how do i know if element in array is there?

Question

So I have an array in my C program

Node *array;
array = (Node*)malloc(53*sizeof(Node));

if(array[5] is empty)
 //Fill array index with element

How do I test if element is there or not? I've tried if(array[5] == NULL) but it keeps giving me an error.

`array[5]` is of type `Node`, and not a pointer to `Node`. It is not comparable to `NULL` — Shloim, Nov 29 '15 at 23:51
after the `malloc` call, there are 53 nodes. There's no such thing as a node being "there or not". — M.M, Nov 29 '15 at 23:56
You need to at least initialise before you check in C. They're there, but they could contain pretty much anything if you don't initialise them. [And please don't cast the result of malloc](http://stackoverflow.com/questions/1565496/specifically-whats-dangerous-about-casting-the-result-of-malloc) — Iskar Jarak, Nov 29 '15 at 23:56

score 1 · Answer 1 · answered Nov 29 '15 at 23:55

I've tried if(array[5] == NULL) but it keeps giving me an error.

This does not work for two reasons:

malloc does not initialize the memory returned to your program. You are responsible for setting memory to zero, and
Node is not a pointer type, so you do not know if a Node is valid or not without knowing the structure of Node itself

You could switch to a double-pointer Node **array, and allocate it like this:

array = (Node**)malloc(53*sizeof(Node*));
memset(array, 0, 53*sizeof(Node*));

However, this means that you would have to array as an array of pointers, with mallocs of individual elements and so on.

If you want an array of Node structures, use a separate array of markers that indicate that a particular element is used or not.

score 1 · Answer 2 · answered Nov 30 '15 at 00:35

All of the elements in the array exist. There's no way to check whether an element exists, because elements can't not exist.

If you want to have some concept of a "nonexistent element", you have to make it yourself. A straightforward way to implement this would be to have a field in Node which stores whether the node "exists". You'd set that field to false in every Node after allocating them, and you'd set it to true whenever you want the node to "exist".

Emil Laine · Answer 3 · 2015-11-30T00:10:06.480

0

You could initialize the memory with a sentinel value, such as zero, and then check whether array[5] only contains that value. E.g.:

#define UNINITIALIZED 0

Node *array = calloc(53, sizeof(Node)); // don't cast malloc and friends!
Node null_node;
memset(&null_node, UNINITIALIZED, sizeof(Node));

if(memcmp(&array[5], &null_node, sizeof(Node)) == 0)
    // …

Just make sure a valid Node instance can never equal to that sentinel value.

This is more efficient than dasblinkenlight's solution, because it involves an additional indirection. However, if a safe sentinel value cannot be found, and adding an int uninitialized (or similar) flag field to Node is not possible, the extra pointer is necessary, unless you store the information about which elements are initialized somewhere else.

edited Nov 30 '15 at 00:10

answered Nov 29 '15 at 23:58

Emil Laine

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Although this assumes that a zeroed Node isn't a valid/useful value, of course... which it very well could be. – Iskar Jarak Nov 29 '15 at 23:59
Yes. In that case there could be some other value used to indicate "not initialized". – Emil Laine Nov 30 '15 at 00:00
Perhaps you could add a brief mention of using a sentinel value to your answer. – Iskar Jarak Nov 30 '15 at 00:01

score 0 · Answer 4 · answered Nov 30 '15 at 00:42

I personally think using c++ in such cases is better because then you can use default constructors to initialize. In your case, if you want to check if Node was 'initialized' or not, simply do a calloc and then check value of some integer var.

struct Node {
   //set to 1 if not empty
   char emptyFlag;
   initialize() {

        emptyFlag = 1;

        //other stuff 
   }
}

Node *array;
array = (Node*)calloc(53 , sizeof(Node));

if(array[5].emptyFlag == 0)
 //initialize array[5]

Recommending C++ when the OP indicates they're using C is the same as recommending that they use Javascript. — Jonathon Reinhart, Nov 30 '15 at 00:46

how do i know if element in array is there?

4 Answers4