C++ 2D dynamic array allocation

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?

What is the exact statement of the "pointer error"? You are not allocating memory to each index in 2D array. — Coding Mash
– Coding Mash, Commented Dec 5, 2012 at 17:28

Coding Mash · Accepted Answer · 2012-12-05 17:35:42Z

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];

edited Dec 5, 2012 at 17:35

Coding Mash

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answered Dec 5, 2012 at 17:33

prazuber

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5 Comments

Nathan822 Over a year ago

<code>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; }

Nathan822 Over a year ago

Tried, still troubling. Please see the edit I made in the main question.

prazuber Over a year ago

@Nathan822 Your indexing is very confusing. row_num is usually used to represent the number of rows, and col_num is used to represent the number of columns. While in your code you use 10 for columns number and col_num for rows. Also, the arr variable seems to be unused.

Nathan822 Over a year ago

I am using 10 for row_num, since I want to keep track of the 10 numbers.

prazuber Over a year ago

@Nathan822 What do you mean 'keep track of the 10 numbers'? In your code you create 2D array arr_2 with 10 columns and col_num rows. Also, you create 1D array named arr with 10 * col_num cells. Note, you don't use arr in your function. What is it's purpose?

Donnie · Accepted Answer · 2012-12-05 18:01:50Z

1

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.

edited Dec 5, 2012 at 18:01

answered Dec 5, 2012 at 17:32

Donnie

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1 Comment

Nathan822 Over a year ago

Tried, still troubling. Please see the edit I made in the main question.

JasonD · Accepted Answer · 2012-12-05 20:45:50Z

1

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;
}

edited Dec 5, 2012 at 20:45

answered Dec 5, 2012 at 17:30

JasonD

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3 Comments

Nathan822 Over a year ago

Tried, still troubling. Please see the edit I made in the main question.

JasonD Over a year ago

Your change is not for the better. You now both allocate, and loop, too much.

Nathan822 Over a year ago

Can you write the correct code for this? I am new to this kind of programming and would really appreciate your help!

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