12. Final Assignment : Parsing
12.1 Turtle Graphics
History
Many attempts have been made to create programming languages which are intuitive and
easy to learn.
Turtle Graphics作业代写、代做C/C++语言作业、代写Adding Loops作业、代做C++程序作业
One of the best of these was LOGO which allowed children as young as 3 to learn a
computer language.
A subset of this language involved a “turtle” which could be driven around the screen
using simple instructions. The turtle, when viewed from above, was represented by a
triangle.
An Example
{
FD 30
LT 45
FD 30
LT 45
FD 30
LT 45
FD 30
LT 45
FD 30
LT 45
FD 30
LT 45
FD 30
LT 45
FD 30
LT 45
}
2 Chapter 12. Final Assignment : Parsing
Adding Loops
{
DO A FROM 1 TO 8 {
FD 30
LT 45
}
}
Using Variables
{
DO A FROM 1 TO 100 {
SET C := A 1.5 * ;
FD C
RT 62
}
}
12.1 Turtle Graphics 3
Nested Loops
{
DO A FROM 1 TO 50 {
FD A
RT 30
DO B FROM 1 TO 8 {
SET C := A 5 / ;
FD C
RT 45
}
}
}
4 Chapter 12. Final Assignment : Parsing
The Formal Grammar
<MAIN> ::= "{" <INSTRCTLST>
<INSTRCTLST> ::= <INSTRUCTION><INSTRCTLST> |
"}"
<INSTRUCTION> ::= <FD> |
<LT> |
<RT> |
<DO> |
<SET>
<FD> ::= "FD" <VARNUM>
<LT> ::= "LT" <VARNUM>
<RT> ::= "RT" <VARNUM>
<DO> ::= "DO" <VAR> "FROM" <VARNUM> "TO"
<VARNUM> "{" <INSTRCTLST>
<VAR> ::= [A?Z]
<VARNUM> ::= number | <VAR>
<SET> ::= "SET" <VAR> ":=" <POLISH>
<POLISH> ::= <OP> <POLISH> | <VARNUM> <POLISH> | ";"
<OP> ::= "+" | "?" | "*" | "/"
12.2 Assessment 5
12.2 Assessment
Exercise 12.1 Implement a recursive descent parser - this will report whether or not a given
turtle program follows the formal grammar or not. The input file is specified via argv[1] -
there is no output if the input file is valid. Elsewise, a non-zero exit is made.
Exercise 12.2 Extend the parser, so it becomes an interpreter. The instructions are now
‘executed’. Do not write a new program for this, simply extend your existing parser. Output
is via SDL. You may find the function call SDL_RenderDrawLine useful.
Exercise 12.3 Show a testing strategy on the above - you should give details of unit testing,
white/black-box testing done on your code. Describe any test-harnesses used. In addition,
give examples of the output of many different turtle programs. Convince me that every line
of your C code has been tested.
Exercise 12.4 Show an extension to the project in a direction of your choice. It should
demonstrate your understanding of some aspect of programming or S/W engineering. If
you extend the formal grammar make sure that you show the new, full grammar.
Hints
All four sections above are equally weighted.
Don’t try to write the entire program in one go. Try a cut down version of the grammar
first, e.g.:
<MAIN> ::= "{" <INSTRCTLST>
<INSTRCTLST> ::= <INSTRUCTION><INSTRCTLST> |
"}"
<INSTRUCTION> ::= <FD> | <LT> | <RT>
<FD> ::= "FD" <VARNUM>
<LT> ::= "LT" <VARNUM>
<RT> ::= "RT" <VARNUM>
<VARNUM> ::= number
The language is simply a sequence of words (even the semi-colons), so use fscanf().
Some issues, such as what happens if you use an undefined variable, or if you use a variable
before it is set, are not explained by the formal grammar. Use your own common-sense,
and explain what you have done.
Once your parser works, extend it to become an interpreter. DO NOT aim to parse the
program first and then interpret it separately. Interpreting and parsing are inseparably
bound together.
I’m happy to sort extra help sessions in January - ask.
Submission
Your testing strategy will be explained in testing.txt, and your extension as extension.txt.
For the parser, interpreter and extension sections, make sure there’s a Makefile, so that I can
easily build the code using make parse, make interp and make extension.
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