Pointers and multidimensional arrays in C++

Question

I have this piece of code:

#include <iostream> int main() { int ia[3][4]; // array of size 3; each element is an array of ints of size 4 int (*p)[4] = ia; // p points to an array of four ints p = &ia[2]; // p now points to the last element in ia return 0; }

How does p point to the last element in ia?

I would think it would point to the first element in the third row. How come I don't have any additional dimension to the initiaziler of ia and yet it points to the last element in the last row, not the first? — Jake Wright
– Jake Wright, Commented Jan 6, 2018 at 21:26
It doesn't point to the last element in the last row. The last element of ia is the third row itself. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Jan 6, 2018 at 21:27
The elements of ia are pointers to int[4] arrays. ia has 3 elements. the last one is ia[2]. So the element in your case is not a number, its an array. — StPiere
– StPiere, Commented Jan 6, 2018 at 21:28
Oh, okay, so by "last element" it doesn't mean ia[2][3], but the third row. Thanks a lot! — Jake Wright
– Jake Wright, Commented Jan 6, 2018 at 21:29

eerorika · Accepted Answer · 2018-01-06 21:31:17Z

How does p point to the last element in ia?

ia contains 3 elements. Each element is an array of 4 integers. ia[2] is the last element i.e. the last array of 4 integers.

Killzone Kid · Accepted Answer · 2018-01-07 00:38:05Z

int (*p)[4] = ia; // p points to an array of four

p = &ia[2]; //p now points to the last element in ia

If you have array

int ia[3][4] = { { 1,2,3,4 },{ 5,6,7,8 },{ 9,10,11,20 } };

then after int (*p)[4] = ia; pointer p will be pointing to {1,2,3,4} and after p = &ia[2];, p will be pointing to { 9,10,11,20 }

If you want a pointer to the first element of the last array, from your example:

int ia[3][4] = { { 1,2,3,4 },{ 5,6,7,8 },{ 9,10,11,20 } }; int(*p)[4] = ia; // p points to { 1,2,3,4 } p = &ia[2]; // p points to { 9,10,11,20 } std::cout << *(p[0]) << std::endl; // 9, because p[0] points to the first int of { 9,10,11,20 } std::cout << *(p[0]+3) << std::endl; // 20, because p[0]+3 points to the last int of { 9,10,11,20 }

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