dynamic two dimensional array to static array

Question

any body knows how to convert 2d dynamic array to static so that i can use it in lapacke. dgels function which only take static matrix in c? when i use malloc it does not give correct answer. how can i use malloc so that it works with it.thankyou

#include <stdio.h>
#include <lapacke.h>
#include <conio.h>

int main (int argc, const char * argv[])  
{
    /*double a[5][3] = {1,1,1,2,3,4,3,5,2,4,2,5,5,4,3};*/

    double b[5][2] = {-10,-3,12,14,14,12,16,16,18,16};
    lapack_int info,m,n,lda,ldb,nrhs;
    int i,j;
    double **a;

    a=(double**)malloc(5* sizeof(double*));
    for (i=0;i<5;i++)
    {
        a[i]=(double*)malloc(3* sizeof(double));
    }
    a[0][0]=1; 
    a[0][1]=1;
    a[0][2]=1;
    a[1][0]=2;
    a[1][1]=3;
    a[1][2]=4;
    a[2][0]=3;
    a[2][1]=5;
    a[2][2]=2;
    a[3][0]=4;
    a[3][1]=2;
    a[3][2]=5;
    a[4][0]=5; 
    a[4][1]=4;
    a[4][2]=3;

    m = 5;
    n = 3;
    nrhs = 2;
    lda = 3;
    ldb = 2;

    info = LAPACKE_dgels(LAPACK_ROW_MAJOR,'N',m,n,nrhs,*a,lda,*b,ldb);

    for(i=0;i<n;i++)
    {
        for(j=0;j<nrhs;j++)
        {
            printf("%lf ",b[i][j]);
        }
        printf("\n");
    }
    getch();
    return(info);
}

score 3 · Answer 1 · answered Apr 18 '14 at 15:01

3

I do not know lapacke.dgels but try to change:

double **a;
a=(double**)malloc(5* sizeof(double*));
for (i=0;i<5;i++)
{
    a[i]=(double*)malloc(3* sizeof(double));
}

to:

double (*a)[3];
a = malloc(5 * 3 * sizeof(double));

answered Apr 18 '14 at 15:01

Marian

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2
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1

Exactly, `dgels` needs a contiguous memory area. Intel has some examples [dgels](https://software.intel.com/sites/products/documentation/doclib/mkl_sa/11/mkl_lapack_examples/lapacke_dgels_row.c.htm). – Timothy Brown Apr 18 '14 at 15:03
That's clever, I didn't think of how to get the compiler to do the index calculations for you. – pat Apr 18 '14 at 15:06
@marian: your program works: but in my real program which is big: this initialization of double (*a)[3]; is in the middle of the program: so i have to use specific [designs]. instead of it[3]. so than how should i write malloc? – user3546028 Apr 18 '14 at 16:30
@user3546028 Your malloc shall allocate whole array as one block of memory. You will need to allocate total size at once with `malloc(lines * columns * sizeof(element))`. – Marian Apr 19 '14 at 05:53

pat · Accepted Answer · 2014-04-18T19:38:46.013

a is not a 2d array, it is an array of pointers to separate 1d arrays. Passing *a to LAPACKE_dgels only gives it a pointer to the first row. It will have no way to know where all of the other rows were allocated since they were allocated independently. It wants the entire array to be in a single contiguous block of memory. a must be of type double*, not double**, and you don't dereference it when passing it. You must flatten the 2d indexes into 1d indexes yourself, using either row or column major form (which you tell the function).

EDIT

The following code allocates a flat 1d array with room for m*n doubles. It then fills the array by converting the 2d indices to 1d row-major indices using the formula row * n + col. If we wanted column-major indices, we would use col * m + row.

#include <stdio.h>
#include <lapacke.h>
#include <conio.h>

int main (int argc, const char * argv[])  
{
    double b[5][2] = {-10,-3,12,14,14,12,16,16,18,16};
    lapack_int info,m,n,lda,ldb,nrhs;
    int i,j;
    double *a;

    m = 5;
    n = 3;
    nrhs = 2;
    lda = 3;
    ldb = 2;

    a = malloc(m * n * sizeof(double));
    a[0 * n + 0] = 1;
    a[0 * n + 1] = 1;
    a[0 * n + 2] = 1;
    a[1 * n + 0] = 2;
    a[1 * n + 1] = 3;
    a[1 * n + 2] = 4;
    a[2 * n + 0] = 3;
    a[2 * n + 1] = 5;
    a[2 * n + 2] = 2;
    a[3 * n + 0] = 4;
    a[3 * n + 1] = 2;
    a[3 * n + 2] = 5;
    a[4 * n + 0] = 5; 
    a[4 * n + 1] = 4;
    a[4 * n + 2] = 3;

    info = LAPACKE_dgels(LAPACK_ROW_MAJOR,'N',m,n,nrhs,a,lda,*b,ldb);

    for(i=0;i<n;i++)
    {
        for(j=0;j<nrhs;j++)
        {
            printf("%lf ",b[i][j]);
        }
        printf("\n");
    }
    getch();
    return(info);
}

how can i a flatten them to 1d array using row or column major for? i am not aware of this funtions? can you give the example by taking malloc from above. because i am working in 2d array malloc from the beginning and than i have to create 1d malloc. i dont have any idea: thanx — user3546028, Apr 18 '14 at 16:33

dynamic two dimensional array to static array

2 Answers2