使用递归函数时出现分段错误

Question

I'm quite new to programming and for my Object Oriented class (in C++), we have a 2d array with random groupings of "X". I have to use a recursive function to find the different groupings and clear them. As of right now, I check if the spot is an X, clear it, then check the 8 positions around it (including diagonals), and if one of the positions in an X, I call the function again but on that location. My idea is that if I find one X, I will be able to get all the X's around it in one go, thus, I can count it as a group when I find an X.

At the end of the function, I basically loop through all the spots and call the recursive function again if there is another X. However, I keep getting segmentation faults and I'm not sure why. Any help would be greatly appreciated!

void  Recurssive(string Array[][72],int Pos1, int Pos2)
{

int One=1;
int Two=1;
//cout<<"Test 2";
if(Array[Pos1][Pos2]=="X")
        {
        Array[Pos1][Pos2]="0";
        if(Array[Pos1+1][Pos2]=="X")
                {
                Recurssive(Array,Pos1+1,Pos2);
                }
         if(Array[Pos1-1][Pos2]=="X")
                {
                Recurssive(Array,Pos1-1,Pos2);
                }
         if(Array[Pos1][Pos2+1]=="X")
                {
                Recurssive(Array,Pos1,Pos2+1);
                }
         if(Array[Pos1][Pos2-1]=="X")
                {
                Recurssive(Array,Pos1,Pos2-1);
                }
         if(Array[Pos1+1][Pos2+1]=="X")
                {
                Recurssive(Array,Pos1+1,Pos2+1);
                }
         if(Array[Pos1-1][Pos2-1]=="X")
                {
                Recurssive(Array,Pos1-1,Pos2-1);
                }
         if(Array[Pos1+1][Pos2-1]=="X")
                {
                Recurssive(Array,Pos1+1,Pos2-1);
                }
         if(Array[Pos1-1][Pos2+1]=="X")
                {
                Recurssive(Array,Pos1-1,Pos2+1);
                }

        }

for(int i=1;i<22;i++)
        {
        for(int j=1;j<72;j++)
                {
                if(Array[i][j]=="X")
                        {
                        Recurssive(Array,i,j);
                        }
                }
        }


}

Here is the output of the array I am looping through

            X                                                         
             X                                                        
              X           XXXXXXXXXXXXXXX                             
               X          XXXXXXXXXXXXXXX                             
                X         XXXXXXXXXXXXXXX        XXXX                 
                 XXXX     XXXXXXXXXXXXXXX      XXX  XXX               
                     X                        XXX    XXX              
     XXXXXXXXXXXXXX   X                      XXX      XXX             
     XX          XX    X                      XXX    XXX              
     XX          XX     X                      XXX  XXX               
     XX          XX      X                       XXXX                 
     XX  XXXXX   XX       X                                           
     XX          XX        X                                          
     XX          XX         X                                         
     XXXXXXXXXXXXXX          X                                        
                                          X                           
                                         X                            
                                        X                             
                                       X                              
                                      X 

Answer 1

Let's play computer and go through the motions for Recurssive(Array, 0, 0) . If this position was marked as X , this will make the following array accesses, in order:

Array[1][0]
Array[-1][0]
Array[0][1]
Array[0][-1]
Array[1][1]
Array[-1][-1]
Array[1][-1]
Array[-1][1]

These -1 accesses will go outside the memory defined by Array and may read a random value or may cause a segmentation fault.

In order to fix this, you need to verify that a candidate position is inside the board before you actually access it. Something like:

std::optional<std::string> safe_access(std::string Array[][72], int Pos1, int Pos2) {
  if (Pos1 < 0 || Pos1 >= 71) return {};
  if (Pos2 < 0 || Pos1 >= 21) return {};
  return Array[Pos1][Pos2];
}

You can then call safe_access(Array, -1, -1) == "X" and be guaranteed this will not access outside of Array .