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

All elements must be assigned to something, thus the array always has 100 elements. If you can ensure all the elements are initialized to a special value which means “unassigned” to you (e.g. -1), you can work it out like this:

// fill the array with a special value which means "uninitialized"
const int special_uninitialized = -1;
std::fill(&a[0], &a[100], special_uninitialized);

// set up your values
a[0] = 1;

// count
std::size_t uninitialized_count = std::count(&a[0], &a[100], special_uninitialized);
std::size_t initialized_count = 100 - uninitialized_count;

If you just want to know how many elements are in an array, you have these options:

1. Don’t use an array, use std::vector, which has a size() function, and is generally a better choice than a basic array

2. Keep track of the element count yourself, in a separate variable

3. Use the special “unassigned” value as described above, and use std::find to find the first one, and work out how many are there by subtracting the address of the zeroth element from that. This is a pretty ugly solution.

For a beginner, std::vector is a much better choice. You can use it like this:

std::vector<int> vec;

vec.push_back(17);
vec.push_back(23);
vec.push_back(5);

int x = vec[0]; // x will be 17
vec[0] = 40; // set element 0

size_t s = vec.size(); // s will be 3