macro problem

[stdvu]

how do i write a macro that performs swapping among 3 variables taken as parameters?

[donbock]

Please describe what you want this macro to do. What gets swapped with what? Here's a starting point:
#define MACRO(P1,P2,P3) ...
You can describe the desired behavior in terms of P1, P2, and P3.

[stdvu]

the problem is that the question states exactly the same way as i posted.

[donbock]

You'll have to ask the teacher for clarification.
The intent would be obvious if you were asked to swap two variables. "Swapping among three variables" could mean any number of things.

[Banfa]

Yes unfortunately there are (at least) 6 different ways to do swapping among 3 variables A, B and C all producing different results so if you don't indicate which one (result) you are trying implement it is not going to be possible for us to help you.

The experts and members of this community give their help and time free of charge. You have no right to that help and being rude and displaying a bad attitude is going to make it more likely that you do not receive help.

The question would not have been asked if it was not necessary to know its answer in order to help you.

Banfa
Administrator

[stdvu]

I was not trying to be rude :( I was stating the problem...... i will check with the teacher straight away ..... i myself am confused ..... :(

[stdvu]

Ok my teacher replied that perform swap among 2 variables

[Banfa]

What really? The macro takes 3 variables but must swap between 2 of them? Which 2 of the 3?

[stdvu]

I mailed my teacher to ask him what he meant and he answere me briefly saying :

dear student,

you can do swap operation on two values.

[JosAH]

No need to bother: you can't swap two variables by using a cpp macro; essentially
the macro processor uses a by name parameter passing mechanism which is
well known for the fact that you can't swap two variables using that mechanism.
Have a look:

#define swap(t, a, b) { t x= a; a= b; b= x; }
...
int a[3] = { 2, 0, 1 };
int i= 1;
...
swap(int, i, a[i]);

kind regards,

Jos

[Banfa]

No need to bother: you can't swap two variables by using a cpp macro

Personally I think you are being just a little pedantic (if correct).

stdvu ignoring Jos counter example of how things can go horribly wrong I suggest you may be look at a writing a macro to swap 2 integer (int) variables which is probably what your teacher is asking for.

It isn't unusual for teachers assign simplistic assignments without considering the full implications of what they are asking.

[JosAH]

Personally I think you are being just a little pedantic (if correct).

Simply see to what that macro expands; it doesn't work; nothing pedantic about it.

kind regards,

Jos

[stdvu]

ok so you mean i simply need to swap 2 variables (int) without going into complexity ....... that is just write a simple program that is defined as macro right??? I will post the code, do temme whether I am correct or not..

[Banfa]

Simply see to what that macro expands; it doesn't work; nothing pedantic about it.

No, no I understand that what you have written doesn't work and demonstrates that that type of macro doesn't work. But that is because of your use of an array and I am sure that is not what stdvu's teacher had in mind, hence "pedantic (if correct)".

It is possible to create a macro that, for instance, swaps the values of 2 int variables (as opposed to an int and an array variable indexed by that int). It is not possible to create a macro that works in all cases.

[JosAH]

No, no I understand that what you have written doesn't work and demonstrates that that type of macro doesn't work. But that is because of your use of an array and I am sure that is not what stdvu's teacher had in mind, hence "pedantic (if correct)".

It is possible to create a macro that, for instance, swaps the values of 2 int variables (as opposed to an int and an array variable indexed by that int). It is not possible to create a macro that works in all cases.

Ok, case closed; the mathematical lazy approach isn't appreciated: one
counter example and you can go back to sleep again: the example showed
that it can't be done so why bother ;-)

kind regards,

Jos