[Solved] Comp200 Project 0

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The purpose of Project 0 is to familiarize you with the Scheme programming environment and the resources available to you. The format of this project is different from the more substantive ones you will see later in the term. Those projects focus on extended experience in creating and testing code, and integrating such code with existing computational frameworks. As a consequence, those projects will take a significant amount of time to complete. This project is really just a warm up, and is intended to get you up to speed on the mechanics of using Scheme and the course resources. We expect that this project should not take much time, and while we understand that some of the things we ask you to do seem fairly obvious, we want to make sure you get the mechanics down right. Dont worry; the later projects and problem sets will involve much more extensive thinking and coding!

1. Getting Started

The purpose of this section is to get you started using Scheme as quickly as possible. Start by looking at the handouts available on the course website. If you have a PC capable of running Scheme, we suggest that you install it there, since it will be convenient for you to work at home. ENG-B19 is the best places to work. You can also contact TAs by their email.

Remember, whether youre working with your machine or the labs, the source of all useful information is the Comp200 web page.

Starting Scheme

The first thing to do is to get an implementation of Scheme and start using it:

• ▪ In ENG-B19: A good way to get started learning Scheme is to do this project in the Lab. You can login to the system (Windows) with your Novell username andpassword and you can immediately start DrRacket from All Programs .
• ▪ If you have problems logging in contact to CIT. After logging in, you can prefer opening the command-line Racket interpreter or you can directly use the core compiler

of DrRacket.▪ On your home computer Follow the instructions in the Working from Home

handout on the course web page for downloading and installing Scheme (either Linux or Windows versions), and/or connecting to the linuxlab machine using ssh.

Getting Started with Racket

On Windows, you can start DrRacket from the Racket entry in the Start menu. In Windows Vista or newer, you can just type DrRacket. You can also run it from its folder, which you can find in Program Files Racket DrRacket.

On Mac OS X, double click on the DrRacket icon. It is probably in a Racket folder that you dragged into your Applications folder. If you want to use command-line tools, instead, Racket executables are in the bin directory of the Racket folder (and if you want to set your PATH environment variable, youll need to do that manually).

On Unix (including Linux), the drracket executable can be run directly from the command-line if it is in your path, which is probably the case if you chose a Unix-style distribution when installing. Otherwise, navigate to the directory where the Racket distribution is installed, and the drracket executable will be in the bin subdirectory.

Depending on how you look at it, Racket is

• a programming languagea dialect of Lisp and a descendant of Scheme;
• a family of programming languagesvariants of Racket, and more; or
• a set of toolsfor using a family of programming languages.Where there is no room for confusion, we use simply Racket.

• Rackets main tools are
• racket, the core compiler, interpreter, and run-time system;
• DrRacket, the programming environment; and
• raco, a command-line tool for executing Racket commands that install packages, buildlibraries, and more.Most likely, youll want to explore the Racket language using DrRacket, especially at the beginning. If you prefer, you can also work with the command-line racket interpreter and your favorite text editor; see also Command-Line Tools and Your Editor of Choice. The rest of this guide presents the language mostly independent of your choice of editor.If youre using DrRacket, youll need to choose the proper language, because DrRacket accommodates many different variants of Racket, as well as other languages. Assuming that youve never used DrRacket before, start it up, type the line#lang racketin DrRackets top text area, and then click the Run button thats above the text area. DrRacket then understands that you mean to work in the normal variant of Racket (as opposed to the

smaller racket/base or many other possibilities).

If youve used DrRacket before with something other than a program that starts #lang, DrRacket will remember the last language that you used, instead of inferring the language from the #lang line. In that case, use the Language|Choose Language menu item. In the dialog that appears, select the first item, which tells DrRacket to use the language that is declared in a source program via #lang. Put the #lang line above in the top text area, still.

Scheme Expression Evaluation

Scheme programming consists of writing and evaluating expressions. The simplest expressions are things like numbers. More complex arithmetic expressions consist of the name of an arithmetic operator (things like +, -, *) followed by one or more other expressions, all of this enclosed in parentheses. So the Scheme expression for adding 3 and 4 is (+ 3 4) rather than 3 + 4.

An expression may be typed on a single line or on several lines; the Scheme interpreter ignores redundant spaces and line breaks. It is to your advantage to format your work so that you (and others) can read it easily. It is also helpful in detecting errors introduced by incorrectly placed parentheses. For example, the two expressions

(* 5 (+ 2 (/ 4 2) (/ 8 3)))(* 5 (+ 2 (/ 4 2)) (/ 8 3))

look deceptively similar but have different values. Properly indented, however, the difference is obvious.

(* 5(+ 2

      (/ 4 2)      (/ 8 3)))

(* 5(+ 2

      (/ 4 2))   (/ 8 3))

DrRackets editor provides a good environment to pretty-print your code, indenting lines to reflect the inherent structure of the Scheme expressions (see Section B.2.1 of the Introduction to the Scheme Programming Environment manual).

While the Scheme interpreter ignores redundant spaces and carriage returns, it does not ignore redundant parentheses! Try evaluating

((+ 3 4))

Scheme should respond with a message akin to the following:

; application: not a procedure;; expected a procedure that can be applied to arguments ; given: 7; arguments: [none]

Indenting Code

Indentation is very important when you are programming with Scheme due the fact that Scheme is a parenthesis based language and we highly recommend you to indent your codes during all the projects.

DrRacket automatically indents according to the standard style when you type Enter in a program or REPL expression. For example, if you hit Enter after typing (define (greet name), then DrRacket automatically inserts two spaces for the next line. If you change a region of code, you can select it in DrRacket and hit Tab, and DrRacket will re-indent the code (without inserting any line breaks). Editors like Emacs offer a Racket or Scheme mode with similar indentation support.

Re-indenting not only makes the code easier to read, it gives you extra feedback that your parentheses match in the way that you intended. For example, if you leave out a closing parenthesis after the last argument to a function, automatic indentation starts the next line under the first argument, instead of under the define keyword:

(define (halfbake flavor(string-append flavor creme brulee)))

In this case, indentation helps highlight the mistake. In other cases, where the indentation may be normal while an open parenthesis has no matching close parenthesis, both racket and DrRacket use the sources indentation to suggest where a parenthesis might be missing.

Debugging with DrRacket

Rackets built-in debugging support is limited to context (i.e., stack trace) information that is printed with an exception. In some cases, disabling the JIT compiler can affect context information. The errortrace library supports more consistent (independent of the JIT compiler) and precise context information. The racket/trace library provides simple tracing support. Finally, the DrRacket programming environment provides much more debugging support.

Tracing

The bindings documented in this section are provided by the racket/trace library, not racket/base or racket. So, in order to be able to use the trace library you need to type:(require racket/trace)Syntax for using trace function: (trace id )

Each id must be bound to a procedure in the environment of the trace expression. Each idis set!ed to a new procedure that traces procedure calls and returns by printing the arguments and results of the call via current-trace-notify. If multiple values are returned, each value is displayed starting on a separate line.

When traced procedures invoke each other, nested invocations are shown by printing a nesting prefix. If the nesting depth grows to ten and beyond, a number is printed to show the actual nesting depth.

The trace form can be used on an identifier that is already traced. In this case, assuming that the variables value has not been changed, trace has no effect. If the variable has been changed to a different procedure, then a new trace is installed.