Programming Forums > Application Development > C++

InfoGeek · May 4th, 2006, 1:13 PM

Read all the posts and you'll get the answer yourself

leo1502 · May 4th, 2006, 2:32 PM

you're right, my mistake

#include <stdio.h>
#define SmartMax(x,y)  (y>x) ? y : x 

int main()
{
 int x=8,y=6;
 SmartMax(x,y)=100;
 printf("%d\n",x);
 return 0;
}

> gcc -x c a.c
a.c: In function `main':
a.c:7: invalid lvalue in assignment

here X and Y are lvalues, what am I missing here?

andro · May 4th, 2006, 2:55 PM

You're not returning a pointer to x, you're returning the value of x (I guess?)

To demonstrate, do: printf("%d\n", SmartMax(x, y));

You're basically saying "8 = 100", which is why you get the error.

DaWei · May 4th, 2006, 5:53 PM

The trail to the answer (putting the expanded code into place in the source) was provided in post #2. Actual specific answers have occurred throughout the thread. If a person wants to try to set the value, 3 (or 'true', or 'yummy'), to 100, I doubt it's going to turn the world's math departments upside down. In some cases explanations are worthless and a strait-jacket is called for.

Twilight · May 4th, 2006, 5:53 PM

Yeah, just give addresses for the program to assign to, and it'll work fine.

The Dark · May 4th, 2006, 8:58 PM

Or just write it as a function

(Toggle Plain Text)

void SmartMax(int &x, int &y, int val)
{
  if (x > y)
    x = val;
  else
    y = val;
}

On any modern compiler, this would probably be inlined (or you could tell the compiler to iniline it if you want) and would produce about the same amount of code as the macro version.

leo1502 · May 5th, 2006, 1:36 PM

You mean I should return a pointer and then do *SmartMax(x,y)=100 ?
If so, that's not what I wanted, I need the syntax to be SmartMax(x,y)=100

Or maybe I didn't follow you right, can you give an example then?

about the function idea, I need it to be a macro (that's the assignment's requirements)

Ooble · May 5th, 2006, 3:31 PM

(Toggle Plain Text)

#include <stdio.h>
#define SmartMax(x,y)  (y>x) ? y : x 

int main()
{
 int x=8,y=6;
 SmartMax(x,y)=100;
 printf("%d\n",x);
 return 0;
}

Let's expand this:

(Toggle Plain Text)

#include <stdio.h>

int main()
{
 int x=8,y=6;
 (y>x) ? y : x=100;
 printf("%d\n",x);
 return 0;
}

That look legit to you?

leo1502 · May 5th, 2006, 4:51 PM

(Toggle Plain Text)

#include <stdio.h>
#define SmartMax(x,y)  int *z,*w; z=&x; w=&y; ((y>x) ? *w : *z)



int main()
{
 int x=2,y=6; 
 SmartMax(x,y)=100;
 printf("%d\n",y);
 return 0;
}

> g++ a.cpp
> ./a.out
100

now it works
but why does it work here and not when I use the macro:

(Toggle Plain Text)

#define SmartMax(x,y)  ((y>x) ? (y) :(x))

aren't x and y lvalues? shouldn't both macros return the same?

can you show me how to write this macro without using new vars?

grumpy · May 5th, 2006, 5:51 PM

Quote:

Originally Posted by leo1502

[code]
but why does it work here and not when I use the macro:

(Toggle Plain Text)

#define SmartMax(x,y)  ((y>x) ? (y) :(x))

aren't x and y lvalues? shouldn't both macros return the same?

can you show me how to write this macro without using new vars?

I thought this thread would be well and truly dead by now.

This last form should work as you intend. If it is not, you are doing something else wrong. In particular, if you take out one of the brackets or leave in a semi-colon, it will not work.

May 4th, 2006, 1:13 PM	#21
InfoGeek Professional Programmer Join Date: Jun 2005 Location: India, The great. Posts: 435 Rep Power: 4	Read all the posts and you'll get the answer yourself __________________ PFO - My daily dose of technology.

