accessing array's last element when passing array in function argument shows garbage value

Question

I'm passing an array in function argument and when I tried to access the last element using arr[-1] in the function's body I'm getting some garbage. Can someone explain me why is that so?

Here is the code:

#include<stdio.h>

using namespace std;

int binsearch(int nelement, int arr[],int maxsize)
{
    int low = 0;
    int high = maxsize;

    printf("%d",arr[-1]);
    return 0;       
}

void main()
{
    int arr[] = {1,2,3,4,5,6,7,8,9};
    int flag = 3;
    printf("%d\n",arr[8]);
    flag = binsearch(flag,arr,sizeof(arr));
}

And here is the output:

9
-858993460

Negative indicies are already discussed here: stackoverflow.com/questions/3473675/negative-array-indexes-in-c — Darko
– Darko, Commented Jan 22, 2013 at 17:46
What's up with that code? You're using namespaces, which is a C++ feature; and using <stdio.h>, which is C... You want to do C? Ditch the namespaces. — netcoder
– netcoder, Commented Jan 22, 2013 at 17:54
Unless this code is for an embedded system, it is not valid and will not compile in standard C nor C++, for several reasons. — Lundin
– Lundin, Commented Jan 22, 2013 at 19:13

Eric Postpischil · Accepted Answer · 2013-01-22 18:09:36Z

3

arr[-1] does not access the last element. Conceptually, it access the space before the first element, but the behavior is not defined.

To access the last element, use arr[nelement-1] or arr[NumberOfElements-1], depending on what you mean by the last element. (Are you considering the array to contain only nelement elements, or do you want the last element in the entire allocated array? If the latter, you will need to know the number of elements in the array in order to calculate the index of the last one.)

edited Jan 22, 2013 at 18:09

answered Jan 22, 2013 at 17:26

Eric Postpischil

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3 Comments

Hemant Over a year ago

actually when I use python we usually do this to access the last element of array and when i read it into my school notes i found the same, that is why i got confused why it is showing me garbage value. Anyways thanks for clarifying it.

alk Over a year ago

Using arr[maxsize-1] will run into undefined behaviour for sure as well, as depending on the size of an int maxsize is twice or fours or eights times as large as the array carries int elements.

Eric Postpischil Over a year ago

@alk: Fixed; I mistook their maxsize for the maximum number of elements, rather than the array’s size in bytes.

alk · Accepted Answer · 2013-01-22 18:07:09Z

2

Array indicies shall be unsigned. Using a negative value as index to an array provokes undefined behaviour.

As a side note:

This code won't pass the number of (int) elements down to the function:

int arr[] = {1,2,3,4,5,6,7,8,9};
...
flag = binsearch(flag, arr, sizeof(arr));

As the sizeof operator returns the number of bytes used by arr.

To pass the number of ints the array provides you might like to used the following construct:

flag = binsearch(flag, arr, sizeof(arr)/sizeof(arr[0]));

edited Jan 22, 2013 at 18:07

answered Jan 22, 2013 at 17:42

alk

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Comments

sr01853 · Accepted Answer · 2013-01-22 18:09:19Z

0

Since maxsize is sizeof(arr) and arr is an array of int - it is the (sizeof(int) * no_of_elements_of_arr)

it must be arr[(maxsize/sizeof(int)) - 1] in place of arr[-1]

arr[-1] is expected to print some garbage value

edited Jan 22, 2013 at 18:09

answered Jan 22, 2013 at 17:32

sr01853

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1 Comment

alk Over a year ago

Using arr[maxsize-1] will run into undefined behaviour for sure as well, as depending on the size of an int maxsize is twice or fours or eights times as large as the array carries int elements.

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