Start up DrRacket. If this is the first time you've brought up DrRacket,
you will be asked to choose a language. From the Language menu,
choose Choose Language. From "How to Design Programs", choose
"Beginning Student", and click "OK". Now, in the definitions window, write a comment that
includes your name and your username. From the File menu, choose
Save Definitions As.... Name your file yourLastName-hw1, where
yourLastName is your surname. Click the Run button.
In the Interactions window, type in the number 7, then hit the Enter
key. DrRacket gives you back the number 7. This is an example of an
atomic expression. We can also write compound expressions,
which consist of a left parenthesis, an operator, some more expressions (either atomic or compound), and a right parenthesis. Here are some examples,
each conforming to the definition of "expression" by repeatedly applying
the two definitions given above. Type each expression into the Interactions
window and make sure you understand the results returned by DrRacket.
7
5
(+ 7 5)
(- (+ 7 5) 3)
(* (- (+ 7 5) 3) 2)
(In the third expression, the operator is +, and the two arguments
required by the + operator are two numbers, 7 and 5.)
In class
on Thursday and Friday we played with some built-in functions in the image
library. Add the following line to the Definitions window:
(require 2htdp/image)
Now click the Run button. The functions in the image library are now available
to your program.
From the Help menu, choose Help Desk. A browser window will open.
Under the Teaching heading, choose How to Design Programs Teachpacks.
Click on entry number 2.2 (image.ss). Here
you will find a description of all the pre-defined functions (operators) in the
image.ss
library. Notice the ...search manuals... box at the top of the page. Type in
image-width to find information about that operator. When the DrRacket Helpdesk provides the matches for image-width, choose the one from "2htdp/image".
Here is an expression that will display a solid red circle of radius 25:
(circle 25 "solid" "red")
Notice that this conforms to the definition of a compound expression.
(The operator is circle. The three arguments supplied to the
circle operator are a number, 25, and two strings,
"solid", and "red".)
In the Interactions window,
write a compound expression that will display two solid circles stacked on top
of each other. Both circles should have a radius of 25. The circle on
the top should be red, and the circle on the bottom should be yellow.
(Hint: look at the definitions of the operators circle and
above in the
help desk).
In the Interactions window,
write a compound expression that will display three solid circles on top
of each other, the top one being red, the middle one being yellow, and the
bottom one being green. Use a radius of 25 for all three circles.
As you answer each of the remaining problems, use comments in your file to clearly indicate the number of the problem
you are solving.
It's so dull always having traffic lights in the same three colors. Let's
define a function that allows us to stack three solid circles (of radius 25) on top of
each other, using any three colors we choose. When we call the
function, we will provide the three colors we want as arguments to the function.
When we define the function, we will need to reserve 3 empty places
(3 parameters) that will hold the three arguments we provide when we
call the function. Here are the contract and purpose for our function:
;; custom-traffic-light: string string string -> image
;; consumes three strings representing the colors of the top, middle, and
;; bottom lights and produces an image of a traffic light in those colors
Copy the contract and purpose into the definitions window. Now
write the function definition for
custom-traffic-light. (Hint: here is the first line of the function definition,
with parameter names top, middle, and bottom:)
(define (custom-traffic-light top middle bottom)
Hit the Run button. If DrRacket reports any errors, try to figure
out what's wrong, make corrections, and hit the Run button again. In the
Interactions window, call the function custom-traffic-light
to create an
image of a traffic light. Call the function with arguments "chartreuse", "magenta", and "brown".
