Array size determination within a function

Question

In the 'main' part of the programm I declared an array of N elements. I would like to send it to the function which make some statistical calculation on it. How is it possible to detemine the size of this array (N) within the function without sending the number explicitely?

Answer 1

You can use templates, the compiler will automatically deduce the size for you.

#include <iostream>

template<class T, std::size_t sz>
void PrintArray(T (&arr)[sz])
{
for (auto& i : arr)
std::cout << i << "\n";
}

int main()
{
int arr[] = {4, 3, 1, 5, 8};
PrintArray(arr);
}

https://code.sololearn.com/c1wqPSpEJnuT

Answer 2

Use an inbuilt container? (vector, array)

http://www.cplusplus.com/reference/array/array/

https://www.sololearn.com/learn/261/?ref=app

Answer 3

Note: I believe the generic method will not work with pointers. (just in case this is what you were planning)

Answer 4

Thanks a lot! It works well:
https://code.sololearn.com/c6nxF46PmV76/#cpp

#include <iostream>
using namespace std;

template<class T, std::size_t sz>
void PrintArray(T (&arr)[sz])
{
std::cout << "Size of an element: " << sizeof(arr[0]) << "\n";
std::cout << "Size of the whole array: " << sizeof(arr) << "\n";
std::cout << "Number of elements: " << sizeof(arr)/sizeof(arr[0]) << "\n";
std::cout << "Elements of the array: ";
for (auto& i : arr)
std::cout << i << " ";
std::cout << "\n " << "\n";
}

int main()
{
int arr[] = {4, 3, 1, 5, 8};
PrintArray(arr);

float farr[] = {1.2, 3.5, 8.1, 9.0, 3.9, 6.1};
PrintArray(farr);

double darr[] = {1.22, 3.51, 8.17, 9.05, 9.00, 3.94, 6.18, 5.283};
PrintArray(darr);

char carr[] = {'a', 'd', 'f', 's'};
PrintArray(carr);

char sarr[] = "just an example"; // string as an array
PrintArray(sarr);
std::cout << sarr << "\n";
}

Output ==========================================

Size of an element: 4
Size of the whole array: 20
Number of elements: 5
Elements of the array: 4 3 1 5 8

Size of an element: 4
Size of the whole array: 24
Number of elements: 6
Elements of the array: 1.2 3.5 8.1 9 3.9 6.1

Size of an element: 8
Size of the whole array: 64
Number of elements: 8
Elements of the array: 1.22 3.51 8.17 9.05 9 3.94 6.18 5.283

Size of an element: 1
Size of the whole array: 4
Number of elements: 4
Elements of the array: a d f s

Size of an element: 1
Size of the whole array: 16
Number of elements: 16
Elements of the array: j u s t a n e x a m p l e

just an example

=====================================================

Answer 5

Unfortunately such a construction sizeof(arr)/sizeof(arr[0]) doesn't work within a function. In the example below the sizeof(arr) operator returns just a size of the pointer itself and the value of aSize is always 1. So it is necessary to pass the size of the array explicitely, The question is how to avoid this?

float rootMeanSquer(int *arr, int arrSize, float aver) {
int i = 0;
float rootMeanSquer = 0;
int aSize;

aSize = sizeof(arr)/sizeof(arr[0]); // A try to determine the number of elements
cout << aSize << endl;

for( int i = 0; i < arrSize; i++) {
rootMeanSquer += sq(arr[i] - aver);
};

rootMeanSquer /= (arrSize - 1);
return sqrt(rootMeanSquer);

} // End rootMeanSquer