How to convert an array decay to a pointer into a vector without knowing the array size?

Asked Nov 12 '20 at 14:13

Active Nov 12 '20 at 14:13

Viewed 23 times

I have an array which I want to convert in vector. I have this code :

int function(int A[], int K){
    vector<int> v,temp1,temp2;
    for(int i = 0; A[i] != 0; ++i) v.push_back(A[i]);
    sort(v.begin(), v.end());
    int n = v.size();
    cout << n << endl;
}

And I try this code with the following test :

int main(){
    int tab[] {4,9,8,2,6};
    int sol = function(tab,3);
    cout << sol << endl;
    return 0;
}

And it displays 18 while I expect 5 as a value, I don't understand why

asked Nov 12 '20 at 14:13

Elezan Hita

You cannot do that because the size is not known. – MikeCAT Nov 12 '20 at 14:14
inside `function` the parameter `A` is just a pointer, any information on size or where the array ends is lost. – 463035818_is_not_an_ai Nov 12 '20 at 14:15
you can copy `tab` to a vector in `main` – 463035818_is_not_an_ai Nov 12 '20 at 14:15
I stop the for loop when I have a 0, it isn't enough to stop the loop and get the good number of element in A which is non null ? – Elezan Hita Nov 12 '20 at 14:16
2

there is no `0` in the array, hence you access elements beyond the size of the array and invoke undefined behavior – 463035818_is_not_an_ai Nov 12 '20 at 14:16
1

`tab` has no zeros. Why do you expect `A` to have them? – Evg Nov 12 '20 at 14:16
Okay, thank you, I think I will just use the STL – Elezan Hita Nov 12 '20 at 14:17
A workaround is to add a `0` at the end of the array. Better is to provide the size of the array to the function – Damien Nov 12 '20 at 14:17
@Damien the better is to not use a c-array in the first place – 463035818_is_not_an_ai Nov 12 '20 at 14:17
why do you pass `K` and then do nothing with it? – Artefacto Nov 12 '20 at 14:17
Is it possible to use an array from the STL without knowing the size ? I see we need to know the size by initialisation, for example array for an array with 4 element – Elezan Hita Nov 12 '20 at 14:18
Because I selected a piece of code, I just forgot to remove K @Artefacto – Elezan Hita Nov 12 '20 at 14:19
btw as `tab` is an array (not just a pointer to one), there is also the possibility to pass the array via reference. However, its a bit advanced, it requires you to understand quite a bit for little benefit over a `std::vector` or `std::array`,https://stackoverflow.com/questions/5724171/passing-an-array-by-reference – 463035818_is_not_an_ai Nov 12 '20 at 14:20
for dynamically sized arrays (ie size only known at runtime) use `std::vector` – 463035818_is_not_an_ai Nov 12 '20 at 14:21

How to convert an array decay to a pointer into a vector without knowing the array size?

0 Answers0