. Just include the header file "ctype.h"

edit : run this code and see if it works :

Code:

#include <stdio.h>
#include <ctype.h>

int main()
{
char ch;

printf("enter a character :");
scanf("%c", &ch);

if(isalpha(ch))
   printf("input is alpha\n");
else
   printf("input is not alpha\n");

  return 0;
}

**ggs** · 07-16-2004

i'm fairly certain it is standard, i've never used a compiler without it in any case
and in the worst case, you can just write isalpha yourself
don't settle for ugly macros
if you do, don't use hex, just use 'literals'

e;f,b

**Kinasz** · 07-16-2004

oh ok, well thanks guys, i feel a bit silly now but it is good to know, ggs please expand on how it would be done with 'literals'?

**jim mcnamara** · 07-16-2004

FWIW - the isxxxx functions or macros
Here is an edited part of how an old C compiler did it:

create an array called _ctype[]:

Code:

static char _ctype[256];
static char _ctype_exists=0;
void _define__ctype(void){
  int i=0;
  if(_ctype_exists==1) 
         return;
  _ctype_exists=1;
  for(i=129;i<256;i++)
         _ctype[i]=0;
  _ctype[0] = 0; 
  _ctype[1] = 32; 
  _ctype[2] = 32; 
  _ctype[3] = 32; 
  _ctype[4] = 32; 
  _ctype[5] = 32; 
  _ctype[6] = 32; 
  _ctype[7] = 32; 
  _ctype[8] = 32; 
  _ctype[9] = 32; 
  _ctype[10] = 33; 
  _ctype[11] = 33; 
  _ctype[12] = 33; 
  _ctype[13] = 33; 
  _ctype[14] = 33; 
  _ctype[15] = 32; 
  _ctype[16] = 32; 
  _ctype[17] = 32; 
  _ctype[18] = 32; 
  _ctype[19] = 32; 
  _ctype[20] = 32; 
  _ctype[21] = 32; 
  _ctype[22] = 32; 
  _ctype[23] = 32; 
  _ctype[24] = 32; 
  _ctype[25] = 32; 
  _ctype[26] = 32; 
  _ctype[27] = 32; 
  _ctype[28] = 32; 
  _ctype[29] = 32; 
  _ctype[30] = 32; 
  _ctype[31] = 32; 
  _ctype[32] = 32; 
  _ctype[33] = 1; 
  _ctype[34] = 64; 
  _ctype[35] = 64; 
  _ctype[36] = 64; 
  _ctype[37] = 64; 
  _ctype[38] = 64; 
  _ctype[39] = 64; 
  _ctype[40] = 64; 
  _ctype[41] = 64; 
  _ctype[42] = 64; 
  _ctype[43] = 64; 
  _ctype[44] = 64; 
  _ctype[45] = 64; 
  _ctype[46] = 64; 
  _ctype[47] = 64; 
  _ctype[48] = 64; 
  _ctype[49] = 2; 
  _ctype[50] = 2; 
  _ctype[51] = 2; 
  _ctype[52] = 2; 
  _ctype[53] = 2; 
  _ctype[54] = 2; 
  _ctype[55] = 2; 
  _ctype[56] = 2; 
  _ctype[57] = 2; 
  _ctype[58] = 2; 
  _ctype[59] = 64; 
  _ctype[60] = 64; 
  _ctype[61] = 64; 
  _ctype[62] = 64; 
  _ctype[63] = 64; 
  _ctype[64] = 64; 
  _ctype[65] = 64; 
  _ctype[66] = 20; 
  _ctype[67] = 20; 
  _ctype[68] = 20; 
  _ctype[69] = 20; 
  _ctype[70] = 20; 
  _ctype[71] = 20; 
  _ctype[72] = 4; 
  _ctype[73] = 4; 
  _ctype[74] = 4; 
  _ctype[75] = 4; 
  _ctype[76] = 4; 
  _ctype[77] = 4; 
  _ctype[78] = 4; 
  _ctype[79] = 4; 
  _ctype[80] = 4; 
  _ctype[81] = 4; 
  _ctype[82] = 4; 
  _ctype[83] = 4; 
  _ctype[84] = 4; 
  _ctype[85] = 4; 
  _ctype[86] = 4; 
  _ctype[87] = 4; 
  _ctype[88] = 4; 
  _ctype[89] = 4; 
  _ctype[90] = 4; 
  _ctype[91] = 4; 
  _ctype[92] = 64; 
  _ctype[93] = 64; 
  _ctype[94] = 64; 
  _ctype[95] = 64; 
  _ctype[96] = 64; 
  _ctype[97] = 64; 
  _ctype[98] = 24; 
  _ctype[99] = 24; 
  _ctype[100] = 24; 
  _ctype[101] = 24; 
  _ctype[102] = 24; 
  _ctype[103] = 24; 
  _ctype[104] = 8; 
  _ctype[105] = 8; 
  _ctype[106] = 8; 
  _ctype[107] = 8; 
  _ctype[108] = 8; 
  _ctype[109] = 8; 
  _ctype[110] = 8; 
  _ctype[111] = 8; 
  _ctype[112] = 8; 
  _ctype[113] = 8; 
  _ctype[114] = 8; 
  _ctype[115] = 8; 
  _ctype[116] = 8; 
  _ctype[117] = 8; 
  _ctype[118] = 8; 
  _ctype[119] = 8; 
  _ctype[120] = 8; 
  _ctype[121] = 8; 
  _ctype[122] = 8; 
  _ctype[123] = 8; 
  _ctype[124] = 64; 
  _ctype[125] = 64; 
  _ctype[126] = 64; 
  _ctype[127] = 64; 
  _ctype[128] = 32; 
 
