Significance of "

Question

I was looking through the solutions of a problem on Topcoder, and came across this one:

http://community.topcoder.com/stat?c=problem_solution&rm=249419&rd=9996&pm=6621&cr=309453

At present I am not interested to know how the algorithm works, but what's the usage of "< ?=" operator in the code? I tried compiling the code on my machine, and also on ideone.com, but to my surprise, the code fails to compile. What does the operator do, and why doesn't it work on standard compilers like ideone, and the code passes system tests on Topcoder?

The code is here:

using namespace std;

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <ctype.h>
#include <string.h>
#include <string>
#include <sstream>
#include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <algorithm>
#include <functional>

#define PB push_back
#define SZ size()
#define REP(v, hi) for (int v=0; v<(hi); v++)
#define REPD(v, hi) for (int v=((hi)-1); v>=0; v--)
#define FOR(v, lo, hi) for (int v=(lo); v<(hi); v++)
#define FORD(v, lo, hi) for (int v=((hi)-1); v>=(lo); v--)

typedef vector <int> VI;
typedef vector <VI> VVI;
typedef vector <VVI> VVVI;

/* ############################ THE REAL CODE ############################ */

class RoboRace {
  public:
  int startTime(vector <string> m, vector <string> _c) {
    string c;
    REP(i,_c.SZ) c+=_c[i];
    int N=c.SZ;
    int Y=m.SZ, X=m[0].SZ;
    VVVI best(Y, VVI(X, VI(N+1, 99999)));
    int ey=-1,ex=-1;
    int yy=-1,yx=-1;
    int fy=-1,fx=-1;
    REP(y,Y) REP(x,X) {
      if (m[y][x]=='Y') { yy=y; yx=x; m[y][x]='.'; }
      if (m[y][x]=='F') { fy=y; fx=x; m[y][x]='.'; }
      if (m[y][x]=='X') { ey=y; ex=x; m[y][x]='.'; }
    }
    REP(n,N+1) best[ey][ex][n]=n;
    REPD(n,N) REP(y,Y) REP(x,X) {
      if (m[y][x]=='#') continue;
      best[y][x][n] <?= best[y][x][n+1];
      if (c[n]=='N' && y>0)   best[y][x][n] <?= best[y-1][x][n+1];
      if (c[n]=='S' && y<Y-1) best[y][x][n] <?= best[y+1][x][n+1];
      if (c[n]=='W' && x>0)   best[y][x][n] <?= best[y][x-1][n+1];
      if (c[n]=='E' && x<X-1) best[y][x][n] <?= best[y][x+1][n+1];
    }

    REP(n,N) if (best[yy][yx][n] < best[fy][fx][n]) return n;
    return -1;
  }
};

WTH should the [tag:c++]`=` operator be?? Never heard about that one ... — πάντα ῥεῖ, Feb 08 '14 at 21:04
@πάνταῥεῖ i am not sure its an operator, but it seemed more of a operator for a novice like me, similar to the "+=" operator. — shivshnkr, Feb 08 '14 at 21:06
As jrok's answer sounds for me you'd be better off not using this! — πάντα ῥεῖ, Feb 08 '14 at 21:08
i don't understand why i get -ve votes for my not knowing something!! — shivshnkr, Feb 08 '14 at 21:08

jrok · Accepted Answer · 2014-02-08T21:12:13.507

6

That's a gcc extension, compound minimum operator, a binary operator that assign the minimum of its operands to the left-hand operand. It's mentioned in the deprecated features list of gcc manual.

A <?= B means assign the minimum of A and B to A.

There are (were) also

<?     minimum operator
>?     maximum operator
>?=    compound form of maximum operator

Sane code nowadays would use std::min instead.

edited Feb 08 '14 at 21:12

answered Feb 08 '14 at 21:06

jrok

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but why doesn't this code compile on my machine or ideone.com, though it passes system tests? – shivshnkr Feb 08 '14 at 21:13
@shivshnkr, It's a ***deprecated*** GCC *extension*. – chris Feb 08 '14 at 21:13
@chris it means none but topcoder still supports deprecated things? – shivshnkr Feb 08 '14 at 21:19
@shivshnkr, I suppose so. – chris Feb 08 '14 at 21:21

Significance of "

1 Answers1