Interpreter in Java
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"The pattern doesn't address parsing." "When the grammer is very complex, other techniques (such as a parser) are more appropriate."
public class InterpreterDemo {
public static boolean precedence( char a, char b ) {
String high = "*/", low = "+-";
if (a == '(') return false; // if (a == '(' && b == ')') return false;
if (a == ')' && b == '(') { System.out.println( ")-(" ); return false; }
if (b == '(') return false;
if (b == ')') return true;
if (high.indexOf( a ) > -1 && low.indexOf( b ) > -1) return true;
if (high.indexOf( a ) > -1 && high.indexOf( b ) > -1) return true;
if (low.indexOf( a ) > -1 && low.indexOf( b ) > -1) return true;
return false;
}
public static String convertToPostfix( String in ) {
StkChar opstk = new StkChar();
StringBuffer out = new StringBuffer();
String opers = "+-*/()";
char topsym = '+';
boolean empty;
for (int i = 0; i < in.length(); i++)
if (opers.indexOf( in.charAt(i) ) == -1)
out.append( in.charAt(i) );
else {
while ( ! (empty = opstk.isEmpty())
&& precedence( topsym = opstk.pop(), in.charAt(i) ))
out.append( topsym );
if ( ! empty) opstk.push( topsym );
if (empty || in.charAt(i) != ')') opstk.push( in.charAt(i) );
else topsym = opstk.pop();
}
while ( ! opstk.isEmpty()) out.append( opstk.pop() );
return out.toString();
}
public static int evaluate( String in ) {
StkInt stack = new StkInt();
String opers = "+-*/";
for (int a, b, i=0; i < in.length(); i++)
if (opers.indexOf( in.charAt(i) ) == -1)
stack.push( in.charAt(i)-48 );
else {
b = stack.pop(); a = stack.pop();
if (in.charAt(i) == '+') a = a + b;
else if (in.charAt(i) == '-') a = a - b;
else if (in.charAt(i) == '*') a = a * b;
else if (in.charAt(i) == '/') a = a / b;
stack.push( a );
}
return stack.pop();
}
public static void main( String[] args ) {
System.out.print( args[0] );
String postfix = convertToPostfix( args[0] );
System.out.print( " -- " + postfix );
System.out.println( " -- " + evaluate( postfix ) );
}
class StkChar {
private char[] arr = new char[9];
private int sp = -1;
void push( char ch ) { if ( ! isFull()) arr[++sp] = ch; }
char pop() { if (isEmpty()) return '\0'; return arr[sp--]; }
boolean isFull() { return sp == arr.length-1; }
boolean isEmpty() { return sp == -1; }
}
class StkInt {
private int[] arr = new int[9];
private int sp = -1;
void push( int ch ) { if ( ! isFull()) arr[++sp] = ch; }
int pop() { if (isEmpty()) return 0; return arr[sp--]; }
boolean isFull() { return sp == arr.length-1; }
boolean isEmpty() { return sp == -1; }
}}
2+3*4-5+6 -- 234*+5-6+ -- 15
(2+3)*4-5+6 -- 23+4*5-6+ -- 21
2+3*(4-5)+6 -- 2345-*+6+ -- 5
2+3*((4-5)+6) -- 2345-6+*+ -- 17
(3-(4*(5+6))/(7-8))*9/4 -- 3456+*78-/-9*4/ -- 105
