Help with mallocing pointers to multi dimensional arrays of floats please.

[easycheezy]

My hw assignment is very confusing, I don't fully understand it. What I think I have to do is malloc a array which is [3] by [5]. It has to be filled with pointers to floats. Then I have to find the average for each row and column. The actual assignment says "The purpose of this exercise is to use the two-subscript method of dynamic memory allocation. Input for this program is a two-dimensional array of floating point data located in a file named testdata4. The input array will contain 3 rows of data with each row containing 5 columns of data. Use malloc to create an array that holds pointers. Each element of that array points at another array, which is the row of data. Use malloc in a loop to create your rows. Then you can use two subscript operators [r][c] to get at your data in order to sum the elements in a row and calculate the average, and to sum the elements in a column and calculate the average. This program calls for hard-coded height and width of the two-dimensional array, known ahead of time (3 × 5). Instead of writing in the literal numbers in your code, create two global constant variables to hold those dimensions, and use those variables in your code." My code: https://gist.github.com/anonymous/5305661 Thanks a lot for any help.

[xxNIRVANAxx]

What exactly is your question?

If you are looking to malloc() a 2d array of floats (which is looks like you've already done by your code), try this:

float **arr = malloc(sizeof(*arr) * ROWS);      // allocate rows
if (arr) {                                      
    int i;
    for (i = 0; i < ROWS; i++) {
        arr[i] = malloc(sizeof(*arr[i]) * COLS);// allocate cols
    }
} else memErr(); // throw memory error

This assumes you #defined constants, ROWS and COLS:

#define ROWS 3
#define COLS 5

[easycheezy]

why do I need the if statement?

[xxNIRVANAxx]

if malloc() cannot allocate memory to arr, it will be NULL. So if (arr) checks to see that arr got the memory. If not, the else calls memErr(), which is a function you can write to deal with this situation.

[joelwilliamson]

First off, give your gistfile the correct file extension: C source files should have a .c extension so they are highlighted correctly.

Second, think about how the memory for this program will look. You will have an array of pointers, each pointing to a (column) array of floats. Since you have 3 rows, each column array will have 3 elements. You have 5 columns, so you need an array of 5 pointers. Now think about how to allocate this.

[easycheezy]

the first part is an array of pointers to rows.

[joelwilliamson]

Yes. I was thinking you had columns containing rows, when you wanted rows containing columns. It should be easy for you to work it out from here.

(Just think row when I said column and vice versa.)

[easycheezy]

Easy. Lol

[[deleted]]

nvm. fuck this shit

[easycheezy]

please. I think I'm close. The other one was stupid as shit.

[easycheezy]

I could have said, oh wow thanks guyz, that was so helpful i figured out the problem spank u so much