passing pointer argument

**timhxf** · 01-08-2007

hey, it is me again. I have another question in my mind, ok, this is the code:

Code:

#include<stdio.h>
#define max 15
void average5(double a[]);
int main(void)
{
    
    static double rain[3][5]={
    {1,1,1,1,1},
    {2,2,2,2,2},
    {3,3,3,3,3}};
    average5(rain);
    
    system("pause");
    return 0;
}
void average5(double a[])
{
     int a1;
     double subtot;
     for(a1=0;a1<15;a1++)
      {
       if(a1%5==0)
        subtot=0;
       subtot+=*(a+a1);
       if(a1%5==4)
        printf("the 5 average is %.1lf ",subtot/5);
      
      }
}

I pass the array name "rain" to average5() which takes a double pointer argument. The compiler only told me this is an imcompatible pointer type, but the program itself still can run and showing the correct result. any idea about this?

**Happy_Reaper** · 01-08-2007

What exactly does the compiler say ?

Also, I don't think your program does exactly what you expect it to. But then again, I don't quite know what you expect it to do either...

Oh, one last thing, you can change that system("PAUSE") for a getchar() and maintain portability.

**vart** · 01-08-2007

rain has a type double[][5]
and it is different from the double[]

if you want to read two-dimentional array as one-dimentional pass pointer to the first element:
&rain[0][0]

and make a function prototype as:

Code:

void average5(double* a)

**kermit** · 01-08-2007

You will want to pass in a size for the second subscript. The compiler can't guess at what size the second (or subsequent arrays, if you had more than two) array is. As it is, you have only passed a pointer to the first element of the array a, but each element of a also contains a vector of 5 elements.

**Salem** · 01-08-2007

Exactly the same as your previous question.
http://cboard.cprogramming.com/showthread.php?t=87215

**timhxf** · 01-08-2007

I compiled this code, and I only got an warning saying passed the imcompatible pointer type, but the program works!

**vart** · 01-08-2007

Originally Posted by timhxf

I compiled this code, and I only got an warning saying passed the imcompatible pointer type, but the program works!

So what's the question? Why your wrong code works on your compiler? Just luck... Don't you expect that you'll be always so lucky with your errors?

**timhxf** · 01-08-2007

thanks all. I know that my code is wrong. I just wondered why it worked at my machine. I am a new comer for C, do not know how the compiler works. So, just a bit curious

**whiteflags** · 01-08-2007

I just wanted to weigh in on this issue here.

Originally Posted by timhxf

thanks all. I know that my code is wrong. I just wondered why it worked at my machine. I am a new comer for C, do not know how the compiler works. So, just a bit curious

Dense arrays can be allocated in a number of ways, but what the compiler should do in this case

unsigned int x = 2, y = 3;
double rain[MAXROWS][MAXCOLS];

is simply allocate a huge block of memory of MAXROWS * MAXCOLS doubles.

What changes most is the way that the array is indexed. One sane array implementation may ensure that if you laid out the array visually--maybe on a graph--you would have to move x rows and y columns to get to the right space. So, when the parser reaches a statement like

rain[x][y] = 42;

The address which is dereferenced gets computed like so x * MAXROWS + y. Pointer implementations must be able to do this sort of arithematic, given the size of the object in bytes to which they point and that all objects in the array are stored immediately after one another, if you step enough bytes forward or backward, you can reach an address which stores something valid.

What is remarkable about your code is that you pass a simple double pointer and "walk" through the array, but, pedantically

Code:

int average5( double *a1 );
...
average5(rain);  /* this points to the whole array; much bigger in bytes than a double. */

Which is why a lot of people yelled at you about changing your prototype. While passing a pointer to such a huge space definitely ensures that you could access the whole array, you may not dereference it in the right places. Undefined behavior is the result.

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