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switch { } - case for range


I was checking out the C-FAQ and read here (
http://c-faq.com/misc/nonconstcase.html ) that:
" case labels are limited to single, constant, integral expression ".

However, I have been using case with ranges for a long while ( gcc,
VC++) so either the FAQ calls for an update or those two compilers
provide this functionality as an extension.

example;
switch (a)
{
case 1 ... 10:
// code
break;

case 11 ... 50:
// code
break;

case 800:
// code
break;

default:
// code
break;
}

Sep 6 '06
21 52356
CBFalconer wrote:
sv***********@g mail.com wrote:
>CBFalconer wrote:
>>Why in heavens name use it and become non-portable, when the
portable code (following) is clearer:

if ((a >= 1) && (a <= 10)) firstcode();
else if ((a 10) && (a <= 50)) secondcode();
else if (a == 800) thirdcode();
else defaultcode();
Too bad C didn't keep the range syntax from BCPL. Then you could
have used the even clearer:

if (1 <= a <= 10) firstcode();
else if (10 < a <= 50) secondcode();
else if (a == 800) thirdcode();
else defaultcode();

I disagree. The grammar needed to parse such constructs is a
horror, and the result is all sorts of silly errors. SPL (Algol
based HP System Programming Language for the HP3000) had that
construct, and it created nothing but trouble.
It could be done with a preprocessor for C. This needs a lot more work
before use in production code.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
char *p, *q;
int pe, qe;
char line[1024];

while(fgets(lin e, sizeof line, stdin))
{
p = strstr(line, "<");
if(p)
{
q = strstr(p+1, "<");
if(q)
{
*p = 0;
*q = 0;
if(p[1] == '=') { pe = '='; p++; } else pe = ' ';
if(q[1] == '=') { qe = '='; q++; } else qe = ' ';
printf("%s<%c%s && %s<%c%s", line, pe, p+1, p+1, qe, q+1);
}
else
{
printf("%s", line);
}
}
else
{
printf("%s", line);
}
}
return 0;
}

Caveats: Only works once per line. Knows nothing of C syntax. Does not
detect comments or string literals.

Example:

C:\docs\prog\c> type test.c
#include <stdio.h>

void test(int a, int b, int c)
{
printf("%d lt %d lt %d = %d\n", a, b, c, a < b < c);
printf("%d lt %d le %d = %d\n", a, b, c, a < b <= c);
printf("%d le %d lt %d = %d\n", a, b, c, a <= b < c);
printf("%d le %d le %d = %d\n", a, b, c, a <= b <= c);
printf("\n");
}

int main(void)
{
test(1,2,2);

return 0;
}

C:\docs\prog\c> gcc -ansi -pedantic -Wall -W -O2 rangepp.c -o rangepp

C:\docs\prog\c> rangepp < test.c test2.c && gcc test2.c && a
1 lt 2 lt 2 = 0
1 lt 2 le 2 = 1
1 le 2 lt 2 = 0
1 le 2 le 2 = 1

--
Simon.
Sep 10 '06 #21
CBFalconer wrote:
sv***********@g mail.com wrote:
>>
CBFalconer wrote:
>>Why in heavens name use it and become non-portable, when the
portable code (following) is clearer:

if ((a >= 1) && (a <= 10)) firstcode();
else if ((a 10) && (a <= 50)) secondcode();
else if (a == 800) thirdcode();
else defaultcode();

Too bad C didn't keep the range syntax from BCPL. Then you could
have used the even clearer:

if (1 <= a <= 10) firstcode();
else if (10 < a <= 50) secondcode();
else if (a == 800) thirdcode();
else defaultcode();

I disagree. The grammar needed to parse such constructs is a
horror,
Not true. Why do you think this?

(The BCPL parser has something in the nature of a hack, but not
a nasty hack - the nasty bit was the code generation, which,
at least in the compiler I tinkered with, re-evaluated middle
expressions, so if they had side-effects URGH.

I've also implemented long relational expressions myself. The
grammar is peanuts. The tricky bit is just arranging to not
re-evaluate the middle expressions, and that's one of those
jobs where, once you know you have to do it, you just do it.)

--
Chris "Spice" Dollin
A rock is not a fact. A rock is a rock.

Sep 11 '06 #22

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