Macrozzz

[sandy]

Hi,
While reading about MACRO's and C preprocessor,
I came across something.

You need to write a simple macro CHAR(x) which takes a character x and
converts it into ASCII value of x.

printf("%c",CHAR(a)) ==a
printf("%c",CHAR(X)) ==X

No, this simple definition doesn't work:
#define CHAR(x) 'x'


Cheers,

Sandeep.

[Richard Heathfield]

sandy said:

Hi,
While reading about MACRO's and C preprocessor,
I came across something.

You need to write a simple macro CHAR(x) which takes a character x and
converts it into ASCII value of x.

If you're on an ASCII system, this is easy:

#define CHAR(x) x

If you're on an EBCDIC system, this is still easy:

#define CHAR(x) convert_ebcdic_to_ascii(x)

but writing convert_ebcdic_to_ascii might slow you down a little.

And if you're on some other system foo, s/ebcdic/foo/ in the previous
sentence.

For portability, you might want to write yourself a
convert_native_to_ascii() function, which on ASCII systems would be a NOP,
but which would do the necessary on other systems. The macro then becomes

#define CHAR(x) convert_native_to_ascii(x)

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[Marc Boyer]

Le 05-12-2006, sandy <sa***********@gmail.coma écrit*:

You need to write a simple macro CHAR(x) which takes a character x and
converts it into ASCII value of x.

First, ASCII is not the only possible encoding.
Second, 'x' is a numerical value.

printf("%c",CHAR(a)) ==a
printf("%c",CHAR(X)) ==X

No, this simple definition doesn't work:
#define CHAR(x) 'x'

#include <assert.h>
#define CHAR(X) (#X)[0]

int main(){
assert( CHAR(x) == 'x' );
}

How does it works ? #X ask the preprocesseur to transform
#foo into "foo". And then, (#foo)[0] --"foo"[0] == 'f'

Marc Boyer

[santosh]

Marc Boyer wrote:

Le 05-12-2006, sandy <sa***********@gmail.coma écrit :

You need to write a simple macro CHAR(x) which takes a character x and
converts it into ASCII value of x.

First, ASCII is not the only possible encoding.
Second, 'x' is a numerical value.

printf("%c",CHAR(a)) ==a
printf("%c",CHAR(X)) ==X

No, this simple definition doesn't work:
#define CHAR(x) 'x'

#include <assert.h>
#define CHAR(X) (#X)[0]

int main(){
assert( CHAR(x) == 'x' );
}

How does it works ? #X ask the preprocesseur to transform
#foo into "foo". And then, (#foo)[0] --"foo"[0] == 'f'

This assumes that ASCII is the character encoding for the system. The
OP gave no such assurance.

[Marc Boyer]

Le 05-12-2006, santosh <sa*********@gmail.coma écrit*:

>
Marc Boyer wrote:

>Le 05-12-2006, sandy <sa***********@gmail.coma écrit :

You need to write a simple macro CHAR(x) which takes a character x and
converts it into ASCII value of x.

First, ASCII is not the only possible encoding.
Second, 'x' is a numerical value.

printf("%c",CHAR(a)) ==a
printf("%c",CHAR(X)) ==X

No, this simple definition doesn't work:
#define CHAR(x) 'x'

#include <assert.h>
#define CHAR(X) (#X)[0]

int main(){
assert( CHAR(x) == 'x' );
}

How does it works ? #X ask the preprocesseur to transform
#foo into "foo". And then, (#foo)[0] --"foo"[0] == 'f'

This assumes that ASCII is the character encoding for the system. The
OP gave no such assurance.

Yes, I wrote 'First, ASCII is not the only possible encoding.'
But I was assuming (given its example) that the real need of
the OP is to transform x into 'x', not to get an ASCII value.

Marc Boyer

[santosh]

sandy wrote:

Hi,
While reading about MACRO's and C preprocessor,
I came across something.

You need to write a simple macro CHAR(x) which takes a character x and
converts it into ASCII value of x.

printf("%c",CHAR(a)) ==a
printf("%c",CHAR(X)) ==X

No, this simple definition doesn't work:
#define CHAR(x) 'x'

If your system is an ASCII based one then this is as easy as:
#define CHAR2ASCII(ch) ((int)(ch))

But if not, then some of conversion is necessary. There's no one
'catch-all' macro that can do the job.

[Marc Boyer]

Le 05-12-2006, santosh <sa*********@gmail.coma écrit*:

sandy wrote:

>Hi,
While reading about MACRO's and C preprocessor,
I came across something.

You need to write a simple macro CHAR(x) which takes a character x and
converts it into ASCII value of x.

printf("%c",CHAR(a)) ==a
printf("%c",CHAR(X)) ==X

No, this simple definition doesn't work:
#define CHAR(x) 'x'

If your system is an ASCII based one then this is as easy as:
#define CHAR2ASCII(ch) ((int)(ch))

Why did you do such cast ?

Marc Boyer

[Barry Schwarz]

On 4 Dec 2006 23:38:18 -0800, "sandy" <sa***********@gmail.comwrote:

>Hi,
While reading about MACRO's and C preprocessor,
I came across something.

You need to write a simple macro CHAR(x) which takes a character x and
converts it into ASCII value of x.

printf("%c",CHAR(a)) ==a
printf("%c",CHAR(X)) ==X

No, this simple definition doesn't work:
#define CHAR(x) 'x'

Of course it doesn't. There is no printable character whose ASCII
value can be expressed in a single character. Therefore, using %c is
doomed to failure, whether you use a macro or not.

Change your printf to %d or %x and it works fine.
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