Thread: Some help needed with structures and pointer

Hello Guys and Gals,

I am a new to the forum and haven't used C beyond basic programming before, hence I am still learning and need some help with a particular problem. I am trying to create a data structure that will be partially dimensioned at run time, hence using pointers instead of arrays or malloc functions.

The data structure needs two dimensions, i.e. rows and columns.
The number of columns are defined at run time but the number of rows are not, and as the program runs the number of rows will grow and be added to with sample values.
I am compiling C using the Microchip C30 and MPLAB IDE for PIC microcontrollers. I would be very grateful if you could advise on the method for achieving this, since I run into various compile errors such as pointer not declared, error before = etc.

The sort of code I am using is below:

Code:

//structure declarations


//rows of this struct that can grow
typedef struct rowStruct{             
int index;
int value;
}row,*rowPtr;

//columns 
typedef int column, *columnPtr;               

//combines row and column
struct{                                               
row *rowPtr;
column *columnPtr;
} dataSet, *dataSetPtr


void structInit()
{

int i=1;
//initialise the first row values for each column
   while (dataSetPtr->channel<=16)  //16 is actually an ext variable
   {
          dataSetPtr->channel=i;
         rowPtr=&(dataSetPtr->row);//get syntax error before '=' here.
         rowPtr->Index=0;
         rowPtr->value=0;
   
        i++;
     }
}
//end

As said I can't get this to compile let alone test the two dimensionality that I need, and there may be much better ways of doing this, any help would be much appreciated!!

What is the data that you want to put into each element of your 2D array?
I see this:

Code:

typedef struct rowStruct{
    int index;
    int value;
} row, *rowPtr;

Is that your data? index and value? Or do you have "index" there as part of your attempt to implement the array?

I find the code you've posted impossible to understand. It's possible that you have a completely misunderstood how to accomplish what you want to do. My question: "is index a data value or just part of your scheme to implement the array"? If it's part of your implementation plan then there's probably a vastly simpler way to do it (if you could even do it the way you're imagining).

Anyway, assuming that both index and value are actual data, this kind of thing would usually be done something like this:

Code:

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

#define NUMBER_OF_COLUMNS        10
#define INITIAL_NUMBER_OF_ROWS  100

struct Element {
    int index;
    int value;
};

// ROWTYPE is a ptr to an array of NUMBER_OF_COLUMNS Elements.
typedef struct Element (* ROWTYPE )[NUMBER_OF_COLUMNS];

int main(void) {
    int size = 0;
    int used = 0;
    int column;
    ROWTYPE a = NULL;

    while (used < 1000) {
        if (used >= size) {
            size += INITIAL_NUMBER_OF_ROWS;
            a = realloc(a, size * sizeof *a);
            if (!a) {
                fprintf(stderr, "realloc failed\n");
                exit(1);
            }
        }
        for (column = 0; column < NUMBER_OF_COLUMNS; column++) {
            a[used][column].index = 1;
            a[used][column].value = 2;
        }
        used++;
    }

    return 0;
}