61 lines
1.8 KiB
Java
61 lines
1.8 KiB
Java
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/*
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* Name: Mike Cifelli
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* Course: CIS 443 - Programming Languages
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* Assignment: Lisp Interpreter 1
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*/
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package eval;
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import parser.*;
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/**
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* <code>MINUS</code> represents the '-' function in Lisp.
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*/
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public class MINUS extends LispFunction {
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public SExpression call(Cons argList) {
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// make sure we have received at least one argument
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if (argList.nullp()) {
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Cons originalSExpr = new Cons(new Symbol("-"), argList);
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throw new RuntimeException("too few arguments given to -: " +
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originalSExpr);
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}
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SExpression argFirst = argList.getCar();
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Cons argRest = (Cons) argList.getCdr();
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// make sure that the first argument is a number
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if (argFirst.numberp()) {
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LispNumber num1 = (LispNumber) argFirst;
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if (argRest.nullp()) {
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// there is only one argument, so return the additive
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// inverse of the number
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return new LispNumber(- num1.getValue());
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}
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SExpression argSecond = argRest.getCar();
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// make sure that the next argument is a number as well
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if (argSecond.numberp()) {
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LispNumber num2 = (LispNumber) argSecond;
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LispNumber difference = new LispNumber(num1.getValue() -
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num2.getValue());
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SExpression argCddr = argRest.getCdr();
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if (argCddr.consp()) {
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return call(new Cons(difference, argCddr));
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}
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return difference;
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}
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throw new RuntimeException("-: " + argSecond + " is not a number");
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}
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throw new RuntimeException("-: " + argFirst + " is not a number");
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}
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}
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