Values in an array (which is an attribute of the object) replaced with garbage after returning the reference

Question

I'm trying to implement sparse matrix addition(code attached), as c++ doesn't like 2d dynamic arrays I am using three different array each to represent RowIndex, ColumnIndex, and the corresponding values.

say i added A and B to get C.

C = A.add(B)

in the add member function, I am returning the address to the newly created C matrix.

Everything works fine before returning C, as A->C has expected values inside the arrays, but once I store the C in another identifier in the main function and then print the same array, via new object I find garbage in some of the arrays.

what i've tried :

• At first I was creating the object inside the Add function and then returning the object, tried it with/without the new keyword.
• I thought it might be a problem with the scope so, Now I am using an attribute of matrix A to instantiate the new C matrix, to store the newly-created matrix and then returning the address to it.

Debugging :

• The arrays are located at the same address before and after the .add() function's use still different values are printed.

class sparse{
        public:
            int rows , cols, len ; 
            int arr;
            int *rindex, *cindex, *vals;
            sparse *c; 

            sparse(){};
            sparse( int r, int c, int nzs ,  int *rn ,  int *cn, int *values){
                this->rows = r;
                this->cols = c;
                this->rindex = rn;
                this->cindex = cn;
                this->vals   = values;
                this->len    = nzs;
            }
        sparse* add(sparse b){
            int ap=0  , bp =0, cp =0; // pointers that help with merging in A,B,C matrices
            int crindex[this->len+ b.len], 
                ccindex[this->len+ b.len], 
                cvals[this->len+ b.len];
            int crows = this->rows, ccols  = this->cols;  

            int cnzs = cp; //non-zero values in C
            int crf[cnzs], ccf[cnzs], cvf[cnzs];

             //merge sort approach to add two sparse matrices


            for (int i=0; i< cp ; i++){ // to redcuce size individual arrays
                crf[i] = crindex[i];
                ccf[i] = ccindex[i]; 
                cvf[i] = cvals[i];
            }

            this->c =  new sparse(crows, ccols, cnzs, crf, ccf, cvf );
            this->c->print();

            // debugging statements
            printf("\narray before recieving: ");
            for (int i =0 ; i<this->c->len; i++ ){
                printf( "%d ", this->c->rindex[i]  );
            }
            printf("\n address :%d \n", this->c->rindex);
            return c;
        }



        void print( ){
            printf( "\nRow | column | value");
            for (int i =0 ; i<this->len; i++ ){
                printf( "\n %d \t %d \t %d", this->rindex[i], this->cindex[i], this->vals[i] );
            }
        }
};


int main(){
    int ars= 20, acs= 15, anzs= 5, bnzs = 5  ;
             //      .
    int ar [anzs] = { 0,0,0,4,7};
    int ac [anzs] = { 0,1,7,1,0};
    int av [anzs] = { 11,11,11,11,11,};
                 //  .
    int br [5] = { 0,1,3,7, 7};
    int bc [5] = { 0,0,5,9,12 };
    int bv [5] = { 22,22,22,22,22};

    sparse a(ars, acs, anzs, ar, ac, av  );
    sparse b(ars, acs,bnzs , br, bc, bv );

    sparse* c = a.add(b);
    c->print();

    printf("\narray after recieving: ");


    for (int i =0 ; i<c->len; i++ ){
        printf( "%d ",c->rindex[i] );
    }
    printf("\naddress : %d", c->rindex);
    return 0;
    }

Here's the output: 

Row | column | value
 0       0       33
 0       1       11
 0       7       11
 1       0       22
 3       5       22
 4       1       11
 7       0       11
 7       9       22
 7       12      22
array before recieving: 0 0 0 1 3 4 7 7 7
 address :6421672

Row | column | value
 -433312354      0       33
 1       1       11
 1996637728      7       11
 0       0       22
 1       5       22
 12      1       11
 0       0       11
 6421672         9       22
 7       12      22
array after recieving: -433312354 1 1996637728 0 1 13 0 6421672 7
address : 6421672 

Answer 1

The problem is in the add() method. You are creating arrays on the stack and passing their addresses to the new sparse instance. After returning from add() these arrays are no longer valid. You want to allocate those arrays on the heap using the new-operator: int * crindex = new int[this->len+ b.len]. But then you also have to free those arrays (with delete[] at some point or you will leak memory).

This is a common mistake when starting with c++. You might want to read more about "stack vs heap allocations". Try for example this question on SO: Stack Memory vs Heap Memory

If you are new to c++ I recommend using std::vector over plain arrays as they are less error prone. So your class would look like this:

#include<vector>
class sparse{
        public:
            //int rows , cols, len ; <-- use rindex.size() instead
            //int arr;
            std::vector<int> rindex, cindex, vals;
}