How do we select the column of a matrix?

[Fiori]

Hello!

I am new to MKL and to C in general. Lets say that we have two matrices A, B of size (n,k). We would like for j =1 until k to subtrack the j column of A from B. In Matlab language the code is: for j=1:k, B(:,j)-A(:,j), end and in R language for(j=1 in k), { B[,j]-A[,j] }. I would like to ask if there is a routine to select the j column (or row) of a matrix? I searched but I had no luck.

Thank you very much.

[mecej4]

Why do you describe the operation in a seemingly more complicated way than just "A =  A - B"? MKL contains BLAS routines for performing matrix-matrix and vector operations. In particular, look at ?axpy routines. For such a simple operation as subtraction, you may also consider using the features that the programming language itself provides, such as those provided by BOOST.

[Fiori]

Thank you for your help. I am confused with this language. I have written a program in Matlab and I want to convert it to C language using the MKL.

The  part of my code is the following

for t=1:T

c1=0; c2=0;

for j=1:p

c1=c1+A(:,:,j) * ( X(:,t-j)-B(:,t-j));

end

end

I haven't known to use the library well yet.  Thanks again.

[Ying_H_Intel · Accepted solution]

Hi GF,

What is the reason of you change the matlab or R to C? and What is your problem size (matrix)?

You may have known, you can use R or matlab directly, and get MKL performance benifit at the same time,  because MKL can be intergarted into  R and Matlab, please see the article  http://software.intel.com/en-us/articles/using-intel-mkl-with-r

So first, i may suggest you to use matlab or R directy. 

Secondly, MKL mainly focus on the computation and the interfaces are mainly based on some kind of standard interface, like BLAS ( matrix, vector operation). So it is not flexiable like Matlab and R.

It hasn't single routines like data operations like extract columns from a Matrix.  But these operations are embeded in the computation.

for example, If you have to convert the matlab code to C/MKL code.  You may first convert them to C code, then call MKL function.

for example, X(:, t-j) - B(:, t-j) , as mecej4 mentioned to use daxpy.

double * Ypointer=&(X[0][t-j]);

double * Xpointer=&(B[0][t-j]);

IncX= number of columns of B

IncY =  number of columns of A;

N= row of column of A;

cblas_daxpy ( N, -1.0, Xpointer, incX, Ypointer,incY);

Then same as for A(:, :, J) * Ypointer.  If A can access by C pointer with regular pattern by  (expressed by incX and lda)

Call cblas_dgemv (const CBLAS_ORDER order, const CBLAS_TRANSPOSE TransA, const MKL_INT
M, const MKL_INT N, const double alpha, const double *A, const MKL_INT lda, const
double *X, const MKL_INT incX, Ypointer, 1).

Last, if you are using Intel C Compiler,  Array notication also is helpful to convert your matlab to C code.

Please See  http://software.intel.com/en-us/articles/about-intel-cilk-plus-and-how-to-get-started.
You can write A[:] - refers the entire array in C code.

Best Regards,

Ying