*** stack smashing detected *** in a C++ array program

Question

I'm writing a simple program which simulates a real world physics problem about electron spin. It should fill an array of 10 with 1 or -1 randomly. then for 100 times choose one element and do the following conversion based on it's neighbors:

+-+ ..... +++
-+- ..... ---
+++ ..... +++
--- ..... ---

and in the following cases randomly pick a result:

-++ ..... --+ or -++
--+ ..... --+ or -++
+-- ..... ++- or +--

etc

but when i run it i get this error: * stack smashing detected * : ./a.out terminated Aborted (core dumped)

I checked and it's on the last "else" two lines. it doesn't give error when i comment them.

My code:

#include<iostream>
#include<fstream>
#include<stdlib.h>
using namespace std;
int main(){
    srand(time(NULL));
    int arr[10],a[2],i,curr,next,prev,j;
    double r;
//Fill arr[10] with random -1 & 1
    a[0]=-1; a[1]=1;
    for(i=0;i<10;i++){
        r=rand()/(RAND_MAX+1.0)*2.0;
        arr[i]=a[int(r)];       
    }
//Cout arr[10]
    for(i=0;i<10;i++)cout<<arr[i]<<" ";
    cout<<'\n';
//Repeat for 100 times
    for(j=1;j<=100;j++){
        r=rand()/(RAND_MAX+1.0);
        r=(r)*11;
        curr=int(r);
        next=(curr+1)%11;
        prev=(10+curr)%11;
        if(arr[prev]==arr[next]){
            if(arr[prev]==1)arr[curr]=1; else arr[curr]=-1;
        }
        else
        {
            r=rand()/(RAND_MAX+1.0)*2.0;
            arr[curr]=a[int(r)];
        }
    }
//Cout arr[10]
    for(i=0;i<10;i++)cout<<arr[i]<<" ";
    cout<<'\n';
    return 0;
}

Thanks in advance.

Answer 1

    r=(r)*11;
    curr=int(r);
    //...
    arr[curr]=//...

r can take a value of 10.X at low probability, causing curr to take the value of 10, assigning to arr at 10, which corrupts memory.