Principles of Computer Science

Question $2(22$ points$)$:

Purpose: To build a simple interpret and test it$.$ To get warmed up with Python, in case you are using it for

the first time.

Degree of Difficulty: Easy. This is just for fun, and to emphasize some of the ideas of Chapter $1.$

References: You may wish to review the following chapters:

$$ General: CMPT $145$ Readings, Chapter $1$

$$ File IO Review: CMPT $141$ Readings: Chapter $12$

Restrictions: This question is homework assigned to students and will be graded. This question shall not

be distributed to any person except by the instructors of CMPT $145.$ Solutions will be made available

to students registered in CMPT $145$ after the due date. There is no educational or pedagogical reason

for tutors or experts outside the CMPT $145$ instructional team to provide solutions to this question

to a student registered in the course. Students who solicit such solutions are committing an act of

Academic Misconduct, according to the University of Saskatchewan Policy on Academic Misconduct.

Note: a$1$q$3$ asks about your work on a$1$q$2$

Keep track of how long you spend working on this question, including time spent reading the ques$-$

tion, reviewing notes and the course readings, implementing the solution, testing, and preparing to

submit. Also keep track of how many times you experienced unscheduled interruptions, delays, or

distractions during your work; include things like social media, phone calls, texting, but not such

things as breaking for a meal, exercise, commuting to or from the university. In a$1$q$3$ you will be

asked to summarize the time you spend working on a$1$q$2,$ and your answer will be more helpful to

you if you have more precise records. Maybe go and read Q$3$ before you start Q$2?$

A simple calculating language: Tho The end goal of this question is a Python program that interprets a

very simple calculating language called Tho. Don$$t worry, this is easy!

Your Python program will be able to open a file, read the commands that are in the file, perform the cal$-$

culations, and will output the numerical result. The file can be considered a program or a script in the

Tho language, and your Python program that reads the Tho script will be considered a very simple Tho

interpreter.

The Tho language is very limited$,$ and that$$s what makes it viable for a first question on a first assignment.

Tho Register The language is not like Python. In Python, we can use as many variables as we need. The

Tho language will use exactly one register, called R$,$ whose value can be modified by the Tho commands.

At the beginning of every script, this register is initialized to be zero.

Tho commands The commands of the Tho language are not like in Python, which has assignment state$-$

ments and expressions, and loops and functions, etc. In Tho, we only have very simple commands. Each

command is rather like an instruction in assembly language, for a very simple kind of CPU.

To describe the commands, we have to give the syntax of the commands, and then clearly describe the

behaviour or effect of the commands. In the description that follows, the command keywords will be

written in upper case, e$.$g$.,$ ASK. Commands sometimes take an argument, which depends on that the script

is trying to do$.$ For arguments, we will use italics, e$.$g$.,$ T $.$

There are only two kinds of arguments for a Tho script. Usually, an argument will be a floating point number.

Sometimes, a Tho command will use a name as an argument. A name is simply a letter or work, similar to

names in Python.

We$$ll start with the ASK and TELL commands, which Tho uses to interact with the user.

Page $3$

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Principles of Computer Science

Command Effect

ASK T Acquire a value from the console, and store it as the value of the name T

TELL Display the value of R on the console

The effect of the ASK T command requires a bit more detail:

$1.$ Display a simple prompt to the user, who will then type in a floating point value, followed by a return.

$2.$ The user$$s value will be stored as the value for T $.$

We can say that the ASK command creates the name, and records its value. After the ASK command, the

name can be used in other commands. It$$s important to understand that a name in Tho is not a variable;

the value associated with a name created by the ASK command cannot be changed. It$$s a named constant

that can be used elsewhere in the Tho script.

The TELL command displays the value stored in the register to the console. The syntax of this command

is simply the word TELL on a line by itself.

The Tho language will also have $4$ arithmetic commands:

Command Effect

ADD e R $=$ R $+$ e

SUB e R $=$ R $$ e

MUL e R $=$ R $\backslash$times e

DIV e R $=$ R$/$e $($floating point division$)$

In this table, the e represents a simple expression. In the Tho language, a simple expression is either a

floating point literal, like $3\mathrm{.}1415,$ or a name created by an