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@moltenform
Last active November 6, 2020 23:53
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A little parser 2. evaluating
/* evaluate a tree of results. */
function evalResult(result) {
if (result.type === exps.NumOrGroup) {
/* ex: (123 + 456) */
return evalResult(result.parts[0]);
} else if (result.type === exps.NumLiteral) {
/* from the string "123" into a number. */
return parseFloat(result.parts[0]);
} else {
/* reduce/evaluate the expression */
let [operator, ...exprs] = result.parts;
let val = evalResult(exprs[0]);
for (let i = 1; i < exprs.length; i++) {
if (operator === "+") {
val += evalResult(exprs[i]);
} else if (operator === "*") {
val *= evalResult(exprs[i]);
} else if (operator === "^") {
val = Math.pow(val, evalResult(exprs[i]));
} else {
throw new Error("Unknown operator.");
}
}
return val;
}
}
/* how to run and evaluate an expression */
function demo() {
let got = evalResult(parseTotal(verySimpleLexer("2 + 3 * 4")));
console.log(`2 + 3 * 4 = ${got}`);
}
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