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c++c++11

Access index in range-for loop

I have a vector of objects and am iterating through it using a range-for loop. I am using it to print a function from the object, like this:

vector<thisObject> storedValues;
//put stuff in storedValues
for(auto i:storedValues)
{
   cout<<i.function();
}

But I want to print the index too. My desired output is: 

1: value
2: value
//etc

I was going to just use a counter that I increased each time, but that seemed very inefficient. Is there a better way?

Solution

  • You can't. The index is a specific notion to a vector, and not a generic property of a collection. The range-based loop on the other hand is a generic mechanism for iterating over every element of any collection.

    If you do want to use the details of your particular container implementation, just use an ordinary loop:

    for (std::size_t i = 0, e = v.size(); i != e; ++i) { /* ... */ }

    To repeat the point: Range-based loops are for manipulating each element of any collection, where the collection itself doesn't matter, and the container is never mentioned inside the loop body. It's just another tool in your toolbox, and you're not forced to use it for absolutely everything. By contrast, if you either want to mutate the collection (e.g. remove or shuffle elements), or use specific information about the structure of the collection, use an ordinary loop.