Using pointers instead of an array [closed] - c

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Closed 10 years ago.
I wrote this program a few weeks ago using arrays and now I need to use pointers instead of using arrays. I'm not exactly sure how to go about doing that so any tips would be appreciated! Thanks! :D
Here is the code:
#include <stdio.h>
int showArray(int row);
int exchangeRow(int row1, int row2);
int x, y;
int array[10][10];
int j;
int k;
int inputrow;
int inputcolumn;
int scanrow;
int temp;
int row1;
int row2;
int main() {
// Initialize array
for(j = 0; j < 10; j++) {
printf("\n");
for(k = 0; k < 10; k++) {
array[j][k] = j * 10 + k;
printf("%d ", array[j][k]);
}
}
printf("\n \n");
// Print out selected row
printf("Type in a number for the corresponding row to be printed \n");
scanf("%d", &inputrow);
if(inputrow >= 0 && inputrow < 10) {
for(j = 0; j < 10; j++) {
printf("%d ", array[inputrow][j]);
}
}
printf("\n \n");
//Print out selected column
printf("Type in a number for the corresponding column to be printed \n");
scanf("%d", &inputcolumn);
if(inputcolumn >= 0 && inputcolumn < 10) {
for(j = 0; j < 10; j++) {
printf("%d ", array[j][inputcolumn]);
}
}
printf("\n \n");
printf("Type in a number for the row that method showArray will print \n");
scanf("%d", &scanrow);
showArray(scanrow);
printf("\n \n");
printf("Type in two numbers for the rows that method exchangeRow will switch \n");
scanf("%d %d", &row1, &row2);
exchangeRow(row1, row2);
printf("\n \n");
system("PAUSE");
}
int showArray(int row) {
for(j = 0; j < 10; j++) {
printf("%d ", array[row][j]);
}
}
int exchangeRow(int row1, int row2) {
if(row1 >= 0 && row1 < 10 && row2 >= 0 && row2 < 10) {
temp = row1;
row1 = row2;
row2 = temp;
printf("The first row now holds the values: ");
showArray(row1);
printf("\n");
printf("The second row now holds the values: ");
showArray(row2);
}
}

Answer 1

I take it you mean "using dynamic memory allocation"...
The way a lot of people do 2D arrays dynamically is like this:
const size_t nrows = 10, ncols = 10;
int **array = malloc( nrows * sizeof(int*) );
for( i = 0; i < nrows; i++ ) {
array[i] = malloc( ncols * sizeof(int) );
}
But I hate this. If you are doing production code, this can be very slow. It's also harder to handle the case where you run out of memory, and there's no guaranteed locality of your array. Plus, it's ugly to free:
for( i = 0; i < nrows; i++ ) free(array[i]);
free(array);
In memory, your static array[10][10] is one contiguous block. So you should do the same:
int **array = malloc( nrows * sizeof(int*) );
array[0] = malloc( nrows * ncols * sizeof(int) );
for( i = 1; i < nrows; i++ ) {
array[i] = array[i-1] + ncols;
}
To free that:
free(array[0]);
free(array);
I often take this a step further, and do a single memory allocation instead of two. That way I have just one pointer. But I won't do that here. You have to be a little conscious of alignment, and the code is a little messier. It's an optimization that usually you don't need.
Hope that helps.

Answer 2

To be more specific, what is required of you is to use pointer notation instead of array notaiton
This would probably mean that your 2 dimensional array should be allocated as an array of pointers. Also it's individual elements should be accessed using pointers rather than the usual array indexing.
For e.g.
int * a = malloc(10 * sizeof(int)); // allocate memory for 10 integers
*a = 1; // assign first element of array
*(a+1) = 2; // assign second
The above was for 1D array. Extend it to multiple dimensions as in your original program.

Answer 3

When you declare an array, such as int arr[4];, arr actually "points" to a location in memory with 4 "spaces" that holds an integer each.
&arr means "address of" arr.
So, a pointer to arr could be defined as:
int *ptr = &arr[0];
But how would you get the data that ptr points to? The answer is:
*ptr;
But, this will only show you the first value of the data stored in the first "space" that arr occupies. To get the others in the array, all you need to do is increment your pointer along the space in memory, like so:
ptr++;
Further information can be found here: http://alumni.cs.ucr.edu/~pdiloren/C++_Pointers/neighbor.htm