How to copy a value inside an array - c++

Question

I have an integer array and the fifth value should always be similar to the first value.
For example, if i have:
int test[] ={1,2,3,4,1}
and if I say:
test[0]= 5
The array should look like: 5 2 3 4 5.
Is there some way to do this? I tried with pointers, but couldn't get a good result.

Answer 1

Test it on GodBolt
Using actual C++ syntax instead of C:
#include <array>
#include <iostream>
void print_list(std::array<int*, 5> const& list){
for( auto const& item : list ){
std::cout << *item << "\t";
}
std::cout << "\n";
}
int main(){
std::array<int, 4> n = {1, 2, 3, 4};
std::array<int*, 5> list = {&n[0], &n[1], &n[2], &n[3], &n[0]};
print_list(list);
*list[0] = 3;
print_list(list);
}

Answer 2

Not possible with integers but pointers would do the trick, see here: https://www.tutorialspoint.com/cplusplus/cpp_array_of_pointers.htm
Something like:
int *sharedNumber = new int(1);
int* ptr[] = {sharedNumber, new int(2), new int(3), new int(4), sharedNumber};
cout<<"values are "<<*ptr[0]<<" "<<*ptr[4]<<"\n";
*sharedNumber = 5;
cout<<"values are "<<*ptr[0]<<" "<<*ptr[4]<<"\n";
Will show 'value is 1' and 'value is 5' for ptr[1] and ptr[4] - just make sure to delete initialised memory to avoid memory leaks.

Answer 3

There is currently no way of doing what you want with the syntax you're providing. Your options, which would (minimally) alter your code are:
Manually setting two elements to the same value (see the comments below your question);
Writing a function for mutating elements that automatically synchronizes the first and fifth elements;
Storing an array of pointers (which seems to be your attempt);
Sharing state between elements of the array so that affecting one element would also change other elements, and that way "synchronizing" their values;
Wrapping your array into a class that manages that array.
It should seem obvious which solutions are, most likely, overkill.
Here is a very simple simple implementation of the second option:
void setElement(int* array, size_t index, int value)
{
array[index] = value;
if (index == 0)
array[4] = value;
if (index == 4)
array[0] = value;
}
Keep in mind that using std::array, std::vector or iterators would probably be a better choice than C-style arrays and raw pointers.

Answer 4

There's a way to achieve this effect for a custom wrapper type, but not for arrays:
Just implement a custom [] operator:
template<class ElementType, size_t arraySize>
class MyArray
{
public:
using value_type = ElementType;
constexpr MyArray() = default;
constexpr MyArray(std::initializer_list<ElementType> arr)
{
std::copy_n(arr.begin(), (std::min)(arraySize, arr.size()), m_data);
}
constexpr size_t size() const
{
return arraySize;
}
constexpr ElementType& operator[](size_t index)
{
return m_data[index % arraySize];
}
constexpr ElementType const& operator[](size_t index) const
{
return m_data[index % arraySize];
}
private:
ElementType m_data[arraySize];
};
int main()
{
MyArray<int, 4> a = { 1, 2, 3, 4 };
std::cout << a[4] << '\n';
a[0] = 5;
std::cout << a[4] << '\n';
}
Note that this just "wraps around"; not only can you access the first element using index 4, but for every index passed the remainder of the division by the array size is used as index. The second element of MyArray<int, 4> could be accessed using indices 1, 5, 9, 4449, ect.