Position 2D array bug as parameter causes memory dumps - c++

Question

This is my program in C++, which accepts an 2D array a[m][n]. If an element a[i][j] is zero, then set all the ith row and jth column elements to zero.
This is code sample:
#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;
class SetZero{
public:
static void setZero(int **, int , int);
};
void SetZero::setZero(int ** a, int m, int n){
int i, j, k;
int ** b = new int *[m]; //flags to identify whether set to zero or not.
for(i = 0; i < m; i++){
b[i] = new int[n];
for(j = 0; j < n; j++)
b[i][j] = 1;
}
for(i = 0; i < m; i++)
for(j = 0; j < n; j++)
if(a[i][j] == 0 && b[i][j]){//DUMP here. If I change it to (a+i)[j], then works.
for (k = 0; k < n; k++){
a[i][k] = 0;//but there is NO dump here. Weird!
b[i][k] = 0;
}
for(k = 0; k < m; k++){
a[k][j] = 0;
b[k][j] = 0;
}
j = n;//break. next row loop.
}
for(int i = 0; i < m; i++)
delete[] b[i];
delete[] b;
}
int main(){
int a[4][5];
srand(time(NULL));
for(int i = 0; i < 4; i++){//create an 2D array
for(int j = 0; j < 5; j++){
a[i][j] = rand() % 100;
cout << a[i][j] << " ";
}
cout << endl;
}
SetZero::setZero((int **)a, 4, 5);//type cast.
cout << endl;
for(int i = 0; i < 4; i++){//print result
for(int j = 0; j < 5; j++)
cout << a[i][j] << " ";
cout << endl;
}
return 0;
}
Environment: WIN8 Visual Studio 2012.
Edit:
The program can compile but cannot execute normally. It will stop when it reaches if(a[i][j] == 0 && b[i][j]){
The error message is:
Unhandled exception at 0x012875DD in CCLC.exe: 0xC0000005: Access
violation reading location 0x0000004B.

Answer 1

SetZero::setZero((int **)a, 4, 5)
a is not an array of pointers, it is simply a 2 dimensional array.
notice how the access violation is reading address 0x0000004B? that's 75, a number between 0 and 99 :) because you are treating a 2 dimensional array (which is just a one dimensional array with a neat way of accessing it) as an array of arrays, it is taking one of the values in your array (75) to be the address of a sub array, then trying to read the non existent array at address 75 (or 0x0000004B)
I suggest that you 'flatten' your arrays and work with them as one dimensional arrays, which I find simpler:
void SetZero::setZero(int * a, int m, int n){
int i, j, k;
int * b = new int [m*n]; //flags to identify whether set to zero or not.
for(i = 0; i < m; i++){
b[i] = new int[n];
for(j = 0; j < n; j++)
b[i*n+j] = 1;
}
for(i = 0; i < m; i++)
for(j = 0; j < n; j++)
if(a[i*n+j] == 0 && b[i*n+j]){//DUMP here. If I change it to (a+i)[j], then works.
for (k = 0; k < n; k++){
a[i*n+k] = 0;//but there is NO dump here. Weird!
b[i*n+k] = 0;
}
for(k = 0; k < m; k++){
a[k*n+j] = 0;
b[k*n+j] = 0;
}
j = n;//break. next row loop.
}
delete[] b;
}
int main(){
int a[4*5];
srand(time(NULL));
for(int i = 0; i < 4; i++){//create an 2D array
for(int j = 0; j < 5; j++){
a[i*5+j] = rand() % 100;
cout << a[i*5+j] << " ";
}
cout << endl;
}
SetZero::setZero(a, 4, 5);//type cast.
cout << endl;
for(int i = 0; i < 4; i++){//print result
for(int j = 0; j < 5; j++)
cout << a[i*5+j] << " ";
cout << endl;
}
return 0;
}

Answer 2

One suggestion about the SetZero(). There is a function called memset() which allows you to set all bytes to a specific value given a starting pointer and the range. This function could make your SetZero() function more cleaner:
void * memset ( void * ptr, int value, size_t num );
Fill block of memory. Sets the first num bytes of the block of memory pointed by ptr to the specified value (interpreted as an unsigned char).
Parameters
ptr: Pointer to the block of memory to fill.
value: Value to be set. The value is passed as an int, but the function fills the block of memory using the unsigned char conversion of this value.
num: Number of bytes to be set to the value, size_t is an unsigned integral type.
For example, the following code block from your program:
for (k = 0; k < n; k++){
a[i][k] = 0;//but there is NO dump here. Weird!
b[i][k] = 0;
}
can be achieved by memset in a cleaner way:
memset(a[i], 0, n * sizeof(int));
memset(b[i], 0, n * sizeof(int));