How to type_cast two dimensional array using pointer?

I want to know whether the typecasting which i did is right or wrong. Through Internet search, i have found out but i am not sure. Please teach and correct me?

Code:

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int Generator_matrix[10][10];
 int (*GM)[10]= Generator_matrix;//Typecasting

 int values[10][10];
 int (*outdata)[10]= values;//Typecasting

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It is not a typecast, but rather an implicit conversion. In most contexts, an array is converted to a pointer to its first element, hence an array of ten arrays of ten int would be converted to a pointer to an array of ten int. Both GM and outdata are pointers to an array of ten ints, so the implicit conversion is correct.

Quote:

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Originally Posted by laserlight

It is not a typecast, but rather an implicit conversion. In most contexts, an array is converted to a pointer to its first element, hence an array of ten arrays of ten int would be converted to a pointer to an array of ten int. Both GM and outdata are pointers to an array of ten ints, so the implicit conversion is correct.

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Thank you for your explanation

Note that

Code:

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int (*GM)[10];

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where GM is a pointer to an array of 10 ints as in laserlight's post - and hence can be initialised to another pointer in int.

But

Code:

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int *GM[10]

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where GM is an array of 10 pointers to int.

These two are not the same but can be confused.

Quote:

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Originally Posted by Thaus

I want to know whether the typecasting which i did is right or wrong. Through Internet search, i have found out but i am not sure. Please teach and correct me?

Code:

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int Generator_matrix[10][10];
 int (*GM)[10]= Generator_matrix;//Typecasting

 int values[10][10];
 int (*outdata)[10]= values;//Typecasting

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This is an example of a typecast that could be relevant here,

Code:

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int* GM = (int*)Generator_matrix; // typecast to int*
//
GM[42] = 13; // access using array syntax

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GM is now a pointer to int and can be thought of as an array GM[100] that is overlaid on Generator_matrix[10][10].

Quote:

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Originally Posted by wolle

This is an example of a typecast that could be relevant here,

Code:

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int* GM = (int*)Generator_matrix; // typecast to int*
//
GM[42] = 13; // access using array syntax

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GM is now a pointer to int and can be thought of as an array GM[100] that is overlaid on Generator_matrix[10][10].

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@wolle

Thanks for your response. As per your note,

Code:

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int *GM = (int*)(Generator_matrix) // typecast to int* , GM is now a pointer to int.

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Actually, Generator_matrix is two-dimensional array i.e. Generator_matrix[10][10]. Then how pointer points to two-dimensional array. Not clear.

Quote:

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Originally Posted by Thaus

Actually, Generator_matrix is two-dimensional array i.e. Generator_matrix[10][10]. Then how pointer points to two-dimensional array. Not clear.

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It's because arrays, one-dimensional as well as multi-dimensional, are laid out linearly in memory one element after the other in a certain known order specified by the C++ standard.

The name of an array is really just a pointer to the first element. It means this will also work for example,

Code:

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int* GM = &Generator_matrix[0][0]; // GM is pointing at the first element of Generator_matrix.

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The above assigns the address of the first element of Generator_matrix to GM which then can be viewed as an array GM[100] that is laid out on top of the array Generator_matrix[10][10] (just like with the type-cast version in my post #5).

Quote:

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Originally Posted by wolle

It's because arrays, one-dimensional as well as multi-dimensional, are laid out linearly in memory one element after the other in a certain known order specified by the C++ standard.

The name of an array is really just a pointer to the first element. It means this will also work for example,

Code:

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int* GM = &Generator_matrix[0][0]; // GM is now pointing at the first element of Generator_matrix.

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The above assigns the address of the first element of Generator_matrix to GM which is a pointer to int and now holds this address.

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Now, i am clear. Thanks

Quote:

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Originally Posted by Thaus

Now, i am clear. Thanks

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Note that this is low-level stuff that is handy on occasion but something you shouldn't do all the time in general code.

I tried with simple example. I need to print H_Matrix value but instead its printing

enter the matrix
-2064509760-2064509736-2064509712-2064509688-2064509664-2064509640
-2064509688-2064509664-2064509640-2064509616-2064509592-2064509568
-2064509616-2064509592-2064509568-2064509544-2064509520-2064509496

Code:

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#include <stdio.h>

int main()
{
    // coding for the general form beginning using rows and cols
    int i,j;
   
    int H_Matrix[3][6]={{1,1,0,0,1,0},{1,0,0,1,0,1},{1,1,1,0,0,1}};
    int (*GM)[3][6]=&H_Matrix;

    printf("enter the matrix");
    for(i=0;i<3;i++)
    {
        for(j=0;j<6;j++)
        {
        printf("%d",*(*(GM+ i) + j));
        }
      printf("\n");
    }
    return 0;
    }

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What wrong ?

