Taking macro argument inside a different macro in C

Question

Why this is not giving the desired output ? For Ex - take a=1,b=2,c=3,d=4 It is giving MAX4() = 2 while it should be 4 in this case.

#include<stdio.h>
#define MAX4(a,b,c,d) MAX2(MAX2(a,b),MAX2(c,d))
#define MAX2(a,b) a>b?a:b 

int a,b,c,d;

int main(){

    /*printf("Please enter 2 integers - \n");
    scanf("%d %d",&a,&b);   
    printf("The maximum number among them is %d\n",MAX2(a,b));
    */
    printf("Please enter 4 integers - \n");
    scanf("%d %d %d %d",&a,&b,&c,&d);   
    printf("The maximum number among them is %d\n",MAX4(a,b,c,d));//,MAX2(a,b),MAX2(c,d));

    return 0;
}

Maxim Egorushkin · Accepted Answer · 2016-04-01 16:24:26Z

3

You need to add parenthesis to enforce the order of evaluation:

#define MAX2(a,b) ((a)>(b)?(a):(b))

The extra parenthesis around arguments are needed to allow for expressions like MAX2(a + 1, b + 2).

edited Apr 1, 2016 at 16:24

answered Apr 1, 2016 at 16:18

Maxim Egorushkin

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1 Comment

Shantanu Singh Over a year ago

Thank you. I didn't know this.

Chris Dodd · Accepted Answer · 2016-04-01 16:32:04Z

3

Macros just do text replacement. So the MAX4 call first expands macros in its arguments, giving

    MAX2(a>b?a:b,c>d?c:d)

which then expands to

    a>b?a:b>c>d?c:d?a>b?a:b:c>d?c:d

When the compiler then parses the expanded macro, the precedence of > (higher than ?:) causes this to not do what you expect. Using extra parentheses in the macro can avoid the precedence problem, but you may still have problems if you call your MAX2 macro on expressions with side effects.

edited Apr 1, 2016 at 16:32

answered Apr 1, 2016 at 16:28

Chris Dodd

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Christopher Schneider Over a year ago

And if you can't do a>b?a:b>c>d?c:d?a>b?a:b:c>d?c:d in your head, you can't call yourself a programmer! Beat me to the answer :-)

Subhiksh · Accepted Answer · 2016-04-01 16:34:22Z

2

In your macro

MAX4(a,b,c,d);

if a=1, b=2, c=3, d=4 then the macro becomes

MAX4(1,2,3,4);

which would in turn will become

MAX2(MAX2(1,2),MAX2(3,4))

So it is evaluated as

MAX2(1>2?1:2,3>4?3:4)

i.e

1>2?1:2>3>4?3:4?1>2?1:2:3>4?3:4

and and according to operator precedence and associativity you get 2 and not 4.

In order to get 4 you should use

#define MAX2(a,b) ((a)>(b)?(a):(b))

answered Apr 1, 2016 at 16:34

Subhiksh

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