7

I'm Java developer, not a C programmer. I'm trying to invoke some function that receives a 2D array, tried like:

#include <stdio.h>
int foo(int mat[][N])
{ 
  printf("%d",N); /*  <--- here error */

}

int main()
{
    int a[5][5] =
    {
        {1, 2, 3, 4,5},    
        {1, 5, 5, 5,2},    
        {1, 4, 30,4,2},
        {1, 2, 2,2, 2},
        {1,1, 1,1,  2 } 
    };

    foo(a);
    return 0;
}

But the compiler says:

'N' undeclared (first use in this function)

I must have the signature: int foo(int mat[][N])

How should I pass the 2D array then?

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10
  • 3
    You should define N. Commented Dec 10, 2018 at 18:47
  • 'N' undeclared. Declare N. Commented Dec 10, 2018 at 18:48
  • But it doesn't know that a is 5X5 matrice ? @EugeneSh. Commented Dec 10, 2018 at 18:48
  • @Malwarey How would it? Put yourself in the place of the compiler. you're generating code for your source file. After the preprocessor finishes sucking in all the stuff from the #includes and performed macro expansions, etc, you're on to actually compiling source and the first thing you see here is int foo(int mat[][N]). Now, how do you know what N is at that time ? It isn't inferred nor deduced by the compiler. That isn't how C works. Commented Dec 10, 2018 at 18:51
  • 3
    C arrays are contiguous blocks of memory. They do not "remember" their size. Commented Dec 10, 2018 at 18:57

2 Answers 2

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1

You can use macros to define constant numbers, i.e. #define N 5. At compile time each appearance of the defined macro name will be replaced with the given value. In our case each N would be replaced with 5. But this would not solve your problem, because it would print 5 every time, even if you change the dimensions of your array.

Pass the dimensions of the array as parameters.

You can pass the 2D array as a pointer.

I created a printing function, with what I can show you accessing the elements from the array. So the function's prototype would look like:

void print2DMatrix(int *matrix2D, const int rowLength, const int columnLength);

where matrix2D is a pointer, there will be passed the address of the array. The other two parameters tell us the dimensions of the array.

We can access the elements in a selected row and in a selected column with matrix2D[selectedRow * maxRowLength + selectedColumn].

When you call the function, you can pass the name of the array, like print2DMatrix(*myMatrix, myRowLength, myColumnLength);, of course you shall declare and initialize it before using. print2DMatrix(myMatrix, myRowLength, myColumnLength); would cause warning, because the passed argument and the waited one have different types, but it will run correctly. The passed one is a double pointer, when the waited one is a single pointer. Because of that you have to use print2DMatrix(*myMatrix, myRowLength, myColumnLength);, where *myMatrix will point to the first row of our array.

I would like to mention that myMatrix and *myMatrix point to the same address, the difference is: myMatrix is looked as a double pointer to an integer, when *myMatrix is looked as a pointer to an integer. Run printf("%d %d %d", myMatrix, *myMatrix, **myMatrix); to see the result. **myMatrix will point to the first element of the first row, what is 11. See the entire code below...

#include <stdio.h>

#define ROW_LENGTH      5
#define COLUMN_LENGTH   5

void print2DMatrix(int *matrix2D, const int rowLength, const int columnLength)
{
    int i;
    for (i = 0; i < rowLength; i++)
    {
        int j;
        for (j = 0; j < columnLength; j++)
        {
            printf("%d ", matrix2D[i * rowLength + j]);
        }
        printf("\n");
    }
}

int main(void)
{
    const int myRowLength = ROW_LENGTH;
    const int myColumnLength = COLUMN_LENGTH;
    int myMatrix[ROW_LENGTH][COLUMN_LENGTH] =
    {
        {11, 12, 13, 14, 15},
        {21, 22, 23, 24, 25},
        {31, 32, 33, 34, 35},
        {41, 42, 43, 44, 45},
        {51, 52, 53, 54, 55}
    };

    print2DMatrix(*myMatrix, myRowLength, myColumnLength);

    return 0;
}
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2 Comments

See also static const vs #define vs `enum.
Thank you so much for your answer, @Norbert Incze. Could you please explain why are you accessing each element with matrix2D[selectedRow * maxRowLength + selectedColumn] ?
0

You are trying to print a non existent value. N is not a declared variable.

The function foo is receiving a pointer. Unlike java, C deals with pointers, not objects, it does not have any information about what it received, except that it is a pointer that contains addresses to ints. Therefore, the function does not know (or care) about your array being of size N (regardless of you mentioning N) .

When you call printf there's nothing in memory called N, which explains the error.

You could always send a second argument which is an int specifying the size and print that one.

void foo(int[][] mat, int matSize){
  printf("%d\n",matSize);
}

Notice how I used int[][] this is to simplify and illustrate the fact that is just a pointer.

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1 Comment

Note that void foo(int [][] mat, int matSize) is not C. The subscripts would appear after mat, and the second one, at least, needs a dimension specified.

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