Foundations of Computer Science Laboratory

Transcription

1 Foundations of Computer Science Laboratory LOGO Programming Purpose In previous labs you experienced an imperative language (Karel) and a logic language (Prolog). In this lab you will learn a functional List-Processing Language LOGO. Next week, you will learn to use LOGO as a language for robot control. In the final lab, you will build a robotic machine and use a computer to control it with the language LOGO as the programming language. Method Read these instructions very carefully, and answer the questions on the answer sheet which your instructor will hand out in lab. Turn in the answer sheet at the conclusion of lab. If you do not finish, complete the questions as homework and turn in the sheet at the beginning of next week s lab. There will not be an opportunity during next week s lab to complete this lab. LOGO - An Overview LOGO is often thought to be a "kid's language" for making "turtle pictures." This is partially true; LOGO has an excellent turtle- graphics facility. But LOGO is more than an electronic etch-a-sketch, it is also a list-processing language, a simplified Lisp. Lisp and Prolog are the two most common AI languages. LOGO has many built-in functions and procedures. They are used to accomplish a task and often to report (return) information to another routine. The language uses lists - groups of objects in a sequence. A list of numbers could be written as [ ] and a list of strings could be [ My name is Bill Taffe]. Lists are created and passed from routine to routine each of which perform some process on the list. Logo is rich in processes and it has a full complement of control structures. Although it can do arithmetic, it does so very differently from procedural languages like BASIC, FORTRAN, C++ or Java. For example, the assignment statement is done as PseudoCode Java LOGO A <- 4 A = 4 Make "A 4 LOGO distinguishes the name of a piece of data from the data itself. When LOGO wants to refer to th name of something it precedes the variable name with a quotation mark (") as in the example above. On the other hand, when it wants to refer to the data stored in a variable's location, it precedes the variable name by a colon (:). So if we continued the example from above where a 4 was stored in location A and we wanted to put that in location B we would write: PseudoCode Java LOGO A <- B B = A Make "B :A LOGO has many other interesting features but rather than explain them - since this is a lab - you will learn them by trying them. Throughout these instructions whenever you see the symbol => it indicates that there is some action for you to take. Frequently it involves answering questions on the answer sheet. If you are not prompted by the symbol => then there is no action other than reading the lab sheet.

2 Turtle Graphics In today's lab, we will use both the turtle graphics and list-processing facilities but we ll use turtle graphics first. Turtle graphics is so named because it makes pictures by giving directions to a turtle who moves about the screen dragging its tail. Wherever the tail is dragged, a line is drawn. The commands a turtle can execute include: (n is a number) home moves the turtle to the center of the screen facing north (upward) rg reset graphics (erases the graphics screen and homes the turtle) fd n move forward n units bk n move backward n units rt n turn right n degrees lt n turn left n degrees pu pen up (lift the turtle's tail) pd pen down (lower the turtle's tail) setpos [x y] sets the x and y positions of the turtle seth n sets the heading of the turtle st show turtle - make the turtle visible ht hide turtle - hide the turtle - makes drawing faster show prints (on the screen) any information given to it such as pos or heading pos reports the turtle's x and y position (used with show as in show pos) heading reports the turtle's heading in degrees (used with show as in show heading) label allows the turtle to write something next to itself for ex. label "hello There are more graphics commands but this subset will suffice for a start. Now it is time to enter the LOGO system. We are using a version of logo developed by LOGO Computer Systems called LEGO TClogo. In the following instructions, anything enclosed in < > is to be typed exactly as printed without the < >. You can start LOGO from the sequence Start -> Class -> Foundations -> Logo. Once running, press enter to get past the opening screen and on the next screen use the down arrow and enter to select New Page. There are two parts to the screen - called pages in LEGO TClogo- the upper or procedure page where LOGO defines new procedures (similar to Define-New-Instruction in Karel), and the lower or command page. There are two cursors; the flashing one indicates the active page. When you first enter, the command page is active and and the procedure page has several procedure names on it. The active page can be changed by using the control key together with U or D. These are written as ^U or ^D. 2

3 Use ^D and ^U to switch active screens from the upper to lower and back again. The procedure page can be changed to the turtle page with the flip key - a control F (^F) which acts as a toggle (toggle means to flip back and forth). Toggle between the procedure and turtle pages. Show the turtle page rather than the procedure page and activate the command page <^D>. Try each of the graphics commands above and after you are familiar with each (about minutes), use these commands to move the turtle and figure out the answers to the Graphics Questions on the Answer Sheet. Procedures (or DEFINE-NEW-INSTRUCTION or teaching the turtle new commands) Change back from the turtle page to the procedure page <^F>. Following is a list of commands you will find useful when creating procedures: newpage creates a new page for procedures namepage "mypage gives the current procedures page the name mypage cleartext erases all text on the procedures page cc erases all text on the command page getpage "mypage saves the current page, clears the procedures page, and loads mypage onto the procedures page load "mypage adds the contents of file mypage to the current procedures page printtext prints all the procedures on the procedures page on the printer printscreen prints the graphics screen setdisk "drive sets the drive letter for saving and loading files, for ex. setdisk "m esc saves the current page and returns to the contents page dos saves the current page (and picture) and escapes to MSDOS Activate the command page <^D>. Create a newpage and name it geometry using the namepage command <namepage "geometry>. Activate the procedure page <^U> and enter the following text to create the procedure (new instruction) square: to square 3