May 4th, 2006, 2:32 PM	#22
leo1502 Newbie Join Date: Apr 2006 Posts: 16 Rep Power: 0	you're right, my mistake (Toggle Plain Text) #include <stdio.h> #define SmartMax(x,y) (y>x) ? y : x int main() { int x=8,y=6; SmartMax(x,y)=100; printf("%d\n",x); return 0; } #include <stdio.h> #define SmartMax(x,y) (y>x) ? y : x int main() { int x=8,y=6; SmartMax(x,y)=100; printf("%d\n",x); return 0; } > gcc -x c a.c a.c: In function `main': a.c:7: invalid lvalue in assignment here X and Y are lvalues, what am I missing here?

May 4th, 2006, 5:53 PM	#24
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	The trail to the answer (putting the expanded code into place in the source) was provided in post #2. Actual specific answers have occurred throughout the thread. If a person wants to try to set the value, 3 (or 'true', or 'yummy'), to 100, I doubt it's going to turn the world's math departments upside down. In some cases explanations are worthless and a strait-jacket is called for. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

May 4th, 2006, 8:58 PM	#26
The Dark Expert Programmer Join Date: Jun 2005 Posts: 893 Rep Power: 4	Or just write it as a function (Toggle Plain Text) void SmartMax(int &x, int &y, int val) { if (x > y) x = val; else y = val; } void SmartMax(int &x, int &y, int val) { if (x > y) x = val; else y = val; } On any modern compiler, this would probably be inlined (or you could tell the compiler to iniline it if you want) and would produce about the same amount of code as the macro version.

May 5th, 2006, 3:31 PM	#28
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	(Toggle Plain Text) #include <stdio.h> #define SmartMax(x,y) (y>x) ? y : x int main() { int x=8,y=6; SmartMax(x,y)=100; printf("%d\n",x); return 0; } #include <stdio.h> #define SmartMax(x,y) (y>x) ? y : x int main() { int x=8,y=6; SmartMax(x,y)=100; printf("%d\n",x); return 0; } Let's expand this: (Toggle Plain Text) #include <stdio.h> int main() { int x=8,y=6; (y>x) ? y : x=100; printf("%d\n",x); return 0; } #include <stdio.h> int main() { int x=8,y=6; (y>x) ? y : x=100; printf("%d\n",x); return 0; } That look legit to you? __________________ Me :: You :: Them

May 4th, 2006, 2:55 PM	#23
andro Professional Programmer Join Date: Oct 2005 Location: California Posts: 319 Rep Power: 4	You're not returning a pointer to x, you're returning the value of x (I guess?) To demonstrate, do: printf("%d\n", SmartMax(x, y)); You're basically saying "8 = 100", which is why you get the error.

May 4th, 2006, 5:53 PM	#25
Twilight Programmer Join Date: Apr 2006 Location: Calgary, Alberta Posts: 67 Rep Power: 3	Yeah, just give addresses for the program to assign to, and it'll work fine.

May 5th, 2006, 1:36 PM	#27
leo1502 Newbie Join Date: Apr 2006 Posts: 16 Rep Power: 0	You mean I should return a pointer and then do *SmartMax(x,y)=100 ? If so, that's not what I wanted, I need the syntax to be SmartMax(x,y)=100 Or maybe I didn't follow you right, can you give an example then? about the function idea, I need it to be a macro (that's the assignment's requirements)

May 5th, 2006, 4:51 PM	#29
leo1502 Newbie Join Date: Apr 2006 Posts: 16 Rep Power: 0	(Toggle Plain Text) #include <stdio.h> #define SmartMax(x,y) int z,w; z=&x; w=&y; ((y>x) ? w : z) int main() { int x=2,y=6; SmartMax(x,y)=100; printf("%d\n",y); return 0; } > g++ a.cpp > ./a.out 100 #include <stdio.h> #define SmartMax(x,y) int z,w; z=&x; w=&y; ((y>x) ? w : z) int main() { int x=2,y=6; SmartMax(x,y)=100; printf("%d\n",y); return 0; } > g++ a.cpp > ./a.out 100 now it works but why does it work here and not when I use the macro: (Toggle Plain Text) #define SmartMax(x,y) ((y>x) ? (y) :(x)) #define SmartMax(x,y) ((y>x) ? (y) :(x)) aren't x and y lvalues? shouldn't both macros return the same? can you show me how to write this macro without using new vars?

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