Lab 2🔗

2.1 Help!🔗

DrRacket has built-in documentation on every function you’ll be using. To access it, put the cursor on an identifier and hit F1. Or choose Help|Help Desk. This documentation is local, and should work fine even without network.

You can also find this documentation online at https://docs.racket-lang.org/, though that source won’t include all of your locally-installed documentation, and it’s obviously not available when you don’t have network.

2.2 Highlighting by holding Shift🔗

When you hold down shift while pressing cursor movement keys, the intervening characters are selected. Try putting the cursor somewhere in your code, and holding shift-down.

I know, you’re not suprised. That’s okay. Unless you were. That’s okay, too!

2.3 Cutting and Pasting using keystrokes🔗

You can use cmd-x or ctrl-x (by platform) and cmd-v or ctrl-v (by platform) to copy and paste. Don’t use the mouse.

Yep, we’re still in Notepad territory.

2.4 Paren Matching🔗

When the cursor is outside of a parenthesis, the matching pair of parentheses are highlighted, along with everything inside. You knew that already, right?

2.5 Moving by s-expressions🔗

You may have noticed that there are a lot of parentheses in Racket. It turns out that this can be a blessing, as well as a curse. By using <special_key>-left and <special_key>-right, you can move the cursor by a matched set of parentheses or datum, also known as an "s-expression". So, for instance, if your cursor is at the beginning of the program, you can use <special_key>-right twice to move down by two full expressions.

On Windows, the special key is alt, and on MacOS the special key is option.

#lang racket

(define (f x)

(* (- (* x x)) (+ x 9) 4))

(define (g x)

(* x x 5))

Use <option>-left and <option>-right to move back and forth through the top-level expressions.

Then, use <option>-left and <option>-right to move back and forth through the arguments to the first * in the definition of x.

The <option>-up combination is also extremely useful; it moves the cursor to the outside of the enclosing expression. Using <option>-up rather than the simple arrow keys means that you’re always in a position to highlight or cut the given s-expression. Also, popping to the outside of a long function with two or three <option>-up’s is much faster than hitting the up key a whole bunch of times.

2.6 Highlighting using s-expressions🔗

You can combine the last two subsections; by holding <shift> while using <option>-left and <option>-right, you can highlight one or more s-expressions. This is most useful when you’re planning to cut them, or replace them (or wrap them in parens... but we’re not there yet).

2.7 Auto-tabbing🔗

Like any other sane editor, DrRacket can indent code for you. When a block of code is highlighted, you can hit <tab> to reindent that block.

Okay, time for a simple exercise.

Here’s a piece of code.

(define (show-example b)

(begin

(printf "magnitude of sum of elements: ~s\n"

(array-map magnitude (array-axis-fold (my-fft b) 0 + 0)))

(printf "sum of magnitude of elements: ~s\n"

(array-axis-fold (array-map magnitude (my-fft b)) 0 + 0))

(plot (points (in-array (array->plottable (my-fft b))))

#:y-max 1500

#:x-max 1024

#:height 300)))

Use the s-expression highlighting keys to highlight just the first printf. Hit <tab>. Note that the printf is now aligned relative to the line before it, but nothing else has changed.

Now, use the s-expression-highlighting keys to highlight the whole function definition. (Not the mouse! Please!) Hit <tab>. See that the whole thing gets re-indented all at once.

Sometimes, you’ll notice that things don’t go where you expect, when you re-indent. This is usually a sign that you have an extra or missing parenthesis, somewhere.

Next simple exercise:

Use the s-expression highlighting expressions to highlight the first printf. Use cmd-x and cmd-v to cut it and paste it outside of the show-example function.

2.8 Automatic Parens🔗

Okay, here’s where the magic gets better. In the Preferences > Editing > General box, check the "Enable Automatic Parentheses" option.

Now, something strange happens; when you type a left-paren ’(’, you’ll see a matching pair of parentheses appear, with the cursor in the middle.

It’s nice not to have to type the closing parenthesis, but it’s even nicer that when you use this binding, your parentheses are never unbalanced.

But wait! What if you want to wrap parentheses around a set of existing expressions? No problem. Just highlight them, using the keystrokes we’ve already described, and then type the left paren. You’ll see a pair of parentheses, wrapped around the highlighted expressions.

The same keystrokes also work for the square bracket, ’[’.

There’s even more in this space, but I think we’d better stop here for a bit.

Here’s a simple exercise; you should be able to complete this without having unbalanced parentheses.

In the following code, change the "if" into a cond. First, change the keyword "if" into "cond". Then, highlight the two following expressions, and wrap them with square brackets. Then, highlight the third one, beginning "append", and wrap it with square brackets, and insert the word "else" following the open bracket. Done!

(define (srl:map-append func lst)

(if (null? lst)

lst

(append (func (car lst))

(srl:map-append func (cdr lst)))))