switch on strings

[Francine.Neary]

Suggestion: do a web search for "perfect hash".

20.17: Is there a way to switch on strings?

A: Not directly. Sometimes, it's appropriate to use a separate
function to map strings to integer codes, and then switch on
those. Otherwise, of course, you can fall back on strcmp()

[snip]

For example....

#include <stdarg.h>
#include <stdlib.h>

/*
** string compare string Match to list of strings starting with Model
** return ordinal of successful match (>=0) or -1 on no match
*/

int WhichString(char *Match, ...)
{
va_list arg;
int ordinal = 0;
char *item;

va_start(arg,Match);
for(item=va_arg(arg, char *);item != NULL; item=va_arg(arg, char *))
{
if (strcmp(Match,item) == 0)
break;
else
++ordinal;
}
va_end(arg);

return (item != NULL ? ordinal : -1 );

}

And used like...

#include <stdio.h>

int WhichString(char *model, ...);

int main(int argc, char **argv)
{
enum {left,right,forward,back,up,down,halt} cmd;

while (--argc)
switch (cmd = WhichString(*++argv,
"left",
"right",
"forward",
"back",
"up",
"down",
"halt",
NULL))
{
case left: /* "left" */
puts("<--");
break;
case right: /* "right" */
puts("-->");
break;
case forward: /* "forward" */
puts(" <>");
break;
case back: /* "back" */
puts(" ><");
break;
case up: /* "up" */
puts(" ^\n |");
break;
case down: /* "down" */
puts(" |\n v");
break;
case halt: /* "halt" */
puts(" @\n !");
break;
case -1:
default:
puts("*** Unknown ***");
}

return 0;

}

Ouch! All those strcmps! Why not a simple hash function?

 

[Lew Pitcher]

how to implement a switch case for stringse...switch should happen
depending on the strings...
Suggestion: do a web search for "perfect hash".
From the C FAQ:
20.17: Is there a way to switch on strings?
A: Not directly. Sometimes, it's appropriate to use a separate
function to map strings to integer codes, and then switch on
those. Otherwise, of course, you can fall back on strcmp()

[snip]

For example....

#include <stdarg.h>
#include <stdlib.h>

/*
** string compare string Match to list of strings starting with Model
** return ordinal of successful match (>=0) or -1 on no match
*/

int WhichString(char *Match, ...)
{
va_list arg;
int ordinal = 0;
char *item;

va_start(arg,Match);
for(item=va_arg(arg, char *);item != NULL; item=va_arg(arg, char *))
{
if (strcmp(Match,item) == 0)
break;
else
++ordinal;
}
va_end(arg);

return (item != NULL ? ordinal : -1 );

[snip]
Ouch! All those strcmps! Why not a simple hash function?

Sure, why not? Care to show an example of how a simple hash function
would be better?

 

[user923005]

how to implement a switch case for stringse...switch should happen
depending on the strings...
Suggestion: do a web search for "perfect hash".
From the C FAQ:
20.17: Is there a way to switch on strings?
A: Not directly. Sometimes, it's appropriate to use a separate
function to map strings to integer codes, and then switch on
those. Otherwise, of course, you can fall back on strcmp()
[snip]
For example....
#include <stdarg.h>
#include <stdlib.h>
/*
** string compare string Match to list of strings starting with Model
** return ordinal of successful match (>=0) or -1 on no match
*/
int WhichString(char *Match, ...)
{
va_list arg;
int ordinal = 0;
char *item;
va_start(arg,Match);
for(item=va_arg(arg, char *);item != NULL; item=va_arg(arg, char *))
{
if (strcmp(Match,item) == 0)
break;
else
++ordinal;
}
va_end(arg);
return (item != NULL ? ordinal : -1 );
}

[snip]
Ouch! All those strcmps! Why not a simple hash function?

Sure, why not? Care to show an example of how a simple hash function
would be better?

In this particular case, a hash function is not needed. Just switch
on the first letter.

 

[Flash Gordon]

user923005 wrote, On 01/10/07 20:45:

In this particular case, a hash function is not needed. Just switch
on the first letter.

A very simple hash function which just takes the first letter as the has
value ;-)

Of course, that could give lots of collisions if the strings are changed
or added to.