Strange result upon pre-increment and post-increment inside an array in C++

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C++

I searched this site for similar question but none dealt with this in-depth. I'm sorry if this is the most overused question ever in this site. But this piece of code is very puzzling.

#include<bits/stdc++.h>
using namespace std;
int main(){
int a[3];
cout<<"Enter number to be inserted: "<<endl;
int num;//Value that is intended to be inserted in a[0].
int index;//To keep track of array index.
cin>>num;

cout<<"Postincrement"<<endl;
index=-1;
a[index++]=num;
cout<<a[index]<<endl;
cout<<index<<endl;

cout<<"Preincrement"<<endl;
index=-1;
a[++index]=num;
cout<<a[index]<<endl;
cout<<index<<endl;
}

What I have tried:

3
Postincrement
-606760192
3
Preincrement
3
0

Preincrement works perfectly. Element is inserted in the a[0] and index is 0.

I don't get why the value of the index becomes the number(3 in this case) on post increment. Shouldn't num be inserted inside the array (a[0]).How could the number be assigned to the index when it's been specified clearly that a[index++]=num. Can anybody help me understand this better?