Description
Principles of Programming Languages
Extending a Programming Language: Statements
For this assignment we will be extending SBML to include
variables, assignment, conditionals, loops, and output. For
now, SBML will have single global, static scope for all
variables.
The code written for this assignment will be reused in future
assignments. Please keep that in mind when working. Keep your
code clean, neat, and modular so that it will be easy to update
and extend in future assignments.
We will use continue to use the parser generator PLY – Python
Lex and Yacc – to specify and implement the SBML language.
SBML extension:
Variables and Assignment:
All variable names begin with an ASCII character, which may be
followed by zero or more ASCII characters, digits, or
underscores.
A regular expression to match this definition of variable names
is: “[a-zA-Z][a-zA-Z0-9_]*”
Expressions from the previous homework (HW 03) must be extended
in the following two ways:
1. Support for assignment to variables must be added. For
example, “x = 1;”. This will include assignment to indexed list
variables. For example, if we have performed the assignment
“array = [1, 2, 3];”, and then perform the assignment
“array[2] = 5”, then the list stored in “array” should now
contain [1, 2, 5].
2. Support must be added for variables used in expressions. For
example, if x was assigned 1, then “print(x);” will print 1.
Similarly, we should evaluate indexed variables if they occur in
a place where their value is needed. For example, if array was
assigned the list [1, 2, 5], then “print(array[0] + array[1] +
array[2]);” should print 8.
Evaluating a variable for its value should have the following
behavior. If the variable has had a value assigned to it, then
the value should be returned. Otherwise, a “Semantic Error”
should be reported and your program should stop.
When an indexed list variable is used in an expression, then
both the list and the index are evaluated to their value, and
then the indexed list expression is evaluated. If the variable
is not a list (or a string), or the index is not an integer,
then a Semantic Error should be reported. If the index is
outside the bounds of the list, then a Semantic Error should be
reported.
New Statement Types:
1. Block: A block statement consists of zero or more statements
enclosed in curly-braces, “{…}”. When the block executes, each
of the statements is executed in sequential order.
2. Assignment: an assignment statement consists of an
expression, an equals sign, an expression, and a semicolon,
“exp1 = exp2;”. When the assignment statement executes the lefthand side expression is assigned the value evaluated for the
right-hand side expression.
3. Print: a print statement consists of the “print” keyword, a
left parenthesis, an expression, a right parenthesis, and then a
semicolon. When the statement executes, the expression is
evaluated for its value. The output displayed should be the same
as that produced by Python for that value.
For example,
>>> print(‘a’)
a
>>> print([‘a’])
[‘a’]
>>> print(1 < 2) True 4. Conditional Statements: A. If Statements: Consist of a keyword “if”, a left parenthesis, an expression, a right parenthesis, and a block statement as the body of the If statement. When the If statement executes, if the expression evaluates to True, then the block statement composing the body is executed. B. If-Else Statements: Consist of a keyword “if”, a left parenthesis, an expression, a right parenthesis, a block statement as the body of the If clause, the keyword “else”, and a bloc statement as the body of the Else clause. When the IF-Else statement executes, if the expression is True, then execute the block statement that is the body of the If clause. Otherwise, execute the block statement that is the body of the Else clause. 5. Loop Statements: A. While Loops: A While statement consists of the keyword “while”, a left parenthesis, an expression, a right parenthesis, and a block statement that is the body of the While statement. Executing the while statement begins by evaluating the condition expression for its value. If the expression evaluates to False, then the While statement terminates. Otherwise, execute the block of statements that compose the body of the While statement, and then repeat the execution of the While statement from the evaluation of the condition expression. Input Programs: An SBML input program for your interpreter consists of a single, outermost Block statement. Executing the program consists of executing this block statement. Your program will be called with a single command-line argument. This argument will be a filename for a file containing an SBML input program. If the program contains any Syntax Errors, then your program should print out “SYNTAX ERROR” and exit. Otherwise, try to execute the input program. If the execution causes a Semantic Error, then your interpreter program should print “SEMANTIC ERROR”, and then exit. Execution of the program may cause Print statements to execute, and the output from Print statements should be sent to Standard Output. No other output should be produced by your program. Suggestions: 1. Create a global map/dictionary to store the values of variables (a very simple Symbol table). 2. The AST node used to represent variables should include an evaluation method that looks up the current value assigned to the variable in the global mapping and return that value. 3. The AST nodes representing statements should have a method to resolve the execution of those statements. Inside the method, you will evaluate any necessary expression, perform branching or looping as required, and produce any side-effects (like sending values to the display in Print statements). Submission Instructions: 1. Please name your program sbml.py 2. Please include your name and student id as comments at the top of your program file. 3. Please collect and submit your program file as a compressed zip file. 4. The title of the compressed file should be: cse307_hw04_LastNameFirstName.zip 5. This is an individual assignment. Any collaboration on writing your programs will be treated as a violation of academic integrity. Example SBML Input Programs: Example1: { number = 33; isPrime = 1; i = 2; while(isPrime == 1 andalso number > i){
if (number mod i == 0) {
isPrime = 0;
}
i = i + 1;
}
if(isPrime == 1){
print(“isPrime is true”);
} else {
print(“isPrime is false”);
}
}
Test and output should look like this:
$ python sbml.py example1
isPrime is False
Example2:
{
data = [ [ 100, 42 ], [ 100, 50 ], [ 123, 456 ], [ 300, 9000 ] ];
result = [ 0, 0, 0, 0 ];
i = 0;
while (i < 4){ a = data[i][0]; b = data[i][1]; if (a > 0){
while (b > 0){
if (a > b){
a = a – b;
} else {
b = b – a;
}
}
}
result[i] = a;
i = i + 1;
}
print(result);
}
Test and output should look like this:
$ python sbml.py Example
[2, 50, 3, 300]
