fork download
copy 
  1. class Source {
  2.  
  3.     private final String str;
  4.     private int pos;
  5.  
  6.     public Source(String str) {
  7.         this.str = str;
  8.     }
  9.  
  10.     public final int peek() {
  11.         if (pos < str.length()) {
  12.             return str.charAt(pos);
  13.         }
  14.         return -1;
  15.     }
  16.  
  17.     public final void next() {
  18.         ++pos;
  19.     }
  20. }
  21.  
  22. class Parser extends Source {
  23.  
  24.     public Parser(String str) {
  25.         super(str);
  26.     }
  27.  
  28.     public final int number() {
  29.         StringBuilder sb = new StringBuilder();
  30.         int ch;
  31.         while ((ch = peek()) >= 0 && Character.isDigit(ch)) {
  32.             sb.append((char) ch);
  33.             next();
  34.         }
  35.         return Integer.parseInt(sb.toString());
  36.     }
  37.  
  38.     // expr = term, {("+", term) | ("-", term)}
  39.     public final int expr() {
  40.         int x = term();
  41.         for (;;) {
  42.             switch (peek()) {
  43.                 case '+':
  44.                     next();
  45.                     x += term();
  46.                     continue;
  47.                 case '-':
  48.                     next();
  49.                     x -= term();
  50.                     continue;
  51.             }
  52.             break;
  53.         }
  54.         return x;
  55.     }
  56.  
  57.     // term = factor, {("*", factor) | ("/", factor)}
  58.     public final int term() {
  59.         int x = factor();
  60.         for (;;) {
  61.             switch (peek()) {
  62.                 case '*':
  63.                     next();
  64.                     x *= factor();
  65.                     continue;
  66.                 case '/':
  67.                     next();
  68.                     x /= factor();
  69.                     continue;
  70.             }
  71.             break;
  72.         }
  73.         return x;
  74.     }
  75.  
  76.     // factor = factor = [spaces], ("(", expr, ")") | number, [spaces]
  77.     public final int factor() {
  78.         int ret;
  79.         spaces();
  80.         if (peek() == '(') {
  81.             next();
  82.             ret = expr();
  83.             if (peek() == ')') {
  84.                 next();
  85.             }
  86.         } else {
  87.             ret = number();
  88.         }
  89.         spaces();
  90.         return ret;
  91.     }
  92.  
  93.     public void spaces() {
  94.         while (peek() == ' ') {
  95.             next();
  96.         }
  97.     }
  98. }
  99.  
  100. public class Main {
  101.  
  102.     static void test(String s) {
  103.         System.out.println(s + " = " + new Parser(s).expr());
  104.     }
  105.  
  106.     public static void main(String[] args) {
  107.         test("1 + 2"        );
  108.         test("123"          );
  109.         test("1 + 2 + 3"    );
  110.         test("1 - 2 - 3"    );
  111.         test("1 - 2 + 3"    );
  112.         test("2 * 3 + 4"    );
  113.         test("2 + 3 * 4"    );
  114.         test("100 / 10 / 2" );
  115.         test("( 2 + 3 ) * 4");
  116.     }
  117. }
  118.  
Success #stdin #stdout 0.14s 320576KB
comments (?)
stdin
copy 
Standard input is empty
stdout
copy 
1 + 2 = 3
123 = 123
1 + 2 + 3 = 6
1 - 2 - 3 = -4
1 - 2 + 3 = 2
2 * 3 + 4 = 10
2 + 3 * 4 = 14
100 / 10 / 2 = 5
( 2 + 3 ) * 4 = 20
https://ideone.com/fuBe5S
Recursive Descent Parser
language:
Java (HotSpot 12)
created:
7 years ago
visibility:
public

Share or Embed source code

Discover > Sphere Engine API

The brand new service which powers Ideone!

Discover > IDE Widget

Widget for compiling and running the source code in a web browser!