C -- number of elements in an array?

Question

Suppose that I have the following c function:

int MyFunction( const float arr[] )
{
    // define a variable to hold the number of items in 'arr'
    int numItems = ...;

    // do something here, then return...

    return 1

}

How can I get into numItems the number of items that are in the array arr?

I remember that I used to make size as another argument for such kind of functions.. — mihsathe, Jul 28 '11 at 16:56
possible duplicate of [Number of elements in static array and dynamic array](http://stackoverflow.com/questions/2577711/number-of-elements-in-static-array-and-dynamic-array) — sth, Jul 28 '11 at 17:07

score 11 · Accepted Answer · answered Jul 28 '11 at 16:57

11

Unfortunately you can't get it. In C, the following 2 are equivalent declarations.

int MyFunction( const float arr[] );
int MyFunction( const float* arr );

You must pass the size on your own.

int MyFunction( const float* arr, int nSize );

In case of char pointers designating strings the length of the char array is determined by delimiter '\0'.

answered Jul 28 '11 at 16:57

Armen Tsirunyan

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Great answer. Which function definition is preferred: the first one (`using arr[]`) or the second one (`using *arr[]`)? – user3262424 Jul 28 '11 at 17:08
@user3262424: You mistyped in your comment - be careful! :) I think choosing between `float arr[]` and `float*` to pass an array is a matter of taste – Armen Tsirunyan Jul 28 '11 at 17:10
Thank you Armen. Sorry for the typo. – user3262424 Jul 28 '11 at 20:27

score 5 · Answer 2 · edited Jul 28 '11 at 16:57

5

Either you pass the number of elements in another argument or you have some convention on a delimiting element in the array.

edited Jul 28 '11 at 16:57

Prof. Falken

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answered Jul 28 '11 at 16:56

Nobody moving away from SE

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Ioan Paul Pirau · Answer 3 · 2011-07-28T17:19:30.563

5

I initially suggested this:

 int numItems = sizeof(arr)/sizeof(float);

But it will not work since arr is not defined in the current context and it's being interpreted as a simple pointer. So in this case, you will have to give the number of elements as parameter. The suggested statement would otherwise work in the following context:

int MyFunction()
{
     float arr[10];
     int numItems = sizeof(arr)/sizeof(float);
     return numItems;// returns 10
}

edited Jul 28 '11 at 17:19

answered Jul 28 '11 at 16:57

Ioan Paul Pirau

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arr is a function argument. It's just a pointer, not an array — won't work. http://ideone.com/sQqWP – sidyll Jul 28 '11 at 16:59
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This is incorrect. `arr` is not of type `float []` when it is a function parameter. Instead it is `float *`. – Susam Pal Jul 28 '11 at 17:04
@Susam: May that's what I said? – sidyll Jul 28 '11 at 17:05
2

@sidyll Yes, we are saying the same things here. +1-ed you. – Susam Pal Jul 28 '11 at 17:08

score 2 · Answer 4 · answered Jul 28 '11 at 16:57

2

You can't. The number will have to be passed to MyFunction separately. So add a second argument to MyFunction which should contain the size.

answered Jul 28 '11 at 16:57

Rudy Velthuis

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score 2 · Answer 5 · answered Jul 28 '11 at 16:58

2

As I said in the comment, passing it as another argument seems to be only solution. Otherwise you can have a globally defined convention.

answered Jul 28 '11 at 16:58

mihsathe

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score 1 · Answer 6 · answered Jul 28 '11 at 16:56

This is not possible unless you have some predetermined format of the array. Because potentially you can have any number of float. And with the const float arr[] you only pass the array's base address to the function of type float [] which cannot be modified. So you can do arr[n] for any n .

score 1 · Answer 7 · answered Jul 28 '11 at 18:47

1

better than

int numItems = sizeof(arr)/sizeof(float);

is

int numItems = sizeof(arr)/sizeof(*arr);

answered Jul 28 '11 at 18:47

user411313

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C -- number of elements in an array?

7 Answers7