Unexplainable behaviour of array

[Parul Bagadia]

hello ppl,

#include<iostream.h>
int main()
{
    int number=0;
    int A[2]={0};
          cout<<"Enter the number of elements you want in SET A"<<endl;
    cin>>number;
    for (int i=0;i<number;i++)
    {
        cout<<"Enter "<<i+1 <<" element"<<endl;
        cin>>A[i];
    }
    return 0;
 
}

Here i know that as i have taken array size as 2 if value of number increases than that of the value of i,program should crash down, but whtas happening here is, if i enter the value of i as 4 its asking for the value of elements 8 times ; when i put the value of i as 3 its asking for value for 9 times, when i=8, elements askes=4, when i=5 elements asked=6.....
I debugged the program but its invain...
Actually here what i want to know is when i have given the limit of array as 2, first of all after 2 elemnts it should crash down; but then its taking garbadge value of i that's explainable to bit extent... but once the for loop is over for e.g when i take value of i as 4, how its getting executed 8 times??

[gpraghuram]

If you specify the size of array as 2 and copy over 5 elements it is array overwrite.
You wont see the crash directly.
If the array overwrites the memory occupied by your program it wont crash . But if it tries to touch anyother process's memory then you will see the crash.


Raghu

[Parul Bagadia]

@gpraghuram
Thanx.
But what's happening in my program is if i take the value of number variable as 4 , the for loop gets executed 8 times this should not happen even if array is getting overwritten right?

[Banfa]

No as soon as you over-write the bounds of the array you invoke undefined behaviour. Undefined behaviour is... well... undefined :D That means anything could happen including a program crash, the program running as expected (this time) or extra iterations of the loop.

There is little point trying to explain undefined behaviour as it may not be the same 2 times in a row.

However in your case a simple guess would be that when you write outside the bounds of your array the next bit of memory you write to is the memory containing number so number is changed and you end up entering a different number of numbers to that requested.