c++ - value of variable change event when don't do operation on them -

i have code:

#include <iostream> #include<cstdio> #include<conio.h> #include<windows.h> #define boardsize 3 using namespace std;  int board[boardsize][boardsize]; int cnt = 0; int mincount = boardsize*boardsize;  void boardswitch(int x, int y){     for(int = x-1; <= x+1; i++){         for(int j = y-1; j <= y+1; j++){             if((i>=0) && (j>=0)){                 board[i][j] = board[i][j] ^ 1;             }         }     } }  bool ison(){     int i, j;      for(i = 0; < boardsize; i++){         for(j = 0; j < boardsize; j++){             if(!board[i][j]){                 return 0;             }         }     }     return 1; }  void boarddisp(){     for(int = 0; <boardsize; i++){         for(int j = 0; j < boardsize; j++){             cout<<board[i][j]<<" ";         }         cout<<endl;     } }  void turn(int x){     int i, j;      j = x%boardsize;     = (x-j)/boardsize;     if(x == 0){         cnt = 0;     }     for(int k = 0; k <=1; k++){         printf("%d \n", cnt);         cout<<i<<" "<<j<<" cnt = "<<cnt<<endl;         //getch();         if(k == 1){             boardswitch(i, j);  //switch on (i,j) position             //cout<<i<<" "<<j<<" cnt = "<<cnt<<endl;             //cnt++;             //cout<<"switch on "<<i<<" "<<j<<" cnt = " <<cnt<<endl;         }         if(x == boardsize*boardsize-1){ //          if(ison()){ //              mincount = mincount < cnt ? mincount : cnt; //          }         }         else{             turn(x+1);         }     }     boardswitch(i, j);  // return status before switch     //cnt--; }   int main(){     for(int = 0; < boardsize; i++){         for(int j = 0; j < boardsize; j++){             board[i][j] = rand()%2;     }     }     boarddisp();     turn(0);     if(mincount == boardsize*boardsize){         //there no way turn board on         cout<<"there no way turn board on";     }else{         cout<<mincount;     }     return 0; } 

in thought cnt global variable , every function can change it's value. there no function change it's value when run code (by code block, visual c++) sometime print out value 1 cnt variable , somtine 0. see no operation can make change value of cnt it's change. why ?

with code commented have it, turn() keeps calling increasing values of x until turn(8).

at point, gets set '2' , j gets set '2'. then, call boardswitch(2,2).

your problem happens in boardswitch(). for() loop keeps going until both , j equal 3, resulting in write board[3][3]. location doesn't exist, writing off end of variable -- @ point, clobbered. in case, cnt getting overwritten.

the main point there nothing prevent writing off end of array in c/c++. start writing memory isn't thought was, , program begins behave unpredictably.

ultimately need fix logic. 1 thing might try in meantime creating function wrap writes array , perform bounds checking you. example:

    void writetoboard(int i, int j, int value) {         if (i >= boardsize) {             printf("error: attempted write outside array.\n");             return;         } else if(j >= boardsize) {             printf("error: attempted write outside array.\n");             return;         } else {              board[i][j] = value;         }                  } 

then replace writes array calls function.

this catch problem when happens in easier debug way.

wrapping board in object, making object private, , exposing sort of thing member function excellent example of encapsulation.

an better solution use standard library containers thing (efficiently). isn't option, , it's know conceptually how sort of thing works. rule, though, there reason implement own bounds checked array. that's why have standard library.