c - How could these case conversion functions be improved? -

as learning exercise, 3 functions—togglecase, lowercase , uppercase—each expect pointer ascii char string, terminated null character; work expected. there more efficient or faster methods of accomplishing task? breaking unspoken rules of c coding? i've made use of macros because, think, makes code better , more efficient function calls. typical or overkill?

please feel free nit-pick , critique code (but nice).

case_conversion.h

#define case_flag 32 #define a_z(c) (c >= 'a' && c <= 'z') #define a_z(c) (c >= 'a' && c <= 'z')  void togglecase(char* c); void lowercase(char* c); void uppercase(char* c); 

case_conversion.c

#include "case_conversion.h"  void togglecase(char* c) {  while (*c)  {   *c ^= a_z(*c) || a_z(*c) ? case_flag : 0;   c++;  } } void lowercase(char* c) {  while (*c)  {   *c ^= a_z(*c) ? case_flag : 0;   c++;  } } void uppercase(char* c) {  while (*c)  {   *c ^= a_z(*c) ? case_flag : 0;   c++;  } } 

there @ least 2 major problems macros. consider happens if call 1 of them like

a_z('a' + 1); 

the call not give correct results due operator precedence. easy fix using brackets:

#define a_z(c) ((c) >= 'a' && (c) <= 'z') 

but can called this:

a_z(i++); 

this call increment i twice! , not fixable (if @ all) in macro. recommend using inline functions instead (if needed - see below).

the fastest way convert between upper/lowercase know of using lookup tables. of course, trades memory speed - pick preference knowing specific platform :-)

you need 2 arrays, 1 either direction. initialize them like

char toupper[128]; // care standard ascii (int = 0; < 128; i++)   toupper[i] = i; toupper['a'] = 'a'; ... toupper['z'] = 'z'; 

and conversion trivial:

char touppercase(char c) {   return toupper[c]; } 

(for production code, should improved extend array possible char values on given platform (or shrink legal values , parameter checking), illustration, do.)