need help with multi line macro usage

Consider the following incorrect code segment. It has two pieces to it: PRINT_THREE_TIMES macro and the code
that uses the macro:

#define PRINT_THREE_TIMES \
printf ("one: %d\n", 1); \
printf ("two: %d\n", 2); \
printf ("three: %d\n", 3); \


if (i == 1)
printf( “Only once: %d\n”, 1 );
else if (i == 2)
printf( “i == 2, we don’t need to print anything here!\n”) ;
else if (i == 3)
PRINT_THREE_TIMES

The code does not quite do what is intended.




show how to do this by changing the macro alone. (You are not allowed to consolidate the three ‘printf’
statements to a single one.)

#define PRINT_THREE_TIMES :

First thing to understand is what happens during the preprocessor phase and what the code looks like afterwards. So lets take a look:

Code:

if (i == 1) 
printf( “Only once: %d\n”, 1 );
else if (i == 2) 
printf( “i == 2, we don’t need to print anything here!\n”) ;
else if (i == 3) 
printf ("one: %d\n", 1); 
printf ("two: %d\n", 2); 
printf ("three: %d\n", 3);

Now if you can't see the problem yet lets add proper indentation

Code:

if (i == 1) 
  printf( “Only once: %d\n”, 1 );
else if (i == 2) 
  printf( “i == 2, we don’t need to print anything here!\n”) ;
else if (i == 3) 
  printf ("one: %d\n", 1); 
printf ("two: %d\n", 2); 
printf ("three: %d\n", 3);

Wait thats not what you wanted. You wanted this:

Code:

if (i == 1) 
  printf( “Only once: %d\n”, 1 );
else if (i == 2) 
  printf( “i == 2, we don’t need to print anything here!\n”) ;
else if (i == 3) 
{
  printf ("one: %d\n", 1); 
  printf ("two: %d\n", 2); 
  printf ("three: %d\n", 3); 
}

So can you guess how you should change the macro?

Sorry, I am a novice to using macro; still don't get it.

http://www.eskimo.com/~scs/C-faq/q10.4.html

Are you saying by adding opening/closing braces that would do the job? I have tried a do while, but program does not compile.
Likewise, do I see a problem without the index been declared?

Macros simply replace every instance of the macro name in your code with what you define the macro as. Try just adding opening/closing braces. Think about where you'd put them.

Code:

#define PRINT_THREE_TIMES \
printf ("one: %d\n", 1); \
printf ("two: %d\n", 2); \
printf ("three: %d\n", 3); \

Also, you shouldn't have the \ on that last line, as it serves no purpose.

We could rewrite that macro many ways:

Code:

#define THRICE \
    printf("uno: %d", 1 ), \
    printf("dos: %d", 2 ), \
    printf("tres: %d", 1 );

There's a fun variant.

Quzah.

Thanks a trillion for the tips guys.

Originally Posted by quzah

Code:

#define PRINT_THREE_TIMES \
printf ("one: %d\n", 1); \
printf ("two: %d\n", 2); \
printf ("three: %d\n", 3); \

Also, you shouldn't have the \ on that last line, as it serves no purpose.

We could rewrite that macro many ways:

Code:

#define THRICE \
    printf("uno: %d", 1 ), \
    printf("dos: %d", 2 ), \
    printf("tres: %d", 1 );

There's a fun variant.

Quzah.

here's another one

Code:

#define THRICE \
	do { \
		printf("uno: %d\n", 1 ); \
		printf("dos: %d\n", 2 ); \
		printf("tres: %d\n", 1 ); \
	} while (0)

also when you forget a semicolon after using THRICE the compiler will complain then because it's missing a semicolon after while(0)

> Sorry, I am a novice to using macro; still don't get it.
And so endeth the first lesson.
Whilst there are some problems which are best solved with a macro, this isn't one of them.