can I print the index of an element in an object vector?

user_18971530 0
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So I know to print something in a loop, the code looks like this:

for(int I = 0; I < num; ++I)...

And to print an object vector:

for(Square sq : squares)...

(if I have a class Square and I create the object sq and squares is the name of the vector)

But how would I go about code if I want my output to look like:

square 1 area: 3 square 2 area: 6 square 3 area: 9

To be more clear: my question is, how do I incorporate "I" like in the first example in a loop where I'm printing objects?

vector loops c++
3 Answers

You can do it like this:

for (size_t idx=0; idx<squares.size(); ++idx)
{
    Square const & sq = squares[idx];

    // Here you can use both:
    //      idx (which is the index in the vector), 
    //      and sq (reference to the element).
}

The fact that sq is a std::vector does not mean you must traverse it using the range-based loop (Range-based for loop).

A std::vector has a method for getting its size (std::vector::size), and operator[] to access an element (std::vector::operator[]).

Note - even if you do use the range-based loop, it is better to use a refernce (or const reference) to avoid unnecessary copies:

for(Square const & sq : squares)   // without `const` if you need to modify the element
{
   //...
}
user_185199210

You can also combine range based for and another index variable

// with c++20 range-based for can have initializer in it
for(auto i=1; auto& s:squares){ 
   std::cout << "square " << i << " area: " << s << '\n' ;
   ++i;
}

https://godbolt.org/z/EfhefdoGP

pre-c++20

{
   auto i=1;
   for(auto& s:squares){ 
      std::cout << "square " << i << " area: " << s << '\n' ;
      ++i;
   }
}
user_59804300

Because the elements of the vector are lined up continuously... It may be possible to do something like this.

class Object
{
public:
    Object( int a, int b ) : a(a),b(b) {}
    int a,b;
};

int main()
{
    std::vector<Object> Vec{ {1,2}, {10,20}, {100,200} };
    for( const auto &obj : Vec )
    {
        auto index = &obj - &Vec[0];  //calculate index of element
        std::cout << index << "  : " << obj.a << "," << obj.b << std::endl;
    }
    return 0;
}
user_183620540
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