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Code Club 3

We are continuing our stroll through the Prologue of HtDP2e.

(When we have finished this stroll, you will know much of the language. Then we will start over and talk about a more systematic way of designing programs.)

Recap of 02

Expressions are like little questions for Racket to answer. We have seen two ways to make expressions. An expression is either:

  1. A value; or

  2. ( function expression1 expression2 ... )

The value of the first kind of expression is itself. To evaluate the second kind of expression, Racket first evaluates all the sub-expressions, then applies the function to the values produced.

We also now know several kinds of value. A value is either:

  1. A number (42);
  2. A string ("Hello, World.");
  3. A "Boolean" (either #true or #false); or
  4. An image

And we know several built-in functions, for example:

  1. Arithmetic functions like +, -, log and so on;
  2. String functions like string-append, string->number and so on;
  3. Boolean functions like and, or, and not; and
  4. Image functions like circle.

Exercises

  1. How many centimetres are there in a foot? (There are 2.54 centimetres in an inch, and 12 inches in a foot.)

  2. How many letters are there in "Supercalifragilisticexpialidocious"? (Hint: remember there is a function string-length)

  3. Look in the documentation for string-ith. What is the 10th letter of "antidisestablishmentarianism"?

    The documentation is under "How to Design Programs Languages", under "1 Beginning Student".

  4. Look in the documentation for string<?. What is (string<? "hello" "goodbye")?

  5. What is (and (or #true #false #false) (or #false #false))? (Try to guess before entering it into Racket.)

  6. Put the following in the definitions window, and click Run (or select Run from one of the menus, or click ctrl-R [or command-R on a Mac]).

(require 2htdp/image)

Now try the next few. (Either in the interactions window or the definitions window.)

(circle 20 "outline" "orange")
(rectangle 10 20 "solid" "brown")
(triangle 20 "solid" "darkgreen")

More about images

Try the following in the definitions window:

(require 2htdp/image)

(above
 (triangle 30 "solid" "darkgreen")
 (rectangle 10 20 "solid" "brown"))

(From now on, I'll leave out the require.)

(beside
 (above
  (triangle 30 "solid" "darkgreen")
  (rectangle 10 20 "solid" "brown"))
 (above
  (triangle 30 "solid" "darkgreen")
  (rectangle 10 20 "solid" "brown")))

Definitions

What if I want lots of trees?

What if I want to make a tree of a given height?

Excerises

Here is the definition of a function to convert inches to centimetres:

(define (inches->cm x)
    (* x 2.54))
  1. Fill in the missing part of this definition to define a function of one argument, feet->inches, that converts feet into inches:
(define (feet->inches x) 
    <missing>)

Check that your function works. Eg,

(feet->inches 1)

should produce the number 12.

  1. Define a function feet->cm using the functions feet->inches and inches->cm.

  2. Define a constant called CM/INCH whose value is 2.54 and replace 2.54 in the defintion of inches->cm with this constant.

Animations

Other exercises

(4) The earth is 24000 miles around the equator. How many meters is that? (There are 1.6 kilometres in a mile).

(5) The speed of light is 3×108 metres per second. How long does it take light to go around the earth?