Passing matrix as a parameter

Question

I am new to c, I am using CUDA compiler. I want to declare five matrices, from a[1][1] to a[5][5]. Initialize them with random values. Print these values. I have shown the whole code I wrote. I face problems

Passing matrix as parameter

Initialize a matrix (should be constant)

#include <stdio.h>
#include <stdlib.h>

void printMatrix(int **a, int rows, int cols)
{
    for(int i=0; i<rows; i++){
        for (int j=0; j<cols; j++){
            printf("%d\t", a[i][j] );
        }
        printf("\n");
    }
}

int main() {
    int N;

    for (int n=0; n<=5; n++)
        N = n;
        int a[N][N];

        for(int i=0; i<N; i++)
            for (int j=0; j<N; j++){
                a[i][j] = rand() % 256;
            }

        printf("Matrix A\n");
        printMatrix(a, 10, 10);
    }
}

This works fine if i define N on the top. If I did that, I couldn't change N value using for loop. How can I correct.

"Initialize a matrix (should be constant)" It is not clear what you mean. What should be constant? The array data? The array size? Why do you need the `N` variable to begin with? Also, please post the actual code that compiles. — Lundin, Feb 22 '16 at 08:36

Vlad from Moscow · Answer 1 · 2016-02-22T16:13:00.350

For starters there is a typo in the program. You omitted the open brace after the loop statement

for (int n=0; n<=5; n++)
                        ^^^^
    N = n;
    int a[N][N];
    //...

There should be

for (int n=0; n<=5; n++) {
                        ^^^^
    N = n;
    int a[N][N];
    //...

Array a is a variable length array. Its dimension may not be equal to 0. So the variable n must to start from 1 as it is written in the assignment

for (int n=1; n<=5; n++) {
         ^^^^

This function call

printMatrix(a, 10, 10);
               ^^  ^^

does not make sense because the number 10 has nothing common with the array.

And the function declaration

void printMatrix(int **a, int rows, int cols);
                 ^^^^^^^

is invalid. There is a mismatch between the argument type and the parameter type and there is no implicit conversion from one type to another type.

The program can look like

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void printMatrix( size_t rows, size_t cols, int a[][cols] )
{
    for ( size_t i = 0; i < rows; i++ )
    {
        for ( size_t j = 0; j < cols; j++ ) printf( "%3d ", a[i][j] );
        printf( "\n" );
    }
}

int main( void ) 
{
    const size_t N = 5;
    const int UPPER_VALUE = 256;

    srand( ( unsigned int )time( NULL ) );

    for ( size_t n = 1; n <= N; n++ )
    {
        int a[n][n];

        for ( size_t i = 0; i < n; i++ )
        {
            for ( size_t j = 0; j < n; j++ ) a[i][j] = rand() % UPPER_VALUE;
        }

        printf( "Matrix A[%zu][%zu]:\n", n, n );
        printMatrix( n, n, a );
        printf( "\n" );
    }        

    return 0;
}

Its output might be

Matrix A[1][1]:
117 

Matrix A[2][2]:
 57 216 
 50 233 

Matrix A[3][3]:
 42 117 215 
177 218  26 
202  81 163 

Matrix A[4][4]:
205 178 157   2 
229 165  93  94 
 91 160  39 205 
 26 242 131  69 

Matrix A[5][5]:
147 248 126 107  42 
103 149 160  62  70 
122  89  17 203 252 
222 125 154 224  98 
 63  61 192 155 222

If the compiler does not support variable length arrays then you have to allocate the arrays dynamically. For example

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void printMatrix( int **a, size_t rows, size_t cols )
{
    for ( size_t i = 0; i < rows; i++ )
    {
        for ( size_t j = 0; j < cols; j++ ) printf( "%3d ", a[i][j] );
        printf( "\n" );
    }
}

int main( void ) 
{
    const size_t N = 5;
    const int UPPER_VALUE = 256;

    srand( ( unsigned int )time( NULL ) );

    for ( size_t n = 1; n <= N; n++ )
    {
        int **a = malloc( n * sizeof( int * ) );

        for ( size_t i = 0; i < n; i++ )
        {
            a[i] = malloc( n * sizeof( int ) );
            for ( size_t j = 0; j < n; j++ ) a[i][j] = rand() % UPPER_VALUE;
        }

        printf( "Matrix A[%zu][%zu]:\n", n, n );
        printMatrix( a, n, n );
        printf( "\n" );

        for ( size_t i = 0; i < n; i++ ) free( a[i] );
        free( a );
    }        

    return 0;
}

Any reason why you would do `int a[][cols]` instead of `int a[rows][cols]`? The array parameter decays into an array pointer in either case, but the latter form might give slightly better type safety. — Lundin, Feb 22 '16 at 09:41
@Lundin I made by analogy void f( int a[], size_t n ); :) Of course it can be written as you showed.:) — Vlad from Moscow, Feb 22 '16 at 09:44
@Lundin For example if the function at first would be only declared I write its declaration the following way void printMatrix( size_t, size_t, int [][*] ); — Vlad from Moscow, Feb 22 '16 at 09:52
I guess it is a matter of style. I always name the parameters in the prototype and make sure they are identical to the function definition. — Lundin, Feb 22 '16 at 09:55
@Vlad from Moscow - Thank you, since I am using CUDA C compiler, this is not compiling. I get "a parameter is not allowed" at line 5 and "expression must be a constant value" at line 23 — Saahithyan Vigneswaran, Feb 22 '16 at 15:52
@SaahithyanVigneswaran If the compiler does not support C99 then you should allocate the array dynamically in the loop. — Vlad from Moscow, Feb 22 '16 at 16:07
@Vlad from Moscow still I face compilation error, (a value of type "void *" cannot be used to initialize an entity of type "int **") at line 23 and (a value of type "void *" cannot be assigned to an entity of type "int *") at line 27 — Saahithyan Vigneswaran, Feb 22 '16 at 17:12
@VladfromMoscow and why are you allocating `n`, since it's a matrix shouldn't we allocate n*n ? — Saahithyan Vigneswaran, Feb 22 '16 at 17:29
@SaahithyanVigneswaran In this case you will not be able to apply twice the subscript operator. — Vlad from Moscow, Feb 22 '16 at 18:24
@VladfromMoscow I still face problem to compile the second code — Saahithyan Vigneswaran, Feb 24 '16 at 08:21

score 0 · Answer 2 · answered Feb 22 '16 at 08:34

for (int n=0; n<=5; n++){  /* <--missing { which causes N to be 6 after last iteration */
 //your code
}

You missed a { after first for loop in main that's why int a[N][N] and other loops are not inside its body (which you probably want "changing value of N ")

Selçuk Cihan · Answer 3 · 2016-02-22T09:56:20.063

It seems that the number of matrices is constant, so just #define it at the top. Without touching your printMatrix method, you can have a main body as follows:

#define N 5
int main(int argc, char ** argv)
{
    int **data[N]; // Array of length N of int **, where each int ** will store a matrix.
    for (int i = 0; i < N; i++) {
        int matrixSize = i + 1; // creating the ith matrix
        data[i] = malloc(matrixSize * sizeof *data[i]);
        for (int j = 0; j < matrixSize; j++) {
            data[i][j] = malloc(matrixSize * sizeof *data[i][j]);
            for (int k = 0; k < matrixSize; k++) {
                data[i][j][k] = rand() % 256;
            }
        }
    }
    // Printing the first one
    printMatrix(data[0], 1, 1);

    // don't forget to loop again to free the buffers allocated...
    return 0;
}

Passing matrix as a parameter

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