number of elements in a static array of a predefined size

Question

I have an array like this:

int a[100]; 

I am filling only the first 4 elements in this array:

a[0] = 1;
a[1] = 2;
a[2] = 3;
a[3] = 4;

When I do sizeof(a)/sizeof(a[0]) it returns 100.

Is there a way I can get number of elements to which I have assinged a value and thus filtering out the remaining 96 unassigned elements?

thanks

Answer 1

No. Nothing keeps track of that.

Answer 2

No. Either use a data structure, such as Vector, that keeps track of length, or pre-fill the array with a value which cannot ever occur as a real value, and test for that value and end of array while looping from start.

Answer 3

Why not use an associative container like std::map in this case? It allows you to test wether it contains an entry with a given key using std::map::find():

std::map<size_t, int> myMap;
myMap[0] = 1;
// ...
bool contains = myMap.find(0) != myMap.end();

Answer 4

Assuming you're a beginner (and thus not jumping into the STL), here's an example of what @Mitch is talking about:

char *Names[100] = {}; // zero init

Names[0] = "hello";
Names[1] = "world";

for (int n = 0; n < 100 && Names[n] != 0; ++n)
    if (!Names[n])
        break;

printf("# of entries: %d", n);

Nowadays you would only do this if had to keep memory usage to an absolute minimum.

Answer 5

In the message title you called your array "static". If this is indeed an array with static storage duration, then its elements are zero-initialized by default at program startup. Since the "assigned" values you used in your example are non-zero, you can determine how many elements you have assigned by finding the first zero element in the array. Of course, this will only work when the "assigned" values are guaranteed to be non-zero, i.e. if zero value can be thought of as a reserved value.

Answer 6

I think using boost::optional as array type can solve the problem:

boost::optional<int> a[100];

So, you can check if element is set like this:

if (! a[i]) {   // element not set
}
else {  // element not set
}

Using boost:array instead of bare static array may be good idea too.