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Code Club 5

Recap

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.

And we have seen examples of defining new functions and new "global constants". For example:

(define THE-ANSWER 42)
(= (* 6 7) THE-ANSWER)
;; Number -> Number
;; Adds 1 to its argument
;; eg, (add1 3) should be 4
(define (add1 x)
    (+ x 1))

Excerises

  1. Complete this function definition by replacing the "..." with the right expression:
;; Number -> Number
;; Convert a distance in inches to centimetres
;; eg, (inches->cm 1) should be 2.54
(define (inches->cm length)
    ... )
  1. Define feet->inches, that converts feet into inches:
;;; Number -> Number
;; Convert a distance in feet to one in inches
;; eg, (feet->inches 1) should be 12
(define ... )

  1. Design 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.

  3. Exercise with strings

;; String -> String
;; Add "Hello, " to a given name

(check-expect (greet "James") "Hello, James")
(check-expect (greet "Tomás") "Hello, Tomás")

(define (greet name)
 ...)

  1. Go back to the conversion functions and replace the examples in comments with check-expect.

;; Number Number -> String
;; Convert a distance in feet and inches to centimetres

(check-expect (feet/inches->cm 5 7) 170.18)

(define (feet/inches->cm ft in)
    ... )

;; Number -> Image
;; Draw a tree of the given height

(define (tree height)