Iterating through c array through function

Question

So I'm trying to figure out a way to iterate though an array passed into a function without knowing the size. I run into infinite loops with my code because the arrays are not NULL terminated. Since the array is turned into a pointer through the function, I cannot use sizeof(Array)/sizeof(int) to get the number of elements. Is there any way of doing this without NULL terminating my arrays?

My find function:

int find(const int* arr, int val)
{
    int pos = 0;
    while (arr != NULL)
    {
        if (*arr == val)
        {
            return pos;
        }
        pos++;
    }
    return -1;
};

My main:

int IntArr[] = { 1, 2, 3, 4, 5 };
int index = find(IntArr, 4);
cout << "find(IntArr, 4)" << endl;
cout << "index: " << index << endl << endl;

If you don't know the size, you are SOL. If you guess, you will guess wrong. You say there's no terminator? Ok, then you have to pass the lenghth some other way, like in an additional argument. — Deduplicator
– Deduplicator, Commented Sep 28, 2014 at 18:33
Pass the size of the array, or use std::array or std::vector and pass by reference, as they know their size (or pass their begin and end iterators) — Neil Kirk
– Neil Kirk, Commented Sep 28, 2014 at 18:35
You can never "NULL terminate" an array of int since NULL is a pointer value. This is the perennial problem with sentinel values, there is nothing you can put into an int that can't be interpreted as representing a valid int. If your array was only supposed to contain positive integers, then you could use a sentinel such as -1, but if it could contain any integer at all then no sentinel value is appropriate. NULL only works for pointers because C demands that 0 can never be a valid address. — Crowman
– Crowman, Commented Sep 28, 2014 at 18:44

Vlad from Moscow · Accepted Answer · 2014-09-28 19:08:49Z

1

For example you could define a template function that accepts an array by reference

template <size_t N>

int find( const int ( & arr )[N], int value )
{
    int pos = 0;

    while ( pos < N && arr[pos] != value ) ++pos;

    return pos == N ? -1 : pos;
}

Take into account that there is standard algorithm std::find declared in header <algorithm>. You could write for example

#include <algorithm>
#include <iterator>

//..

int IntArr[] = { 1, 2, 3, 4, 5 };
auto ptr = std::find( std::begin( IntArr ), std::end( IntArr ), 4 );
cout << "find( std::begin( IntArr ), std::end( IntArr ), 4)" << endl;
cout << "index: " << std::distance( std::begin( IntArr ), ptr ) << endl << endl;

edited Sep 28, 2014 at 19:08

answered Sep 28, 2014 at 19:02

Vlad from Moscow

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1 Comment

user1804599 Over a year ago

Or just use std::find and pass two iterators.

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