assigning a 2D array element to single pointer

Question

I am trying to learn C. I am new to C programming. I have the following function.

/*dA would be a pointer to a 2D array*/
    void normalizeCols(float* dMu, float* dSigma, float* dB, float* dA, int n){
       int col, row;
       for(col=0; col < n; col++)
           /*Step 1: calculating mean*/
            float tempMu = 0.0; 
            dMu = &tempMu;
            for (row=0; row < n; row++){
                /*I am adding all the elements of the column*/
                dMu += *(*(dA+row)+col); //ERROR: operand of * must be a pointer
            }
            /*dividing dMu by number of dimension(square matrix)*/              
            dMu /= (float) n; //ERROR: expression must have arithmetic or enum type
            //More code here
       }
}

I am trying to find the mean of a column. I get those two errors that I have commented in the above snippet. How should I fix this?

Do what? What are you trying to do? Are you trying to shift pointer dA? Or are you trying to add something to the value of *dA? There's at least two completely different ways ways to "fix" your code. Nobody knows which one is the correct one until you explain what you are trying to do. — AnT stands with Russia
– AnT stands with Russia, Commented Mar 10, 2014 at 15:51
think he's trying to add all the elements in the 2d array and find the mean. — tesseract
– tesseract, Commented Mar 10, 2014 at 15:53
All other things aside, no single answer is going to breach the inescapable; you need to spend more time in C-pointer tutorials and texts. The last thing you should do when not crystal-clear on their usage is throw more code at the problem. — WhozCraig
– WhozCraig, Commented Mar 10, 2014 at 16:01

unwind · Accepted Answer · 2014-03-10 15:58:01Z

1

If you know that the matrices are square (i.e. the row-length is n which is also the number of rows), just do the addressing manually.

The inner loop then becomes:

       /*Step 1: calculating mean*/
       float tempMu = 0;
       for (row=0; row < n; row++){
           /*I am adding all the elements of the column*/
           tempMu += dA[col * n + row];
       }
       /*dividing dMu by number of dimension(square matrix)*/              
       tempMu /= (float) n;

Also, make the input arguments const to make it clearer, and switch int to size_t.

Of course, make sure you do the accesses in the proper order (either row-major or column-major) otherwise you'll get horrible cache thrashing.

edited Mar 10, 2014 at 15:58

answered Mar 10, 2014 at 15:38

unwind

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Comments

Emmet · Accepted Answer · 2014-03-10 16:05:29Z

1

On this line:

dMu += *(*(dA+row)+col); //ERROR: operand of * must be a pointer

Note that dA has type float*, therefore *(dA+row) is a float, col is promoted to a float in order to be added to this value, which is now in the outermost parentheses. When you dereference that with the leftmost *, you are attempting to dereference a float, which is the source of your error.

In order for that line to be type correct, dA would have to be float**, but you have other errors: dMu here, for example, is a pointer, which you are incrementing with +=, not a value. Did you mean *dMu += ... ?

answered Mar 10, 2014 at 16:05

Emmet

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Comments

Arjun Sreedharan · Accepted Answer · 2014-03-10 16:12:23Z

1

(dA+row) is a pointer, that is moved from dA a distance of row times the size of the type dA points to.

*(dA+row) gives the value of the location pointed by (dA+row) the pointer

*(dA+row)+col adds that value to col

*(*(dA+row)+col) is illegal because you can only de-reference a pointer which this is not.

Your tempMu should be:

tempMu += *(dA + row * n + col)

edited Mar 10, 2014 at 16:12

answered Mar 10, 2014 at 16:04

Arjun Sreedharan

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1 Comment

Emmet Over a year ago

dMu in this case is also a pointer, so that's not correct either.

IWill80 · Accepted Answer · 2014-03-10 16:00:49Z

0

Not very clear what do you want to do. From the code I see that your are trying to make some "dangerous" operations with pointers.

            /I am adding all the elements of the column**/
            dMu += *(*(dA+row)+col);

You are not adding all the elements in the column but you are moving dMU pointer to another memory location.

*dMU += dA[row][col]
....
*dMu /= (float) n;

it should be correct.

answered Mar 10, 2014 at 16:00

IWill80

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1 Comment

Emmet Over a year ago

It never even gets that far because he's doing a double dereference on a single-level pointer. The error message is coming from the leftmost * dereferencing a float in the outermost parentheses.

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