Pointer arithmetic

sanman10535 · #1 22-Mar-2004, 08:49

I have to write the function vec_to_mat (int *v, int *mat, int n); v is a pointer to the beginning of an integer array (vector) with n x n elements stored in column-major representation (see below for explanation). The function should copy the elements from v to mat (which is a matrix of size n x n). From example,

If v is {1, 2, 3, 4, 5, 6, 7, 8, 9}, the value of mat should be,

1 4 7
2 5 8
3 6 9

The values 1, 2, and 3 go down the first column of matrix mat, and then 4, 5, 6 goes down the second column and so on.

Here's what I have so far:

CPP / C++ / C Code:

# include <stdio.h>

main () {

	int a [] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
	int matrix [3][3];

	vec_to_mat (a, matrix[0], 3);

	for (int i = 0; i < 3; i++) {
		for (int j = 0; j < 3; j++) {
			printf (“%d”, matrix [i][j]);
		}
		printf (“\n”);
	}
}

Also, why do I need to pass matrix[0] to the function vec_to_mat and not just matrix?

Oh yeah, I have to use pointer arithmetic (not array subscripting) to visit the array elements.

JdS · #2 22-Mar-2004, 09:00

Hello sanman10535,

Take a few minutes of your time to read this: http://www.gidforums.com/t-689.html.

sanman10535 · #3 22-Mar-2004, 09:04

I have to write the function vec_to_mat (int *v, int *mat, int n); v is a pointer to the beginning of an integer array (vector) with n x n elements stored in column-major representation (see below for explanation). The function should copy the elements from v to mat (which is a matrix of size n x n). From example,

If v is {1, 2, 3, 4, 5, 6, 7, 8, 9}, the value of mat should be,

1 4 7
2 5 8
3 6 9

The values 1, 2, and 3 go down the first column of matrix mat, and then 4, 5, 6 goes down the second column and so on.

Here's what I have so far:

CPP / C++ / C Code:

# include <stdio.h>

main () {

	int a [] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
	int matrix [3][3];

	vec_to_mat (a, matrix[0], 3);

	for (int i = 0; i < 3; i++) {
		for (int j = 0; j < 3; j++) {
			printf (“%d”, matrix [i][j]);
		}
		printf (“\n”);
	}
}

Also, why do I need to pass matrix[0] to the function vec_to_mat and not just matrix?

Oh yeah, I have to use pointer arithmetic (not array subscripting) to visit the array elements.

sanman10535 · #4 22-Mar-2004, 09:06

Quote:

Originally Posted by sanman10535

I have to write the function vec_to_mat (int *v, int *mat, int n); v is a pointer to the beginning of an integer array (vector) with n x n elements stored in column-major representation (see below for explanation). The function should copy the elements from v to mat (which is a matrix of size n x n). From example,

If v is {1, 2, 3, 4, 5, 6, 7, 8, 9}, the value of mat should be,

1 4 7
2 5 8
3 6 9

The values 1, 2, and 3 go down the first column of matrix mat, and then 4, 5, 6 goes down the second column and so on.

Here's what I have so far:

CPP / C++ / C Code:

# include <stdio.h>

main () {

	int a [] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
	int matrix [3][3];

	vec_to_mat (a, matrix[0], 3);

	for (int i = 0; i < 3; i++) {
		for (int j = 0; j < 3; j++) {
			printf (“%d”, matrix [i][j]);
		}
		printf (“\n”);
	}
}

Also, why do I need to pass matrix[0] to the function vec_to_mat and not just matrix?

Oh yeah, I have to use pointer arithmetic (not array subscripting) to visit the array elements.

AHRRRR, forgot to add some more code

aaroncohn · #5 22-Mar-2004, 09:21

Quote:

Originally Posted by sanman10535

Oh yeah, I have to use pointer arithmetic (not array subscripting) to visit the array elements.

I wish I knew... my best guess from looking is that you need it simply because it's the beginning of the array. What exactly are the parameters required by the function? It might be easier to tell what passing the beginning of the matrix array is for if I know what the other two values are for.

davekw7x · #6 22-Mar-2004, 11:07

Quote:

Originally Posted by sanman10535

Also, why do I need to pass matrix[0] to the function vec_to_mat and not just matrix?

Oh yeah, I have to use pointer arithmetic (not array subscripting) to visit the array elements.

You want to pass the address of matrix to the function (this a pointer to an int, since matrix is a (two-dimensional) array of int).

Two ways of writing address of matrix:

CPP / C++ / C Code:

&matrix[0][0]

or

CPP / C++ / C Code:

matrix[0]

Dave

machinated · #7 22-Mar-2004, 12:08

yes dave is right, it's because your function vec_to_mat's second formal parameter is probably a single dimensional array: int mat1[]; if your second formal parameter was 2d array: int mat1[][3]; then you would have to pass it matrix and not just matrix[0]; for example

CPP / C++ / C Code:

void vec_to_mat(int a1[], int mat1[][3],int x1)
{}
//to call this function you would:
vec_to_mat(a,mat,x);

just think of it as breaking off a row from your 2d matrix and passing it to your vec_to_mat function. you will not just be passing matrix[0] but you will also need to call vec_to_mat function and pass it matrix[1] and then matrix[2] and so on. you will need to call this function as many times as there are rows in the matrix, for each row.

also to do pointer arithmetic, just assign a pointer to the first element of your array and or your array itself just keep incrementing it till you reach the end. for example:

CPP / C++ / C Code:


void vec_to_mat(int a1[], int mat1[], int x)
{
int *ptr = mat1; //or int *ptr = &mat1[0];
ptr++;              //ptr now points to mat1[1];
ptr++;             //ptr now points to mat1[2];
}

Quote:

Originally Posted by sanman10535

I have to write the function vec_to_mat (int *v, int *mat, int n); v is a pointer to the beginning of an integer array (vector) with n x n elements stored in column-major representation (see below for explanation). The function should copy the elements from v to mat (which is a matrix of size n x n). From example,

If v is {1, 2, 3, 4, 5, 6, 7, 8, 9}, the value of mat should be,

1 4 7
2 5 8
3 6 9

The values 1, 2, and 3 go down the first column of matrix mat, and then 4, 5, 6 goes down the second column and so on.

Here's what I have so far:

CPP / C++ / C Code:

# include <stdio.h>

main () {

	int a [] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
	int matrix [3][3];

	vec_to_mat (a, matrix[0], 3);

	for (int i = 0; i < 3; i++) {
		for (int j = 0; j < 3; j++) {
			printf (“%d”, matrix [i][j]);
		}
		printf (“\n”);
	}
}

Also, why do I need to pass matrix[0] to the function vec_to_mat and not just matrix?

Oh yeah, I have to use pointer arithmetic (not array subscripting) to visit the array elements.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Rate This Thread
Rate This Thread:

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Pointers-general confusion of pointer type.	warny_maelstrom	C Programming Language	3	11-Mar-2004 22:26
Pointer values changing unexpectedly	spudtheimpaler	C Programming Language	11	04-Mar-2004 17:37
modulus arithmetic	ewallo	C++ Forum	3	23-Feb-2004 00:35
storing a token pointer as a string	CoreLEx	C Programming Language	1	07-Oct-2003 12:33
convert long to pointer to char	realpopeye	C++ Forum	2	26-Sep-2003 11:22