}

Then in ctype.h you have macros that use _ctype[] :

Code:

#define _IS_SP	1			/* is space */
#define _IS_DIG	2			/* is digit indicator */
#define _IS_UPP	4			/* is upper case */
#define _IS_LOW	8			/* is lower case */
#define _IS_HEX	16			/* [A-F or [a-f] */
#define _IS_CTL	32			/* Control */
#define _IS_PUN	64			/* punctuation */

extern	char _Cdecl _ctype[];	 /* Character type array */

#define isalnum(c)	(_ctype[(c) + 1] & (_IS_DIG | _IS_UPP | _IS_LOW))
#define isalpha(c)	(_ctype[(c) + 1] & (_IS_UPP | _IS_LOW))
#define isascii(c)	((unsigned)(c) < 128)
#define iscntrl(c)	(_ctype[(c) + 1] & _IS_CTL)
#define isdigit(c)	(_ctype[(c) + 1] & _IS_DIG)
#define isgraph(c)	((c) >= 0x21 && (c) <= 0x7e)
#define islower(c)	(_ctype[(c) + 1] & _IS_LOW)
#define isprint(c)	((c) >= 0x20 && (c) <= 0x7e)
#define ispunct(c)	(_ctype[(c) + 1] & _IS_PUN)
#define isspace(c)	(_ctype[(c) + 1] & _IS_SP)
#define isupper(c)	(_ctype[(c) + 1] & _IS_UPP)
#define isxdigit(c)	(_ctype[(c) + 1] & (_IS_DIG | _IS_HEX))

**Emmanuel Delaha** · 07-16-2004

Originally Posted by Kinasz

Is this a proper way to test if a char is an alpha or am I dreaming.
<cut>

No.

Include <ctype.h> and use isalpha(). It belongs to the standard C library.

Have a look to

http://www.dinkumware.com/refxc.html

for a complete list and explanations of standard functions

**ggs** · 07-16-2004

this is really late, but when i said literals i meant instead of using an ugly hex code

#define LOWEST_ALPHA 0x41

you could just

#define LOWEST_ALPHA 'a'

there's not even that much a need for that many layers of macros, you are likely not developing your application for any nonascii systems. but yeah, isalpha..

Thread: is their a better way of checking for alpha?

Thread Tools

Search Thread

Display

is their a better way of checking for alpha?

Similar Threads

Array bounds checking

Profiler Valgrind

Interpreter.c

Forced moves trouble!!

Problems about gcc installation