CSC 533: Programming Languages
Spring 2026

HW3: Implementing If & Break

The SILLY interpreter you wrote for HW2 extended the starter version to include missing features (operators, local variables and repeat statements). For this third SILLY assignment, you are to further extend your interpreter to implement if and break statements.

SAMPLE CODE (output in red)
>>> if true then
      print "howdy"
    endif
"howdy"
>>> var num gets 87
>>> print num
87
>>> if ((num > 90) | (num = 90)) then
      print "A"
    elseif ((num > 80) | (num = 80)) then
      print "B"
    elseif ((num > 70) | (num = 70)) then
      print "C"
    elseif ((num > 60) | (num = 60)) then  
      print "D"    
    else   
      print "F"
    endif
"B"
>>> repeat 5 times
      print 1
      if true then
        print 2
        break
        print 3
      endif
      print 4
    endrepeat
1
2 

>>> repeat 3 times
      print "before"
      var num gets 10
      while (num < 50) do
        print num
        if (num > 20) then
          print "breaking"
          break
        endif
        num gets (10 + num)  
      endwhile
      print "after"
    endrepeat
"before"
10
20
30
"breaking"
"after"
"before"
10
20
30
"breaking"
"after"
"before"
10
20
30
"breaking"
"after"
To complete this assignment, you must do the following:
  1. Define an If class that that inherits from Statement and provides the required methods for an if statement. As you saw from the grammar rules in HW1, an if statement can have an arbitrary number of elseif cases and an optional else case. Your class will have to accommodate all these forms. If the syntax of the statement does not follow the grammar rules, an exception should be thrown with a meaningful (syntax) error message. Similarly, if the test does not evaluate to a Boolean value, an exception should be thrown with a meaningful (runtime) error message.
  2. Define a Break class that that inherits from Statement and provides the required methods for a break statement. Similar to how breaks work in Java, executing a SILLY break statement should terminate the enclosing loop (while or repeat). Hint: this addition will make it clear why the execute method of a statement has a return value. The execute method for Break should return Statement.Status.BREAK to denote that a break has been executed. This return value can then be used by the other statement classes to determine whether execution should be terminated or not.
  3. To test your implementation of these new statements, write a sequence of SILLY statements that initialize a variable to a list of numbers, then processes that list to produce the count and sum of the numbers up to the first negative number. It should also display whether a negative number was encountered. For example, if the first line of your code was:

    nums = [4 2 10 -1 800]

    then your code would display the list:

    [3 16 "yes"]

    If the first line was:

    nums = [1 3 0 5]

    then your code would display the list:

    [4 9 "no"]

    Save your SILLY code in a text file named hw3.txt and submit it along with your project code.