initializing array to zero

[1vf9z8AR]

why doesn't the code work with an & before arr in for loop.(line 8)Shouldn't it start from initial address?

  #include<iostream>
using namespace std;
int main()
{
    int *p;
    const int len=20;
        int arr[len];
    for(p=arr;p<&arr[len];p++)
        *p=0;
}

[Ganado]

I'm not sure what you're asking. What exactly isn't working? How does it differ from what you expect your code to do?

Despite &arr[len] looking like it's trying to access something out of bounds, it is actually technically legal* as long as you just don't dereference arr[len]. arr[len] is the same thing as saying *((arr) + (len)).
https://stackoverflow.com/questions/988158/take-the-address-of-a-one-past-the-end-array-element-via-subscript-legal-by-the

*Edit: Actually the language used in the standard in the above link is quite confusing to me in this regard, and there seems to be a lot of back-and-forth discussion between this, especially regarding ANSI C vs C++. I now don't think it's technically legal. It might be de facto legal, but I would avoid the &arr[len] construct and just replace it with arr + len... (or better yet, avoid using pointers in this manner and just use a regular int for loop!!)

Legal, but unnecessary pointer arithmetic:

#include<iostream>
using namespace std;
int main()
{
    int *p;
    const int len=20;
        int arr[len];
    for(p=arr;p<arr+len;p++)
        *p=0;
}

Much more clear:

#include<iostream>
using namespace std;
int main()
{
    const int len=20;
    int arr[len];
    for(int i = 0; i < len; i++)
        arr[i] = 0;
}

[Thomas1965]

Why do you think it doesn't work ? It looks like you want to initialize all elements in your array to 0.

#include<iostream>

using namespace std;

int main()
{
    int *p;
    const int len=20;
    int arr[len];
    
    for(p = arr; p < &arr[len]; p++)
      *p=0;
        
    int *ip = arr;
    int size = len;
    
    while(size-- > 0)
    {
        cout << "ip = " << *ip << " ";
        ip++;
    }
}

Output

ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0
ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0 ip = 0  

[Cubbi]

Even simpler way to initialize all elements of an array to zero is to use an empty initializer ={} or (as of C++11) {}:

    const int len=20;
    int arr[len] = {};

[Ganado]

+1 Cubbi, the pointer arithmetic confusion made me not even think of that...

[jonnin]

You may also see:
int arr[len] = {0};

and if re-clearing it to re-use it:

memset(arr, 0, len*sizeof(int)); //does not work for everything, but ints & doubles etc are ok.

[JLBorges]

> and if re-clearing it to re-use it:
> memset(arr, 0, len*sizeof(int)); //does not work for everything, but ints & doubles etc are ok.

This would work for everything, and it would call std::memset() if appropriate (for trivial types).

#include <algorithm>

template < typename T > 
void fill( T* arr, std::size_t sz, const T& v ) { std::fill_n( arr, sz, v ) ; }

template < typename T > 
void clear( T* arr, std::size_t sz ) { fill( arr, sz, T() ) ; }

With C++, we can get (and should use) abstraction with zero or minimal run-time cost.

(For some reason, godbolt was failing at creating a short url)
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