Arrays make the modern world happen. The computer itself can be viewed as one big array, lining up all our data, music, photos, and movies on disks and in RAM. However, arrays have their limits. In particular, arrays are fixed in size. We need to know their size when we create them. Sometimes the size is not information we have. Adding something to an array is not possible. It’s also awkward to remove something from an array. We might null or zero out an element, but its shadow is still there. The length property will report the same number throughout the array’s lifetime.
Enter ArrayList. It wraps around an array in Java and makes it feel resizable. Internally, it keeps an array of some fixed size. When it fills up, it creates a new array that’s bigger, copies over the data, and ditches the old array. Just like you wrap around a pair of shoes.
For the most part, we can treat arrays and ArrayLists the same. Their interface is slightly different. The following table contrasts their operations:
operation
array
ArrayList
create
type[] id = new type[#];
ArrayList<Type> id = new ArrayList<Type>();
get count
id.length
id.size()
read item
id[i]
id.get(i)
write item
id[i] = expr;
id.set(i, expr)
append item
not supported
id.add(item)
remove item
not supported
id.remove(item) or id.remove(index)
Let’s see ArrayLists in action through the following exercises:
loading up a word list
playing the card game War
A well-dressed man approaches you with two decks of cards in hand. He says, “Let us play a game. Here’s your deck. Here is mine. We turn over our cards one at a time. If any match, I win and you give me $5. If none match, you win and I give you $5.” Should you play this game?
We’ll solve the last of these through a Monte Carlo simulation. Such a simulation can be summarized as follows: simulate a phenomenon n times and compute your expected value of some measure of interest as the average across all runs. Here’s an interesting historical note about the first formal description of the Monte Carlo simulation from physicist Stanislaw Ulam:
The first thoughts and attempts I made to practice [the Monte Carlo Method] were suggested by a question which occurred to me in 1946 as I was convalescing from an illness and playing solitaires. The question was what are the chances that a Canfield solitaire laid out with 52 cards will come out successfully? After spending a lot of time trying to estimate them by pure combinatorial calculations, I wondered whether a more practical method than “abstract thinking” might not be to lay it out say one hundred times and simply observe and count the number of successful plays. This was already possible to envisage with the beginning of the new era of fast computers, and I immediately thought of problems of neutron diffusion and other questions of mathematical physics, and more generally how to change processes described by certain differential equations into an equivalent form interpretable as a succession of random operations. Later [in 1946], I described the idea to John von Neumann, and we began to plan actual calculations.
Ulam and von Neumann used the ENIAC, one of the first programmable computers, to simulate various physical phenomena for the Manhattan Project and the hydrogen bomb. We’ll just use it to for gambling.
Here’s your TODO to complete before we meet again:
Midterm 2 happens on Monday (November 21). Topics we’ve covered since the last exam include the various classes of operators that deal with booleans, conditional statements, loops, and arrays. The exam is structured much like the last—with some multiple choice and fill in the blanks followed by a few exercises where you must write code.
The amnesty period opens Saturday, November 19, and closes Sunday, November 27. During this time you can resubmit missed homework assignments (only homeworks are eligible, no labs). I will regrade a homework submission under the following conditions:
For each homework you want regraded, you correctly complete one of the tasks below.
You fix your homework submission, commit, and push to Bitbucket.
You email me one message per regrade request with the following: the number of the homework you have resubmitted, your code and a screenshot that shows that you completed the task.
