Dynamic Allocation - user inputs elements without knowing the array size before runtime

Question

so at run time we don't know the size of the array(matrix) and I want the user to input the elements of the array(matrix). is this the proper way to do it?Also did i return the pointer to the array properly?

#define MAX_DIM 10
int main(void)
{
    int done = 0;
    int rows, cols;
    float *dataMatrix;

    while (!done)
    {
    // Prompt user to enter row and column dimensions of matrix (must be > 0)
    do
    {
        printf("Enter row dimension (must be between 1 and %d): ", MAX_DIM);
        scanf("%d", &rows);

    } while(rows <= 0 || rows > MAX_DIM);
    do
    {
         printf("Enter column dimension (must be between 1 and %d): ", MAX_DIM);
         scanf("%d", &cols);
    } while(cols <= 0 || cols > MAX_DIM);

    dataMatrix = readMatrix(rows, cols);
    if (dataMatrix == NULL)
    {
        printf ("Program terminated due to dynamic memory allocation failure\n");
        return (0);
    }


float *matrix(int numRows, int numCols)    
{    
    int i=0;
    float **m= NULL;
    m=malloc(numRows*sizeof(float*));
    if(m==NULL)
    {
       printf("error\n");
       exit(1);
    }
    for(i=0;i<numRows;i++)
    {
       m[i]=malloc(numCols*sizeof(float));
    }
    if(m[i-1]==NULL)
    {
       printf("error\n");
       exit(1);
    }
    printf("Enter values for the matrix: ");
    scanf("%f",m[i]);
    return m[i];
}

R Sahu · Accepted Answer · 2016-07-02 04:24:36Z

is this the proper way to do it?

You are heading in the right direction but not fully there.

Also did i return the pointer to the array properly?

No.

You can allocate memory for a matrix using one of two methods.

Allocate numRows numbers of float*s. For each row, allocate numCols of floats and then return a pointer to the array of float*s. This is what you tried but you didn't do everything right.

Take a closer look at the modified code for reading user data and the return statement.

float **matrix(int numRows, int numCols)    
{    
    int i=0;
    float **m = malloc(numRows*sizeof(float*));
    if(m == NULL)
    {
       printf("error\n");
       exit(1);
    }

    for(i=0; i<numRows; i++)
    {
       m[i] = malloc(numCols*sizeof(float));
       if(m[i] == NULL)
       {
          printf("error\n");
          exit(1);
       }
    }

    printf("Enter values for the matrix: ");
    for (i = 0; i < numRows; ++i )
    {
       for (int j = 0; j < numCols; ++j)
       {
          scanf("%f", &m[i][j]);
       }
    }
    return m;
}

Allocate numRows*numCols of floats. Treat the 1D array as holder of 2D data. Use appropriate offsets to treat the 1D array as a 2D array. Return a pointer to the array of floats.

float *matrix(int numRows, int numCols)    
{    
    int i=0;
    float *m = malloc(numRows*numCols*sizeof(float));
    if(m==NULL)
    {
       printf("error\n");
       exit(1);
    }
    printf("Enter values for the matrix: ");
    for (i = 0; i < numRows; ++i)
    {
       for (int j = 0; j < numCols; ++j)
       {
          int index = i*numCols+j;
          scanf("%f", &m[index]);
       }
    }
    return m;
}

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