Array
Array - A DVD box❓
Suppose you had a bunch of DVDs at home that you wanted to arrange neatly. What would be the ideal choice for storing such a thing? You could find a cardboard box (or some other box) big enough to arrange all of the DVDs neatly, right? It's as simple as that. However, you might want to add a new DVD to the box, or you might want to get rid of the old ones that you've watched a million times over in the past. An important consideration for this box would be that you would only place DVDs in it and nothing else; you wouldn't place your clothes in it, for example. The box would contain multiple items, but all of them would be of the same type. In this case, that type is DVD. Items of the same type share properties. For DVDs, those properties include:
- All the DVDs would be inside a plastic cover.
- The cover would have the name of the movie, the cast, and all sorts of other details.
- All the covers would be of exactly the same size and would contain just one, and only one, DVD.
You might not actually name the DVD box, but when you want your sister to fetch a DVD, you'd tell her that the DVD is inside your "DVD box", and she would instantly know where to find the box. This is a very simple yet realistic scenario that is easy to understand and relate to. So, now let us move over to the world of computers and port this example to programming.
Suppose you were told that you needed to build some software to keep track of all the DVDs in an inventory. This is the exact same scenario that we just described above, but on a much larger scale. So let's imagine the DVD box as a virtual DVD library. For each DVD, you would have certain properties that would be specific attributes of the movies themselves.
In addition to the properties of a DVD, you're also told the maximum number of DVDs that can be stored in the inventory. Obviously, you wouldn't want to store ancient movies from the 1900s unless they were popular ones, right? Say you were told that the requirement is to maintain a maximum inventory of just 100 DVDs. This is an important piece of information because, without this, you wouldn't be able to find the perfectly sized box to fit all the DVDs easily. How could we find a box of a particular size that would be able to fit a maximum of 100 DVDs? Well, lucky for us, we don't need to physically find a cardboard box or anything—there's a programming construct for this purpose. That programming construct is known as an Array.
SO let's Find the Definition:
An Array is a collection of items. The items could be integers, strings, DVDs, games, books—anything really. The items are stored in neighboring (contiguous) memory locations. Because they're stored together, checking through the entire collection of items is straightforward.
So, how can we relate this back to the physical DVDs? Well, do you keep your DVDs all around the house in multiple locations? Hopefully not! Most people keep all of their DVDs right next to one another inside one gigantic box, or perhaps on a bookshelf. We do this so that if we need to find a particular DVD, we can quickly search through all of them without running from room to room.
Creating an Array
On a computer, Arrays can hold up to N items. The value of N is decided by you, the programmer, at the time you create the Array. This is the same as when we found a big enough cardboard box for the DVDs. Additionally, you also need to specify the type of item that will be going into the Array.
In Java, we use the following code to create an Array to hold up to 15 DVDs. Note that we've simple definition of a DVD for clarity.
After running the above code, we now have an Array called dvdCollection, with 15 places in it. Each place can hold one DVD. At the start, there are no DVD's in the Array; we'll have to actually put them in.
The Array can only hold up to 15 DVDs. If we get a 16th DVD, we'll need to make a new Array. We'll look at how we deal with running out of space, in the next chapter.
Before we move onto actually putting some DVDs into the Array, though, one thing you might be wondering is why we'd make an Array with only 15 places. Why not just make it hold 1000000 DVDs so that we know for sure we'll always have enough space?
Well, the reason is the same as it is for the physical box of DVDs. Do you really want to find a box that could hold 1000000 DVDs when you currently only have 15 DVDs and, in fact, never expect to own more than 100 of them? Is it even worth getting a box that could hold 100 DVDs right now, when you only expect to get a few new ones each year? It will take up a lot more space in your home in the meantime.
It's exactly the same with the Array, where the space in your home is analogous to memory on the computer. If you make an Array with 1000000 spaces, the computer will reserve memory to hold 1000000 DVDs, even if you only put 15 DVDs into it. That memory can't be used for anything else in the meantime—just like the space in your house that has been taken over by that huge cardboard box!
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