Can you avoid out of range error if you use range based for loop instead of the standard for loop?

Question

I was experimenting around with range-based for loop found that if you use range-based for loop to loop through a vector it runs into out of range error is there any way to avoid this error if you are using range-based for loop

int main()
{
    vector<int> num{ 1,2,3,4,5 };
    for (auto i : num)
    {
        cout << num[i] << endl;
    }
}

it shows this error

Here you do `cout << num[num[0]] << endl;`, which is problematic for last element — Jarod42, Mar 30 '21 at 14:59
I tend to not use name `i` here, as we might expect `i` for index as you do, `for (auto elem : num)` seems better. — Jarod42, Mar 30 '21 at 15:01
Yes, you can avoid 'out of range' if you are aware of what you're doing. Did you check what values are assigned to `i` in such `for` loop...? Test with a vector e.g. `{1, 2, 0, 0, 0}` — CiaPan, Mar 30 '21 at 15:05

score 5 · Accepted Answer · answered Mar 30 '21 at 15:05

5

It's nothing to do with the type of loop. Your loop looks at all the values i inside the vector num, then it prints num[i]. One of them is 5, so it prints num[5]. But num has 5 elements so they only go up to num[4]. Out of range.

answered Mar 30 '21 at 15:05

user253751

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Thanks alot you cleared up my misunderstanding @user253751 – AUNG Mar 30 '21 at 15:11

score 1 · Answer 2 · answered Mar 30 '21 at 15:40

template<class It>
struct indexer {
  It it;
  It operator*()const{return it;}
  void operator++(){++it;}
  void operator++(int){++it;}
  bool operator==(indexer const& o)const{ return it == o.it; }
  bool operator!=(indexer const& o)const{ return it != o.it; }
};
template<class It>
struct range {
  It b,e;
  It begin()const{return b;}
  It end()const{return e;}
};
range<indexer<std::size_t>> counting( std::size_t begin, std::size_t end ) {
  std::cout << begin << "->" << end << "\n";
  return {{begin}, {end}};
}

template<class X>
auto indexes_of( X const& x ) {
  using std::size;
  return counting(0, size(x));
}

now I can make your code do what you thought it did:

vector<int> num{ 1,2,3,4,5 };
for (auto i : indexes_of(num))
{
    cout << num[i] << endl;
}

Live example.

Your problem was mistaking indexes into the vector for the elements. Range-for returns the elements. indexes_of makes it return the indexes for the container in question.

thanks for showing me a detailed example @Yakk - Adam Nevraumont — AUNG, Mar 30 '21 at 16:46

Can you avoid out of range error if you use range based for loop instead of the standard for loop?

2 Answers2