Indexing of pointer when accessing elements of an array

Question

I'm a beginner and I'm trying to understand how multidimensional arrays work.

So far, I've come this code snippet that i do not understand:

int arr[] = { 3, 5, 6, 7, 9 };
int (*ptr)[5] = &arr;

Ok so. I understand that ptr is a pointer pointing to an array of 5 elements.

But what is the '&' doing?? Is ptr pointing to the beginning address of 'arr'? But isnt 'arr' already an address? Why is there an '&' then? Also. What is the significance of the indexing 5? Because when i try to give any other number besides 5, it throws an error.

I'm sorry if all this sounds silly but I'm so confused! Help!

You do not index with the 5 here, but you declare a pointer to an array with 5 elements. — Ctx
– Ctx, Commented Apr 11, 2019 at 16:33

Ctx · Accepted Answer · 2019-04-11 16:42:18Z

1

Here, you have a variable arr with type array of 5 int:

int arr[] = { 3, 5, 6, 7, 9 };

Here, you declare a pointer ptr to array of 5 int:

int (*ptr)[5]

This gets the address of arr with type pointer to array of 5 int

&arr

and this assignment works, since the pointer types are equal:

int (*ptr)[5] = &arr;

Now if you change the type of ptr to another type, i.e. pointer to array of 6 int:

int (*ptr)[6];

then the assignment no longer works, since the pointer types mismatch now. This is why you get an error.

Note: the [5] in this code does not index anything, but is just a part of the variable declaration (number of elements in the array)

answered Apr 11, 2019 at 16:42

Ctx

18.6k24 gold badges38 silver badges56 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

bobthebuilder Over a year ago

Ok so for int a[3];, the address of a is a pointer to array of 3 elements? (Or does that hold good only when you assign values to the elements at the time of initialisation itself? )

Ctx Over a year ago

@bobthebuilder Yes, &a is of type int (*)[3] then.

bobthebuilder Over a year ago

Ok makes sense. Thank You!

500 - Internal Server Error · Accepted Answer · 2019-04-11 16:32:30Z

0

Your array only has one dimension, so it's not multidimensional.

Your array has 5 elements, and since arrays are zero-based, accessing element [5] (the sixth element) actually results in undefined behavior.

arr is not an address, it's an array. & is the address-of operator.

answered Apr 11, 2019 at 16:32

500 - Internal Server Error

29k8 gold badges64 silver badges78 bronze badges

1 Comment

bobthebuilder Over a year ago

Here's the thing. The second line of the code. Isn't it kinda equivalent to *p=a;?

Collectives™ on Stack Overflow

Indexing of pointer when accessing elements of an array

2 Answers 2

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related