The due date for the bonus assignment cannot be extended. I am away August 12-15.
Last Update: Sunday September 27, 1998 01:06
This assignment allows you to demonstrate your mastery of your interpreter project.
Add the IF/THEN/ELSE/ENDIF control flow statement to your interpreter using the following grammar production:
<if_statement> --> "if" <expression> "then" <statement>* ( "else" <statement>* )? "endif"
IF statements nest, with the optional ELSE clause belonging to the most recent IF. The interpreter must keep track of the nesting level.
Does the "endif" require a semicolon after it?
Should semicolons terminate statements, or separate them? In the following table, semicolons terminate statements in the left example, and they separate statements in the right example:
Terminator Separator if x > 0 then y = 0 ; z = 2 ; else y = 1 ; endif if x > 0 then y = 0 ; z = 2 else y = 1 endif
If semicolons terminate statements, as they do in C, every statement must end with one. If semicolons only separate statements, only when you have two or more statements do you need them. Which method will your interpreter use? You might want to read Brian Kernighan's commentary on the subject in relation to Pascal. (Pascal uses semicolons as statement separators):
You may need to enhance your panic() function so that it stops at control flow keywords as well as semicolons. Consider the following example:
if x < 0 then = 2 + 2 /* syntax error at start of this line */ else y = 1 ; z = 2 ; endif
A panic() function that looked only for semicolons would skip forward into the middle of the ELSE clause of the IF statement, possibly causing further syntax errors, instead of stopping before the ELSE.
Here are two ways (among many) of implementing the above production in your parser.
This method is used by the Unix "C Shell"-style command interpreters:
The "stupid if" format doesn't detect syntax errors in the part of the input that is skipped over, since that part of the input isn't actually parsed according to the grammar. This means code such as the following "works":
if 0 then this junk * ) "xxz" ( is ignored + + + until the parser -/-** sees an else print "This must be correct.\n" ; endif
A method similar to this is used by the Unix "Bourne Shell"-style command interpreters:
The "fancy if" format requires that all input be syntactically correct, even input that is not executed.
This table shows the difference in the two Unix command interpreters' treatment of the input skipped over during control flow handling:
Unix Bourne Shell
detects syntax errors
in FALSE clause
Unix C Shell
ignores syntax errors
in FALSE clause
if false; then ) foo syntax error: `)' unexpected Exit 2 if( 0 ) then ) foo endif
Arrange a time to show me a demonstration of your interpreter. I will hand you a test file to run.
I am attending a professional development seminar in Kingston August 12-15 and will not be in town to see late assignment demos during this time, nor can I see demos while I am proctoring exams.
Ian D. Allen CST 8152 Home Page