loops and macros

Hi all,

Is it possible to include a for or a while loop in a macro? I have an assignment that says to create a macro that will sum the elements of an array, but the book, obviously, didn't show any examples of loops in macros. And before you ask, yes I did read the FAQs, but there weren't any explanations for loops in macros either.

Thanks again.

Quote:

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Originally Posted by ginom71

Hi all,

Is it possible to include a for or a while loop in a macro?

---

Yes.

What is the syntax? Do you write it like you would normally for a function prototype? This is what I have:

Code:

---

#include <stdio.h>
#include <stdlib.h>
#define SUMARRAY(x, y) \
        (while(y!<0){\
        (x[y]) + (x[y-1])\
        y--)

int main(void)
{
        int x[BUFSIZ];

        fgets(x,BUFSIZ,stdin);

        SUMARRAY(x, BUFSIZ);

        system("pause");

        return 0;
}

---

I keep getting a compile error that says "error C2059: syntax error : 'while'"

If you can loop in a macro, what is the problem here?

Quote:

---

Originally Posted by ginom71

What is the syntax?
If you can loop in a macro, what is the problem here?

---

Put in a normal block, using \ at the end of lines:

Code:

---

#define this(x) while(x>0) {printf("%d",x); \
      x--;}

---

I'm not a big user of macros so I can't tell you what caveats may apply here, but I have seen macros do undesirable things when compared to an equivalent function sometimes esp when the caller is recursive, so watch for stuff like that.

Thanks again. I read that people have had trouble with recursive functions in macros. I'm not sure what the benefit is except for maybe simple computations.

So I took your suggestion, and I wrote the following code:

Code:

---

#include <stdio.h>
#include <stdlib.h>
#define SUMARRAY(x,y) \
        while(y > 0){\
        x[y] + x[y-1];\
        y--;}

int main(void)
{
        int x[3] = {1, 2, 3};
        int total;

        total = SUMARRAY(x,3);

        printf("%d", total);

        system("pause");

        return 0;
}

---

The problem is that I get 12 errors after I compile the code, and it starts with the same one "c_13.10.c(13) : error C2059: syntax error : 'while'". The question I have is that the first error points to line 13. Why is that?

Quote:

---

The problem is that I get 12 errors after I compile the code, and it starts with the same one "c_13.10.c(13) : error C2059: syntax error : 'while'". The question I have is that the first error points to line 13. Why is that?

---

Remember that a preprocessor macro involves text substitution, so replace the macro use with the code that you defined the macro with and you will see the problem.

As for a solution: have you learnt how to use pointers yet?

EDIT:
Oh wait, I just realised, since this is a macro, the use of a pointer as an out parameter is unnecessary. Anyway, the point is that you should use total as an argument to the function-style macro.

OK, so that while loop won't compile in the main body of the code. I wouldn't normally use that code to sum the elements in an array anyway. I'm just pretty lost when it comes to defining this in a macro.

I have learned how to use pointers. That's not much different from using arrays.

Quote:

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Originally Posted by ginom71

I'm just pretty lost when it comes to defining this in a macro.

---

Basically, you need to pass three things to the function-style macro: the array name, the array size, and a variable to store the result. To loop, you need a loop counter, and this means defining a variable, hence you should use the do while loop trick, e.g.,

Code:

---

#define MY_MACRO(x) do\
{\
    int i = 0;\
    /* ... */\
} while (0)

---

Quote:

---

Originally Posted by ginom71

Thanks again. I read that people have had trouble with recursive functions in macros. I'm not sure what the benefit is except for maybe simple computations.

So I took your suggestion, and I wrote the following code:

Code:

---

#include <stdio.h>
#include <stdlib.h>
#define SUMARRAY(x,y) \
       

int main(void)
{
        int x[3] = {1, 2, 3};
        int total;

        total = SUMARRAY(x,3);

        printf("%d", total);

        system("pause");

        return 0;
}

---

The problem is that I get 12 errors after I compile the code, and it starts with the same one "c_13.10.c(13) : error C2059: syntax error : 'while'". The question I have is that the first error points to line 13. Why is that?

---

That's because the code expands to:

Code:

---

        total = while(3 > 0){
        x[3] + x[3-1];\
        3--;}

---

The fact that this macro does not return the total is part of the problem, as laserlight points out. If you want the total, you have to add it as a parameter and assign the new value inside the macro.

Then there's the problem that you are using the second parameter like a variable, but passing a constant.

By the way, why are you using a macro anyway? Use a function. Always prefer a function to a macro when you can use either.

