# recursive solution not working as intended/ running into errors

I'm looking for some help on a problem that I vaguely inquired about before, which is solving 15-peg solitaire recursively. I keep getting strange errors when I compile and run it, most of them say "stack overflow" or that I'm getting a seg fault. This is what I have so far, where "board[15]" represents the 15 peg board, and "moves[36]" represents all of the possible moves that can be made. The recursion is supposed to spot when there is only one peg left.

`#include <iostream> using namespace std; void solveGame(int a[15], int b[36][3], int c[15][4]); void chooseMove (int a[15], int b[36][3], int openSpace, int c[15][4]); int findEmpty (int a[15]); int pegCount (int a[15]); bool isPeg (int peg, int a[15]); int usedVals[15] = {-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1}; int d = 0; int index = 0; int main () { int openSpace = 5; int board[15]= {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}; board[openSpace] = 0; int alreadyMoved[15][4]; int moves[36][3] = {{0, 1, 3}, {0, 2, 5}, {1, 3, 6}, {1, 4, 8}, {2, 4, 7}, {2, 5, 9}, {3, 6, 10}, {3, 7, 12}, {3, 1, 0}, {3, 4, 5}, {4, 7, 11}, {4, 8, 13}, {5, 9, 14}, {5, 8, 12}, {5, 2, 0}, {5, 4, 3}, {6, 3, 1}, {6, 7, 8}, {7, 4, 2}, {7, 8, 9}, {8, 4, 1}, {8, 7, 6}, {9, 5, 2}, {9, 8, 7}, {10, 6, 3}, {10, 11, 12}, {11, 7, 4}, {11, 12, 13}, {12, 7, 3}, {12, 8, 5}, {12, 11, 10}, {12, 13, 14}, {13, 8, 4}, {13, 12, 11}, {14, 9, 5}, {14, 13, 12}}; solveGame(board, moves, alreadyMoved); for (int i = 0; i < 13; i++) cout << alreadyMoved[i][0] << " " << alreadyMoved[i][1] << " " < <alreadyMoved[i][2] << endl; return 0; } // main recursive function void solveGame (int a[15], int b[36][3], int c[15][4] { int empSpace; int moveIndex; if (pegCount(a) < 2) { cout<<"game over"<<endl; } else { empSpace = findEmpty(a); chooseMove(a, b, empSpace, c); solveGame(a, b, c); } } // supposed to pick a move that is applicable to the board otherwise it find a new move void chooseMove (int a[15], int b[36][3], int openSpace, int c[15][4]) { int i = 0; while (1) { if (i < 36 && b[i][2] == openSpace && isPeg(b[i][0],a) && isPeg(b[i][1],a)) { a[b[i][0]] = 0; a[b[i][1]] = 0; a[b[i][2]] = 1; c[d][0] = b[i][0]; c[d][1] = b[i][1]; c[d][2] = b[i][2]; c[d][3] = i; d++; index = 0; for (int v = 0; v < 15; v++) usedVals[v] = -1; break; } else if (i > 35) { a[b[c[d-1][3]][0]] = 1; a[b[c[d-1][3]][1]] = 1; a[b[c[d-1][3]][2]] = 0; c[d-1][0] = 0; c[d-1][1] = 0; c[d-1][2] = 0; c[d-1][3] = 0; usedVals[index] = openSpace; index++; int newOpen = findEmpty(a); chooseMove(a, b, newOpen, c); } i++; } } // counts the pegs on the board in order to cancel recursion int pegCount (int a[15]) { int count = 0; for (int i = 0; i < 15; i++) if (a[i] == 1) count++; return count; } // finds an empty space that hasn't already been found faulty int findEmpty (int a[15]) { for (int i = 0; i < 15; i++) { for(int j = 0; j < 15; j++) { if(a[i] == 0 && i != usedVals[j] && usedVals[j] > -1) return i; } } } // tests if current index is a peg bool isPeg (int peg, int a[15]) { return a[peg] == 1; } `

A quick glance shows a lot of potential problems, but I think it probably boils down to the way you are passing arrays. Arrays are passed by reference and not by value, so the recursive function is working with a single copy of the array, which I don't think is what you want. Therefore you are never finding the ending move, which will get you a stackoverflow from unlimited recursion.

Try allocating a new copy of the arrays at each level of recursion. Some people will want you to use `new` or `malloc` for this, because they feel an introduction to C++ should be a trial by fire where you have to master memory management to do anything useful. Instead, I would advise you not to use arrays at all; use a collection class that will work properly when passed by value (I think std::vector of POD will do this) and the collection class will create copies of your arrays the way your code seems to expect.

You may also be having a problem of doing a depth-first search in chooseMove, when you really want a breadth-first search.

Stack overfow when using recursivity is pretty common. This is due to the fact that return values for function calls are stored into the stack, and the stack keeps filling as long as function does not return. If the recursivity goes too deep, you end up filling your whole stack and overflowing it, which also causes SEGV.

Usually you get a stack overflow when your exit condition does not work, but here you are also passing your parameters by value, which might overflow your stack even in normal operation.

I suggest you pass your arrays by reference or better in a std::vector. An std::vector is a small object that holds the real data in a heap allocated space. You can even return those.

I also suggest that you start your program in a debugger, that is the simplest and most effective way to find out what exactly is going wrong.

HTH.

Category:c# Views:0 Time:2012-02-08

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