How to get the index of a value in a vector using for_each?

Question

I have the following code (compiler: MSVC++ 10):

std::vector data;
data.push_back(1.0f);
data.push_back(1.0f);
data.push_back(2.0f);

// lambda          


        

Answer 1

I don't think you can capture the index, but you can use an outer variable to do the indexing, capturing it into the lambda:

int j = 0;
std::for_each(data.begin(), data.end(), [&j](float const& value) {
            j++;
});
std::cout << j << std::endl;

This prints 3, as expected, and j holds the value of the index.

If you want the actual iterator, you maybe can do it similarly:

std::vector<float>::const_iterator it = data.begin();
std::for_each(data.begin(), data.end(), [&it](float const& value) {
            // here "it" has the iterator
            ++it; 
});

Answer 2

Alternatively, you can use &value - &data[0], although it might be a bit more expensive.

std::for_each(data.begin(), data.end(), [&data](float const& value) {
    int idx = &value - &data[0];
});

Answer 3

You could also pass a struct as third argument to std::for_each and count the index in it like so:

struct myStruct {
   myStruct(void) : index(0) {};
   void operator() (float i) { cout << index << ": " << i << endl; index++; }
   int index;
};

int main()
{

   std::vector data;
   data.push_back(1.0f);
   data.push_back(4.0f);
   data.push_back(8.0f);

   // lambda expression
   std::for_each(data.begin(), data.end(), myStruct());

   return 0;
}

Answer 4

Following the standard convention for C and C++, the first element has index 0, and the last element has index size() - 1.

So you have to do the following;-

std::vector<float> data;
int index = 0;

data.push_back(1.0f);
data.push_back(1.0f);
data.push_back(2.0f);

// lambda expression
std::for_each(data.begin(), data.end(), [&index](float value) {
// Can I get here index of the value too?
   cout<<"Current Index :"<<index++; // gets the current index before increment
});