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Get the element index when iterating with an indexed_view

Monday, 25 March 2019

One crucial difference between using an index-based for loop and a range-based for loop is that the former allows you to use the index for something other than just identifying the element, whereas the latter does not provide you with access to the index at all.

The difference between index-based for loops and range-based for loops means that some people are unable to use simple range-based for loops in some cases, because they need the index.

For example, you might be initializing a set of worker threads in a thread pool, and each thread needs to know it's own index:

std::vector<std::thread> workers;

void setup_workers(unsigned num_threads){
    workers.resize(num_threads);
    for(unsigned i=0;i<num_threads;++i){
        workers[i]=std::thread(&my_worker_thread_func,i);
    }
}

Even though workers has a fixed size in the loop, we need the loop index to pass to the thread function, so we cannot use range-based for. This requires that we duplicate num_threads, adding the potential for error as we must ensure that it is correctly updated in both places if we ever change it.

jss::indexed_view to the rescue

jss::indexed_view provides a means of obtaining that index with a range-based for loop: it creates a new view range which wraps the original range, where each element holds the loop index, as well as a reference to the element of the original range.

With jss::indexed_view, we can avoid the duplication from the previous example and use the range-based for:

std::vector<std::thread> workers;

void setup_workers(unsigned num_threads){
    workers.resize(num_threads);
    for(auto entry: jss::indexed_view(workers)){
        entry.value=std::thread(&my_worker_thread_func,entry.index);
    }
}

As you can see from this example, the value field is writable: it is a reference to the underlying value if the iterator on the source range is a reference. This allows you to use it to modify the elements in the source range if they are non-const.

jss::indexed_view also works with iterator-based ranges, so if you have a pair of iterators, then you can still use range-based for loops. For example, the following code processes the elements up to the first zero in the supplied vector, or the whole vector if there is no zero.

void foo(std::vector<int> const& v){
    auto end=std::find(v.begin(),v.end(),0);
    for(auto entry: jss::indexed_view(v.begin(),end)){
        process(entry.index,entry.value);
    }
}

Finally, jss::indexed_view can also be used with algorithms that require iterator-based ranges, so our first example could also be written as:

std::vector<std::thread> workers;

void setup_workers(unsigned num_threads){
    workers.resize(num_threads);
    auto view=jss::indexed_view(workers);
    std::for_each(view.begin(),view.end(),[](auto entry){
        entry.value=std::thread(&my_worker_thread_func,entry.index);
    });
}

Final words

Having to use non-ranged for loop to get the loop index introduces a potential source of error: it is easy to mistype the loop index either in the for-loop header, or when using it to get the indexed element, especially in nested loops.

By using jss::indexed_view to wrap the range, you can eliminate this particular source of error, as well as making it clear that you are iterating across the entire range, and that you need the index.

Get the source from github and use it in your project now.

Posted by Anthony Williams
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