need help with multi line macro usage

This is a discussion on need help with multi line macro usage within the C Programming forums, part of the General Programming Boards category; Consider the following incorrect code segment. It has two pieces to it: PRINT_THREE_TIMES macro and the code that uses the ...

  1. #1
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    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 :

  2. #2
    & the hat of GPL slaying Thantos's Avatar
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    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?

  3. #3
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    Unhappy

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

  4. #4
    Just Lurking Dave_Sinkula's Avatar
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    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  5. #5
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    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?
    Last edited by Cdigitproc1; 04-28-2005 at 05:29 PM.

  6. #6
    Gawking at stupidity
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    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.
    If you understand what you're doing, you're not learning anything.

  7. #7
    ATH0 quzah's Avatar
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    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.
    Hope is the first step on the road to disappointment.

  8. #8
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    Smile

    Thanks a trillion for the tips guys.

  9. #9
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    Quote 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)

  10. #10
    and the hat of int overfl Salem's Avatar
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    > 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.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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