Now its working

int main()

Code:

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{
    // coding for the general form beginning using rows and cols
    int i,j;
   
    int H_Matrix[3][6]={{1,1,0,0,1,0},{1,0,0,1,0,1},{1,1,1,0,0,1}};
    int (*GM)[6]=H_Matrix;

    printf("enter the matrix");
    for(i=0;i<3;i++)
    {
        for(j=0;j<6;j++)
        {
        printf("%d",*(*(GM+ i) + j));
        }
      printf("\n");
    }
    return 0;
    }

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Now, GM is a pointer to an array of 3 arrays of 6 ints. That is, the expression (GM + i) when i == 1 results in a pointer to the next array of 3 arrays of 6 ints... but you only have one such array! If you had an array of N arrays of 3 arrays of 6 ints, then having GM be a pointer to an array of 3 arrays of 6 ints would make more sense.

I am working with XSDK for hardware implementation but all deals with C/C++.

Expected Result: [0 1 1 1 0 0
1 1 0 0 1 0
1 1 1 0 0 1 ]

But if you notice tera console, its showing incorrect answer. I don`t offset and pointer declaration is right or wrong?

Code:

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int main()
{
init_platform();
 //u32 *Results;
 int i,j,k,m;
 int Generator_matrix[3][6];
 int (*GM)[6]=Generator_matrix;
XLdpc_encoding LDPC;
XLdpc_encoding  *LDPCPTR=&LDPC;
XLdpc_encoding_Initialize(LDPCPTR,XPAR_LDPC_ENCODING_0_DEVICE_ID);

 print("\r\n---FOR HW---\r\n");

 XLdpc_encoding_EnableAutoRestart(LDPCPTR);

 XLdpc_encoding_Start(LDPCPTR);
 while (!XLdpc_encoding_IsDone(LDPCPTR));
for(i=0;i<3;i++)
 {

        for(j=0;j<6;j++)
          {

//while (!XLdpc_encoding_IsDone(LDPCPTR));
XLdpc_encoding_Read_Generator_Words(LDPCPTR, 0, GM, 6);  /// XLdpc_encoding_Read_Generator_Words(LDPCPTR, int offset, int *data, int length);
xil_printf("%d\t", *(*(GM+ i) + j));
            }
            printf("\n");
        }

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Attachment 34889

You should indent your code properly, e.g.,

Code:

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int main()
{
    init_platform();
    //u32 *Results;
    int i,j,k,m;
    int Generator_matrix[3][6];
    int (*GM)[6] = Generator_matrix;
    XLdpc_encoding LDPC;
    XLdpc_encoding  *LDPCPTR=&LDPC;
    XLdpc_encoding_Initialize(LDPCPTR,XPAR_LDPC_ENCODING_0_DEVICE_ID);

    print("\r\n---FOR HW---\r\n");

    XLdpc_encoding_EnableAutoRestart(LDPCPTR);

    XLdpc_encoding_Start(LDPCPTR);
    while (!XLdpc_encoding_IsDone(LDPCPTR));
    for (i = 0; i < 3; i++)
    {
        for (j = 0; j < 6; j++)
        {
            //while (!XLdpc_encoding_IsDone(LDPCPTR));
            XLdpc_encoding_Read_Generator_Words(LDPCPTR, 0, GM, 6);  /// XLdpc_encoding_Read_Generator_Words(LDPCPTR, int offset, int *data, int length);
            xil_printf("%d\t", *(*(GM + i) + j));
        }
        printf("\n");
    }

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Next, change this:

Code:

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XLdpc_encoding_Read_Generator_Words(LDPCPTR, 0, GM, 6);
xil_printf("%d\t", *(*(GM + i) + j));

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to:

Code:

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XLdpc_encoding_Read_Generator_Words(LDPCPTR, 0, Generator_matrix, 6);
xil_printf("%d\t", Generator_matrix[i][j]);

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Do you get the expected output? If not, then the error may lie with populating Generator_matrix.

No, i didn`t get expected result. Let`s will try and update you.