For this assignment, you are to complete an interpreter for a simple, procedural programming language named SILLY (Simple, Interpreted, Limited Language for You). The grammar rules for the SILLY language are as follows:
Note that carriage returns are required at the end of each statement, as denoted by
NEWLINE. For simplicity's sake, you may assume that each token in the language is separated by
spaces and/or tabs.
The SILLY language is case sensitive, so variables a and A are considered unique. Variables are not explicitly declared, and are assumed to have initial values of 0 if not previously assigned. An assignment statement assigns an integer value to a variable. An output statement displays a single value on a line by itself. If statements and while loops are controlled by an expression, where any nonzero value is considered to be true. A for loop iterates a set number of times, with the loop control variable counting down from its initial value until it reaches 0. Finally, a quit statement terminates the interpreter.
SAMPLE CODE (output in red) >>> x = 4 >>> output x 4 >>> y = x + 1 >>> x = 1 + 5 - 2 >>> output y + x 9 >>> y = z + z + 1 >>> output z 0 >>> output y 1 >>> val1 = 1023 >>> val2 = 1024 >>> output val1 % 2 1 >>> output val2 / 10 % 2 0 >>> output 65 - 1 / 8 / 2 4 >>> quit BYE >>> num1 = 4 >>> if num1 output num1 else output 0 end 4 >>> x = 10 >>> y = 1 >>> while x - y x = x - 1 y = y + 2 end >>> output x 7 >>> for countdown = 10 if countdown % 2 output countdown end end 9 7 5 3 1 >>> quit BYE
An incomplete version of the SILLY interpreter is provided for you via the following classes/files:
To produce a fully functional interpreter for the SILLY language, you will need to augment/modify the provided code so that it:
*'), integer division ('
/'), and remainder ('
%'). All of these operators have the same precedence and left associativity, so expressions can be evaluated in a simple left-to-right order.
end' keyword for an if statement - if there is an else case, the keyword '
else' separates the statements to be executed if the test expression evaluates to true (i.e., non-zero) or false (i.e., zero).
Note that in making these additions, the main progam file (Interpreter.java) need not be modified at all. Instead, you will primarily be making changes to the various Statement classes, with some minor modifications to Statement and Token. As you add new features, be sure that syntax errors result in an informative message being displayed.