4 Activate the command page and the turtle page Type the command <square> and watch the turtle! Move the turtle to position [-20-20] and execute square again Reset the graphics page <rg> and execute square again Create two more figures - to triangle and to trapezoid - test them and, when they work correctly, write the steps for "to triangle" on the question sheet under the Procedures Section of the Answer Sheet under column Original (don't worry about the "With Parameters column - that will come later). Unlike Karel, LOGO uses data - numbers, strings and lists. It is possible to create a procedure and give it information. For example we will create a square of arbitrary size and when square is executed, the size will be "passed" to the procedure. It is done like this. to square :size The information passed is called a parameter and the list of parameters passed (only :size in this case) is called a parameter list. Modify square as shown and try it Modify procedure triangle to include parameter passing and test it Write the changed procedure alongside the original in the procedures section of the questions under column With Parameters Control Structures LOGO has two branching instructions if <test> [true_list] ifelse <test> [true_list] [false_list] where <test> is a boolean test and true_list and false_list are lists of actions. LOGO has three logical instructions to enable the performance of complex boolean tasks. (read this part carefully) and (test1) (test2) or (test1) (test2) not (test) 4

5 These may be combined to create logical ANDs and ORs. For example, to write If A is greater than B and A is less than C you would write if and (:A > :B) (:A < :C) Thus we could create a procedure which operates only if the parameters were not too large. For example, we could have the turtle execute a square only if it were less than 75 on a side by: to square :size if :size < 75 [ ] Modify "to square" in this manner and try it Modify "to triangle" so that it executes only if the side is > 25 and < 75 and test it. {Hint: reread the syntax of the AND operator from the previous page.} Print the procedure on the printer by typing <printtext> - attach the output to your page question LOGO has two commands which are used for looping repeat n [list] stop repeats the instructions in the list n times stops the execution of the procedure and returns control to whatever called it The repeat command can be used to simplify our square procedure as follows: Original Simpler to square :size to square :size repeat 4 [] Modify square in this manner and try it Modify triangle, test it LOGO does not have a while <test> do <action>. Instead, it uses recursion and the stop command. If we wanted to create a whole group of squares, one inside the other we could create a procedure "to box" as follows: to box :side if :side < 10 [stop] square :side box :side - 5 5

6 If box is less than 10, the procedure stops executing and reverts to however it was called, but if it isn't the procedure draws a square and calls box again but with a side that is 5 smaller than the last call. Create a box using the above procedure It's time to the turtle graphics so Return to the contents page by pressing the escape key List Processing Now we will explore the list processing aspects of LOGO. List processing can be combined with turtle graphics to create power-turtles but for now we will explore list processing without graphics. LOGO is a simple version of the powerful LISt Processing language LISP. A useful subset of LOGO's list processing commands are: make "listname [list] creates a list with name "listname" thing "listname reports the contents of "listname" show :listname shows the contents of "listname" first :listname reports the first item in "listname last :listname reports the last item in "listname butfirst :listname reports the contents of "listname except for the first item butlast :listname reports the contents of "listname except for the last item word "one "two combines the strings one and two into one string sentence "one "two combines "one" and "two" into a list sentence :one :two combines the contents of lists "one" and "two" into one list fput "front :mylist reports a new list with "front" as a new item in front of "mylist" lput "back :mylist reports a new list with back as a new item at the back of "mylist" Now, practice some of these list processing commands by doing the following: Select newpage and name it lists <namepage "lists>. Create a list (named "mylist")... items of data, separated by spaces, enclosed in square brackets [] - <make "mylist [ ]>. Answer questions 1-6 in the List Processing Section of the Answer Sheet. There are also several commands to process items in a list. A subset of these commands is: count :listname counts the number of items in "listname empty? :listname reports whether the list is empty (true or false) identical? :list1 :list2 reports whether two lists are identical (true or false) item n :listname reports the nth item in "listname" member? "stuff :listname reports if "stuff" is a member of the list "listname" output :value causes a procedure to report a value - see example on p7 Answer questions 7-11 in the List Processing Section of the Answer Sheet. They will help you further investigate these commands. 6

7 Create the following procedure which adds the numbers in a list on the procedure page and answer question 12 in the List Processing Section of the Answer Sheet to add make "thelist [ ] make "sum 0 repeat 5 [ make "number first :thelist make "sum :sum + :number make "thelist allbutfirst :thelist] show :sum Now try a slight variation on this procedure. Change procedure add to: to add "thelist :sum make "sum 0 repeat 5 [make "number first :thelist make "sum :sum + :number make "thelist butfirst :thelist] output :sum Answer question 13. List I/O It is possible to cause lists to be typed or input from the command center. The commands to do this are: type :alist readlistcc causes the contents of "alist" to be printed in the command center reads a list from the command center and stores it temporarily in its own name Now for the last task. Modify procedure add so that you can put in the list from the command center using the "readlistcc" command. to add :sum make "sum 0 type [enter your list...] make "thelist readlistcc repeat 5 [ make "number first :thelist make "sum :sum + :number make "thelist butfirst :thelist] output :sum Answer question 14. 7