Thread: warnings- dynamic memory allocation - malloc

I have huge problem with pointers from few days, i have written the transpose function in two ways, the one just below the main with malloc never works and the one without malloc in quotes works perfect, i have to write a program using malloc, please can any one help me.

Code:

//the .h file is as follows
int size_b_row1=4;
int size_b_col1= 4;
void transpose();
//void transpose(double matrix[size_b_row1][size_b_col1], double matrix_t[size_b_col1][size_b_row1]);


//the .c file is as follows 
int main() 
{
    double C[4][4] = {{1,3,5,2},{7,6,2,2},{1,2,7,3},{2,3,5,3}};
    double TC[4][4];


    transpose(C, TC);

    printf("%f", TC[3][3]);
    puts("!!!Hello World!!!"); /* prints !!!Hello World!!! */
    return EXIT_SUCCESS;
}



void transpose()
{
    int i,j,k;
    double **matrix;
    double **matrix_t;


        matrix=(double **) malloc(sizeof(double *)*size_b_row1);
        for( k=0;k<size_b_row1;k++)
        {
            matrix[k]=(double *) malloc(sizeof(double*)*size_b_col1);
        }
        matrix_t=(double **) malloc(sizeof(double *)*size_b_row1);
        for( k=0;k<size_b_row1;k++)
        {
            matrix_t[k]=(double *) malloc(sizeof(double*)*size_b_col1);
        }


    for(i=0;i<size_b_row1;i++)
    {
        for(j=0; j<size_b_col1;j++)
            matrix_t[j][i] = matrix[i][j];
    }
  
/*

void transpose(double matrix[size_b_row1][size_b_col1], double matrix_t[size_b_col1][size_b_row1])
{
    int i,j;
    for(i=0;i<size_b_row1;i++)
    {
        for(j=0; j<size_b_col1;j++)
            matrix_t[j][i] = matrix[i][j];
    }
}
*/

Actually, neither of your functions will work if size_b_col1 is not equal to size_b_row1. You also need to make up your mind whether you want to pass data to and from your functions using function arguments, or using global variables. Global variables are generally not considered a good idea.

But, to answer your question ....

At no point in the (uncommented) function are any values assigned to elements of matrix. Transposing an uninitialised array gives an uninitialised array. You have have no code that copies C (from main()) to matrix (in transpose()), nor do you have any code that copies matrix_t (in transpose()) back to TC (in main()). In contrast, your commented function is modifying the arrays in main() directly (since they are passed as arguments).

The type conversions (double *) or (double **) are not required on a malloc() call and, in fact, it is a bad idea to use them. Make sure you have done #include <stdlib.h> if you are using malloc().

Why would you think it makes sense to allocate "matrix" inside the transpose function?
Where the heck is the data supposed to come from if the array to hold it doesn't even exist until transpose is called?
How do you think it is going to be useful to pass arguments to a function that specifies that it does not specify the need for any arguments?

Hint: lines 11 and 12 are wrong.

Originally Posted by raviraj123

this is just a part of my code , i dont think so you really understood the problem, i have to write it like this because i was told to write like this. i found an another way by using malloc in it ,the code is in my first thread-post , have a look at it it just returns zero, if you think the lines 11 and 12 are wrong copy paste the code in your compiler and run it you will get the out put as 3.00000 , and now coming to the point i was asking the question about malloc not the one which runs perfectly.

No, I have not misunderstood anything.
No I did not say lines 11 and 12 would not compile. Of course they will compile. The problem is that you can't do it that way if you are meant to use malloc to allocate these using the array of arrays technique.

If my questions didn't give you a serious wake-up call as to what kinds of things you have very wrong, go back and ponder on them again until you understand your mistakes. Then post some updated code.