C++ weird array behaviour [duplicate]

Question

I was debugging my code for key index counting and found this problem. I do not understand what is happening here. I've looked at the code for too long to see if I am missing something very obvious, but it does not seem like it.

 int main()
{
    const int r=7,len=10;
    int arr[10]={1,4,6,2,0,4,3,6,5,2};
    int count[r+1]={0};
    for(int i=0;i<len;i++)
    {
        count[arr[i]+1]++;
    }
    cout<<arr[0]<<" ";

    for(int i=0;i<r+1;i++)
    {
        count[i+1]+=count[i];
    }
    cout<<arr[0]<<" ";
    return 0;
}

This is the kind of a mock up code which generates the same bug.

Output:-

1 11

I am not changing value of arr anywhere in my program and still it shows 11 instead of 1 in the output.

if I comment out count[arr[i]+1]++; or count[i+1]+=count[i]; or both it gives the correct output.

1 1

What is happening please explain. (comment if I am doing something silly).

Edit: This is only happening with arr[0].

Typical buffer overflow issue. Use a debugger to step through the code, statement by statement while monitoring the values of all variables. It helps if you break up complex expression into simpler expressions (like count[arr[i]+1]++; could be broken into int temp = arr[i]; count[temp + 1]++;). — Some programmer dude
– Some programmer dude, Commented Jan 2, 2021 at 10:54
As a hint, think about what possible values i can be in the second loop. And what index i + 1 then would be. — Some programmer dude
– Some programmer dude, Commented Jan 2, 2021 at 10:56
@Poseidon I believe this compiler warning pretty well explains what's going on. Futile to investigate why code which has undefined behavior, behaves like it does. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jan 2, 2021 at 10:59
@Someprogrammerdude Oh, Thank you I knew I was doing something silly!!!! — Poseidon Broger
– Poseidon Broger, Commented Jan 2, 2021 at 11:00

lucidbrot · Accepted Answer · 2021-01-02 10:59:31Z

1

Compiling it with g++ -Wall -Wextra you get this warning:

rando_so.cpp: In function 'int main()':
rando_so.cpp:15:19: warning: iteration 7 invokes undefined behavior [-Waggressive-loop-optimizations]
         count[i+1]+=count[i];
         ~~~~~~~~~~^~~~~~~~~~
rando_so.cpp:13:18: note: within this loop
     for(int i=0;i<r+1;i++)

This hints you to look a bit closer at that second loop. Your variable i goes up to the highest possible index of count - and then you add 1. That is undefined behaviour. In your case, it is likely that you happen to be writing into the first element of arr now, because of how it's laid out on the stack. But as far as I know, anything could happen as a result of this.

answered Jan 2, 2021 at 10:59

lucidbrot

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1 Comment

Poseidon Broger Over a year ago

Got it. Thank you. the 'anyhting can happen' part is the scariest thing as it might not have affected the program at all(at least that is what i understand) and I would've proceeded with my day.

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