C++ 2D dynamic array allocation - c++

Question

I have a float** array that contains num_rows rows and num_cols columns. I'd like to determine the number of occurrences of every number between 0-9 columnwise. To do this, I thought of using another 2D array of size [10][num_cols], so that for each column the number corresponding to an element is the number of occurrences of that number in the original table.
Example: if the original table contains 1 2 3 1 1 in the fifth column, then in the second column, the values should be like: 1-> 3, 2 -> 1, 3 -> 1.
I tried using the function as follows, but it gives me a pointer error. I tried using vectors but that too brings no luck.
int ** attribute_count(float * * input, int row_num, int col_num) {
int ** arr_2 = new int * [10];
int * arr = new int[10 * col_num];
int counter = 0;
for (int i = 0; i < 9; i++) {
for (int j = 0; j < col_num; j++) {
arr_2[i][j] = 0;
}
}
for (int i = 0; i < 9; i++) {
for (int j = 0; j < col_num; j++) {
int temp = input[i][j];
arr_2[temp][j]++;
}
}
return arr_2;
}
EDIT:
I tried your suggestions. The new code is:
int** attribute_count(float** input, int row_num, int col_num) {
int** arr_2 = new int* [10];
int* arr = new int[10 * col_num];
int counter = 0;
for (int i = 0; i < 11; i++) {
for (int j = 0; j < col_num; j++) {
arr_2[i] = new int[col_num];
}
}
for (int i = 0; i < 11; i++) {
for (int j = 0; j < col_num; j++) {
int temp = input[i][j];
arr_2[temp][j]++;
}
}
return arr_2;
}
This still gives me memory errors. The function is being called in the .cpp like this:
int** attr = attribute_count(training_data, 10, num_cols_train);
cout<<attr[5][1];
Any idea what I'm doing wrong even now?

Answer 1

I think your problem is in incorrect allocation of the 2D array. Try
int ** arr_2 = new int* [row_num];
for (int i = 0; i < row_num; i++)
arr_2[i] = new int[col_num];

Answer 2

You've only allocated one dimension of arr_2. You need to loop through and allocate an array of ints on each one to get the 2nd dimension.
EDIT: Also, what's up with arr? You allocate it, never use it, don't return it, and don't deallocate it. That's how we spell memory leak.

Answer 3

arr_2 is defined and allocated as an array of pointers to int, but you don't actually assign/allocate those pointers.
Here's a stab at correcting your code - however I'm not convinced you have rows and columns the right way around...
int ** attribute_count(float ** input, int row_num, int col_num)
{
int ** arr_2 = new int * [10];
for (int i = 0; i < 10; i++)
{
arr_2[i] = new int[col_num];
for(int j = 0 ; j < col_num ; j++)
{
arr_2[i][j] = 0;
}
}
for (int i = 0; i < row_num; i++)
{
for (int j = 0; j < col_num; j++)
{
int temp = input[i][j];
arr_2[temp][j]++;
}
}
return arr_2;
}