Racket Definitions Video 4/4 - Define function, by design

Here's Doctor Racket! You'll require picturing-programs. It provides, among other things, some examples of images. Click Run, and we'll see all's well, and what those images look like. Now hide the interactions pane, so we're working just in definitions... We are going to define a function rosette in a more systematic way and we did last time. So, first we start off with a contract... input and output both images in this case. Next, a purpose statement. these lines that begin with semicolons are ignored by Doctor Racket... but they're really important to human beings who are reading and planning programs. First, let's check what we expect rosette to do. Here's an example. What do we expect rosette do do with the image of hacker? Well, it's going to have to put two flipped versions of hackers side by side. So one flipped version puts hacker above its vertically flipped self... And that's how we do that. That's enough typing that we should probably copy it for later use. So, what we're going to do is put that beside it's horizontally-flipped self. There, that completes our expected version of (rosetta pic:hacker) What do we expect rosette to do with the calendar image? Well, here's what we expect: something very similar to what it did with the hacker image, except calendar's gonna appear everywhere that hacker occurred before. So, once again we're putting the vertically flipped pair of calendars beside a horizontally flipped version of themself. I can put that on another line... Dr Racket will just indent it dr racquetball discontented where it takes we mean it to be indented. Now, I define the function header... ...key word define... left paren... the name of the function and any placeholders, or parameters I'll need. The function's name is rosette. We're gonna use the pic placeholder everywhere we have an image. So, it would stand for calendar, if we were applying rosette to calendar. It would stand for hacker if we were applying rosette to hacker. To start with, we'll just do a stub; we'll do something totally stupid and useless that couldn't possibly be what we expected just to get our foot in the door. So, if I run this... not completely surprisingly... it doesn't work; it reports that I failed all the tests. It does better than that, it tells me how I failed. It says the actual value, that empty circle is different from what was expected, that 2x2 array of hackers. The second test failed for similar reasons: the actual value doesn't match 2x2 array of calendars. So, we'll go back and try to fix this. Again, we'll work just in the definitions pane, and we get some inspiration by looking at our examples. They're very similar form of instructions. The only thing that really changes: the actual images that are provided. So, we can emulate the approach in our examples to create the function. Same sequence of instructions, except everywhere that hacker or calendar appeared we put our placeholder, pic. Fast to type, because pic's nice and short too. Done. so now we run this... And it reports that we passed tests; that we are in fact correctly producing the rosette of hackers and the rosette of calendars. yippee isn't legal, but it's fun to type sometimes. Let's review this recipe for functions. We need a contract. We need a statement of purpose(s). We need to state our expectations using check-expect. We need to define a function header... that defines the name of the function, rosette in this case, and any parameters we'll need as placeholders. Finally, we need a function body. It should either work or at least be a stub, to hold the place for something that does work.