Your assignment is to write a Python program that uses a recursive descent parser to translate the language defined below into code for the stack machine defined below. When submitted, your translator should run as a script (ie contain the required #! statement as the first line). Your translator is to read from the file which specified as the command line argument when your translator is invoked and it is to write to standard output.
-- Input a value n, sum from 1 to the absolute value of n
-- Output sum and whether sum is even or odd
i:=1;
sum := 0;
get n;
if n < 0 then
n := 0 - n;
end if;
while i <= n loop
sum := sum + i;
i:=i + 1;
end loop;
print sum;
newLine;
if 2 * (sum / 2) = sum then
print "Even";
else
print "Odd";
end if;
If the program above were in a file called sample1 then a
sample run would be very similar to this (the blank lines are added for readability;
you may choose whether to generate blank lines, but I recommend it):
>translate sample1 push 1 store i push 0 store sum get store n push n push 0 lt branchIfZero AfterThen1 push 0 push n minus store n branchAlways AfterElse1 LABEL AfterThen1 LABEL AfterElse1 LABEL loopTop2 push i push n le branchIfZero LoopEnd2 push sum push i plus store sum push i push 1 plus store i branchAlways loopTop2 LABEL loopEnd2 push sum print newLine push 2 push sum push 2 divide times push sum eq branchIfZero AfterThen3 push "Even" print branchAlways AfterElse3 LABEL AfterThen3 push "Odd" print LABEL AfterElse3
push operand: Puts the value of operand on the stack.
operand is a variable, an integer literal, or a string literal.
plus, minus, times, divide: replace top two elements
with the result of the operation.lt, le, gt, ge, eq, ne:
replace top two integer elements with boolean result
(1 for true, 0 for false)store operand: store top value into variable
operand and pop stack print: sends the string or value on top of stack to
standard output and then pops the stack newLine: sends a newline character to standard outputbranchAlways location: transfers control to the specified locationbranchIfZero location: transfers control to the specified location if top of
stack is 0 and pops stackLABEL location: marks the specified location in the stack
machine programfileName
using the command interp fileName fixed width font. The symbols "{" and
"}" denote 0
or more occurrences and the symbols "[" and "]" represent 0 or 1 occurrences.
The symbol "|" represents a choice.
0 1 2 3 4 5 6 7 8 9 a..z and A..Z.
The language is case sensitive. ( ) := ; + - * / < <= > >= = /=
get print newLine if then else end while loop Program -> StatementList
StatementList -> Statement {Statement}
Statement -> SimpleStmt | IfStmt | WhileStmt
SimpleStmt -> Variable := Expr ;
| print Expr ;
| print String ;
| get Variable ;
| newLine ;
Variable -> Identifier
IfStmt -> if BoolExpr then
StatementList [else StatementList] end if ;
WhileStmt -> while BoolExpr loop
StatementList end loop ;
Expr -> Term {AddOp Term}
Term -> Factor {MulOp Factor}
Factor -> Number | Variable | ( Expr
)
BoolExpr -> Expr RelOp Expr
if and while
statements.
if and while statements have the usual
semantics.:= Expr
assigns the value of Expr to Variable
print Expr sends to standard output
the value of Expr.print String sends String to standard output.
newLine sends a newline character to standard output. import scanner in your program.
initialize(filename),
currentLexeme(), and
advanceToken().
hasMoreTokens. currentLexeme() and currentToken() return strings.
The list of tokens returned can be found in the python program.
if condition then statment)
and ifRest (which has else statement end if;). See the example in the notes.translate.
The file should have no extension.