Question

I am reading about queues in robert sedwick book on algorithms

When the items in the data structure are themselves array indices, so
we refer to such item as “index items”. Typically, we have a set of M
objects, kept in yet another array, that we need to pass through a
generalized queue structure as a part of a more complex algorithm.
Objects are put on the queue by index and processed when they are
removed, and each object is to be processed precisely once. Usually
array indices in a queue with no duplicates accompliches this goal
directly.

My question in last sentence “Objects are put on the queue by index and processed when they are removed, and each object is to be processed precisely once” ? we are using only one array instead of two arrays?

What does author mean by “Usually array indices in a queue with no duplicates accompliches this goal directly.” ?

Thanks for your time and help

Answer 1

Well, the author wants to address the algorithmic task of processing data that’s stored in an array:

       +-----+-----+---------+-----+
Data = | Foo | Bar | Grandma | Zip |
       +-----+-----+---------+-----+

We need to process this data in some sort of order that’s determined by our algorithm, and there’s some sort of “to do” queue of items we want to process next. Copying the actual data objects may be undesirable or impossible (objects may be large or uncopyable). A queue of indexes does the trick:

             --+---+---+--\
ToDo = [2] --> | 0 | 3 | -----> (1)
             --+---+---+--/

The queue tells us that Data[1] is the next item to be processed. Data[3] and Data[0] are waiting, and we just decided that Data[2] comes in as the most recent task.

(For example, queues are used in a breadth-first search of a tree structure: You pop the node that you want to visit next off the queue on one side and push the children of that node in as future work on the other side. Each node should be visited exactly once. The above index queue lets you store the actual tree elements in the Data array, and refer to them by a light-weight index only.)