Just write the code as you normally would, make sure it compiles and works.
Then take that code and make it into a macro, substituting certain parameters inside the code with parameters that the macro takes.
It's very simple.
But a friendly advice, though. Don't normally do this with macros; use functions instead. You only have to use macros here because the assignment demands it.

OK, thanks everyone for the help. So you're saying that I need to write it out like I would for a function, which I did (just for practice). See below:

Code:

---

#include <stdio.h>
#include <stdlib.h>
#define SIZE 4

int SUMARRAY(int y[], int size);

int main(void)
{
        int x[] = {1, 2, 3, 4};
        int sum;

        sum = SUMARRAY(x, SIZE);

        printf("%d\n", sum);

        system("pause");

        return 0;

}
int SUMARRAY(int y[], int size)
{
        int i;
        int total = 0;

        for(i = (SIZE - 1); i >= 0; i--){
                total = total + y[i];
        }
        return total;
}

---

And what I thought all of you said was to write the function out like I normally would, so this is what I wrote:

Code:

---

#include <stdio.h>
#include <stdlib.h>
#define SUMARRAY(x, y) \
        int i;\
        int total = 0;\
        for(i = (y - 1); i >= 0; i--){\
                total = total + x[i];\
        }\
        return total;\
}

int main(void)
{
        int x[] = {1, 2, 3, 4};
        int sum;

        sum = SUMARRAY(x, 4);

        printf("%d\n", sum);

        system("pause");

        return 0;

}

---

Only, I keep getting the same errors:
c_13.10.c(17) : error C2059: syntax error : 'type'
c_13.10.c(17) : error C2143: syntax error : missing ';' before 'type'
c_13.10.c(17) : error C2065: 'i' : undeclared identifier
c_13.10.c(17) : error C2065: 'i' : undeclared identifier
c_13.10.c(17) : error C2065: 'i' : undeclared identifier
c_13.10.c(17) : error C2065: 'total' : undeclared identifier
c_13.10.c(17) : error C2065: 'total' : undeclared identifier
c_13.10.c(17) : error C2065: 'i' : undeclared identifier
c_13.10.c(17) : error C2065: 'total' : undeclared identifier
c_13.10.c(19) : error C2143: syntax error : missing ')' before 'string'
c_13.10.c(19) : error C2143: syntax error : missing '{' before 'string'
c_13.10.c(19) : error C2059: syntax error : '<Unknown>'
c_13.10.c(19) : error C2059: syntax error : ')'
c_13.10.c(21) : error C2143: syntax error : missing ')' before 'string'
c_13.10.c(21) : error C2143: syntax error : missing '{' before 'string'
c_13.10.c(21) : error C2059: syntax error : '<Unknown>'
c_13.10.c(21) : error C2059: syntax error : ')'
c_13.10.c(23) : error C2059: syntax error : 'return'
c_13.10.c(25) : error C2059: syntax error : '}

Does this have something to do with the VB C++ compiler?
Is it not possible to declare variables in a macro? If not, then I'm assuming that you declare them in the main body, and then pass them to the macro.

So then I tried declaring the variables in the main body, like this:

Code:

---

#include <stdio.h>
#include <stdlib.h>
#define SUMARRAY(x, y, z, t) \
        for(z = (y - 1); z >= 0; z--){\
        t = t + x[z];\
        }\
        return t;\
}

int main(void)
{
        int x[] = {1, 2, 3, 4};
        int i = 0;
        int total = 0;
        int sum;

        sum = SUMARRAY(x, 4, i, total);

        printf("%d\n", sum);

        system("pause");

        return 0;

}

---

It still doesn't work.

Quote:

---

Originally Posted by ginom71

And what I thought all of you said was to write the function out like I normally would, so this is what I wrote:

---

No, I told you that you need to pass a variable to the macro to store the result. Consider this macro:

Code:

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#define FOO(x) return (x);

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If you tried to use it as:

Code:

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int a = 0;
int b = FOO(a);

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it would actually be as if you wrote:

Code:

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int a = 0;
int b = return (a);

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Laserlight, thanks for you help. I removed the line:

Code:

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sum = SUMARRAY(x, 4, i, total);

---

and replaced it with;

Code:

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SUMARRAY(x, 4, i, total);

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That cleared everything up, is that because I already had a return value?

Quote:

---

Originally Posted by ginom71

That cleared everything up, is that because I already had a return value?

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More like because function-style macros are not functions, they do not have return values in the way that functions do.