159.341 – 2021 Semester 1 Massey University
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Assignment 1
Deadline: Hand in by 5pm on Monday 29th March 2021
Evaluation: 10 marks – which is 10% of your final grade
Late Submission: 1 mark off per day late
Work: This assignment is to be done individually – your submission may be checked for
plagiarism against other assignments and against Internet repositories. If you adapt
material from the internet you must acknowledge your source.
Purpose: To reinforce ideas covered in the lectures for understanding and using imperative
programming languages and language design concepts.
Problem to solve:
Write a parser/interpreter for a simple, imperative string-processing language.
Requirements:
The interpreter must read input from the standard input (stdin) and write output to the standard output (stdout)
and should read statements from the user one at a time and perform the associated action immediately. If invalid input
is provided, the interpreter should attempt to recover and try to provide as useful an error message as possible.
The grammar for the strings processing language is given below:
program := { statement }
statement := 'append' identifier expression ';'
| 'list' ';'
| 'exit' ';'
| 'print' expression ';'
| 'printlength' expression ';'
| 'printwords' expression ';'
| 'printwordcount' expression ';'
| 'set' identifier expression ';'
| 'reverse' identifier ';'
expression := value { '+' value }
value := identifier | constant | literal
constant := 'SPACE' | 'TAB' | 'NEWLINE'
literal := '"' { letter | digit | punctuation } '"'
identifier := letter { letter | digit }
letter := 'A' | 'B' | ... | 'Z' | 'a' | 'b' | ... | 'z'
digit := '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
punctuation := '.' | ',' | ':' | ';' | '?' | '!' | ...
The intended behavior of each instruction is given in the following table:
Command Parameters Behaviour
append identifier expression Evaluate the expression and append it to the contents of the variable.
list List all variables and their contents.
exit Exit the interpreter
print expression Evaluate and print the expression
printlength expression Evaluate the expression and print its length
printwords expression Evaluate the expression and print the individual words
printwordcount expression Evaluate the expression and print the number of words
set identifier expression Set (create if necessary) the contents of the variable to the expression reverse identifier Reverse the order of the words in the contents of the variable.
159.341 – 2021 Semester 1 Massey University
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Sample input might be:
set one "The cat";
set two "sat on the mat";
set sentence one + SPACE + two;
append sentence " by itself.";
print sentence;
printwordcount sentence;
printwords sentence;
printlength sentence;
list;
reverse one;
print one;
exit;
From which output like the following could be expected:
159.341 2021 Semester 1, Assignment 1
Submitted by Rick Deckard, 20191187
The cat sat on the mat by itself.
Wordcount is: 8
Words are:
The
cat
sat
on
the
mat
by
itself.
Length is: 33
Identifier list (3):
one: "The cat"
two: "sat on the mat"
sentence: "The cat sat on the mat by itself."
cat The
Your program can just read from stdin and write to stdout, it does need to open and close any files. You might want
to use stderr for error messages if it finds input situations it cannot handle or runs out of resources.
One of the first things you should do in your program design is decide what data structures you will use and how to
organise them. You will need some way of storing a symbol table which will store all of the variables in your program
(and a way of accessing their contents). This language has only one implicit type (a string) so your symbol table does not
need to store any information about the type of the variable.
You should also think about what functions/procedures you will need. You may want to consider a recursive descent
parser as a base for your interpreter. Each statement starts with a command, so you could start by writing a function to
parse this command. Based on which command it is, your parser can work out what is expected to follow.
For example, if you parse the command 'list' then you expect it to be followed by a ';'. If it is not followed by a
semicolon then an error has occurred. Likewise, if you parse the command 'set' you will then need to parse an identifier
name and then an expression followed by a semicolon.
There are some features that have been left (intentionally) unspecified. For example, how are you going to deal with
special characters inside a string literal? What if that special character is a double quote? What are you going to do if the
user types in invalid input? One approach is to ignore input until the next end of statement character (in this language the
semicolon ';'). The definition of a word has also been left vague – should a word be separated by a space or would any
non-letter character count? The decision of how to deal with this is left to you.
The language has only specified a single operator, the binary + operator for concatenation. Think of + as a function that
takes two strings and returns a third which is the concatenation of the two arguments. This operator is associative (it does
not matter whether you resolve the concatenations left-to-right or right-to-left. Expressions may have any number of terms
connected with + operators.
159.341 – 2021 Semester 1 Massey University
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The language specifies three constants:
SPACE as " "
TAB as "\t"
NEWLINE as "\n"
These could be parsed separately by your interpreter or you could treat them as identifiers and store them as variables in
your symbol table (you would need to have some way of preventing the user from changing their values).
Some sample input (along with possible output) will be provided on the stream site that you can use to test your
interpreter.
You must follow the next two specifications in each and every assignment for this course
- Place the following comments at the top of your program code and provide the appropriate information:
/ Family Name, Given Name, Student ID, Assignment number, 159.341 /
/ explain what the program is doing . . . / - Ensure that your program prints this information to the console. You might use code like:
"----------------------------------------"
" 159.341 Assignment 1 Semester 1 2021 "
" Submitted by: Rick Deckard, 20191187 "
"----------------------------------------"
Hand-in: Submit your program and documentation (a zip file is acceptable) electronically through the form on the
stream site.
Marks will be allocated for: correctness, fitness of purpose, sensible use of data structures and algorithms, utility, style,
use of sensible comments and program documentation, and general elegance. Good comments will help me to award
you marks even if your code is not quite perfect.
If you have any questions about this assignment, please ask the lecturer.