Is it possible to swap the occurences of two variables using macros in C++?

Question

I was working on a contest problem, where I had to initialize a vector in the following way:

vector<pair<int,int>> moves({{i,j}, {i,-j}, {-i,j}, {-i,-j}, 
                             {j,i}, {j,-i}, {-j,i}, {-j,-i}});

Although I know that this is not the best way and am aware of a number of different ways to accomplish this with minimal effort, I wonder if there is a way the C++ macros allow me to simply copy-paste the first four elements of the above vector to achieve the same in the following way:

vector<pair<int,int>> moves({{i,j}, {i,-j}, {-i,j}, {-i,-j},
#define i j 
#define j i
                             {i,j}, {i,-j}, {-i,j}, {-i,-j}
#undef i
#undef j
                             });

The above code obviously does not work because of circular referencing of the variables.

P.S. : Although Matteo Italia's answer is absolutely correct in the above context, I am more interested in knowing whether swapping occurences of variables using macros is possible in C++ or not and how, if possible?

The problem of Cartesian product can be solved very well without any macros: https://stackoverflow.com/questions/29451291/cartesian-product-in-c — Yksisarvinen, Aug 30 '19 at 22:21
@Yksisarvinen As I wrote in the question, I am aware of that. — Shubham Avasthi, Aug 30 '19 at 22:21

Matteo Italia · Answer 1 · 2019-08-30T22:26:09.237

If you really think it's such a saving, you can use a function-style macro and invoke it twice, swapping its arguments.

#define SIGN_COMB(a, b) {(a), (b)}, {(a), -(b)}, {-(a), (b)}, {-(a), -(b)}
vector<pair<int,int>> moves({SIGN_COMB(i, j), SIGN_COMB(j, i)})

The parentheses in the macro expansion as usual are to avoid surprises if expressions are passed as argument (if one passed in e.g. x+y, the negated terms would be -x+y instead if -(x+y)).

JaMiT · Accepted Answer · 2019-08-31T21:47:49.787

Macros are Evil and not a good idea in this context. However, since that appears to be known and the question is more of an academic nature: think "define" rather than "swap". You can use neither "i" nor "j" as a macro's name since you want both to appear in the processed code. So introduce two new identifiers for your macros. Once you update the first four elements to use the new identifiers, you get to copy-and-paste them as requested.

vector<pair<int,int>> moves({
#define a i
#define b j
                             {a,b}, {a,-b}, {-a,b}, {-a,-b},
#undef a
#undef b
#define a j
#define b i
                             {a,b}, {a,-b}, {-a,b}, {-a,-b}
#undef a
#undef b
                             });

Is it possible to swap the occurences of two variables using macros in C++?

2 Answers2