vector list initializer and assignment

Question

For the two snippet of code, i don't understand the second one

1st: it prints 0 1 2 as expected

  vector<int> ivec;
  ivec={0,1,2};
  for(auto i: ivec){
    cout<<ivec[i]<<endl;
  }

2nd: it prints 0 0 0 5. How is this random value generated?

  vector<int> ivec;
  ivec={0,1,2};
  ivec={4,5,9,1};
  for(auto i: ivec){
    cout<<ivec[i]<<endl;
  }

Answer 1

This is undefined behavior because ivec has size 4 yet you are indexing elements such as 4 , 5 , and 9 which are out of bounds.

for(auto i : ivec){
  cout << ivec[i] << endl;
}

Instead you should be printing the elements themselves

for(auto i : ivec){
  cout << i << endl;
}

To be clear, in your range-based for loop i is the actual element not the index of the element. If you wanted to use indices you would not use a range-based for loop

for(std::size_t i = 0; i < ivec.size(); ++i){
  cout << ivec[i] << endl;
}

Answer 2

this loop here:

for(auto i : ivec)

is not the same as

for(int i =0; ...)

when you do

for(auto i: ivec){ // with ivec value to {4,5,9,1};
   cout<<ivec[i]<<endl;
} 

then you do:

 cout << ivec[4] << endl; // Undefined Behav...
 cout << ivec[5] << endl; // Undefined Behav...
 cout << ivec[9] << endl; // Undefined Behav...