Basic help needed with pointers (double indirection)

Question

Hi, i asked some time ago on an account i cant remember how to manipulate basic pointers and someone gave me a really good demo

for example

char *ptr = "hello" (hello = a char array)

so now *ptr is pointing at h

ptr++ = moves the ptr to point at the next element, to get its value i do *ptr and that gives me e

ok so far i hope :D but now i need to manipulate a char **ptr and was wondering how I do this in a way that mimmicks the effects of a 2d array?

some basic tips would be much appreciated as I need to do an assignment that has a **ptr to imitate a 2d array and without knowing how it does this first means I cant even solve it on paper (for example, how do you dereference a **ptr, how do you get [x][y] values etc)

thanks

Answer 1

A pointer to a pointer is just that. For example:

// Declare our double-indirection pointer.
char** ptr;
// And initialize it:
char s1[] = "hello";
char s2[] = "world";
ptr = malloc(sizeof(char*) * 2);
ptr[0] = s1;
ptr[1] = s2;
// ptr now points to a pointer that points to 'h'.
char* ptr2 = *ptr;
// ptr2 points to 'h'.
char* ptr3 = *(ptr + 1);
// ptr3 points to "w".
char c = **ptr; // could be written as *(*ptr)
// c = 'h'.
char c2 = *(*(ptr + 1));
// c2 = 'w'.
char c3 = *(*(ptr) + 1);
// c3 = 'e'.

Answer 2

You may use them as you would a normal two-dimensional array. (Since effectively, that's what they are)

char** ptr = {"lorem", "ipsum", "dolor"};
char* s1 = ptr[0]; //Points to the beginning of "lorem"
char* s2 = ptr[1]; //Points to the beginning of "ipsum"
char c = ptr[2][4]; //Contains 'r'

This is due to the fact that:

int *a;
//...
int i = a[6];
int j = *(a + 6); //Same as previous line!

Cheers,

Amit Ron--