Parse postfix arithmetic expressions
import java.io.IOException;
public class ParsePost {
private Stack theStack;
private String input;
public ParsePost(String s) {
input = s;
}
public int doParse() {
theStack = new Stack(20);
char ch;
int j;
int num1, num2, interAns;
for (j = 0; j < input.length(); j++) {
ch = input.charAt(j);
theStack.displayStack("" + ch + " ");
if (ch >= '0' && ch <= '9') // if a number push it
theStack.push((int) (ch - '0'));
else // it's an operator
{
num2 = theStack.pop();
num1 = theStack.pop();
switch (ch) {
case '+':
interAns = num1 + num2;
break;
case '-':
interAns = num1 - num2;
break;
case '*':
interAns = num1 * num2;
break;
case '/':
interAns = num1 / num2;
break;
default:
interAns = 0;
}
theStack.push(interAns);
}
}
interAns = theStack.pop();
return interAns;
}
public static void main(String[] args) throws IOException {
String input = "1-2+3*4+5/6-7+8*9";
int output;
ParsePost aParser = new ParsePost(input);
output = aParser.doParse();
System.out.println("Evaluates to " + output);
}
class Stack {
private int maxSize;
private int[] stackArray;
private int top;
public Stack(int size) {
maxSize = size;
stackArray = new int[maxSize];
top = -1;
}
public void push(int j) {
stackArray[++top] = j;
}
public int pop() {
return stackArray[top--];
}
public int peek() {
return stackArray[top];
}
public boolean isEmpty() {
return (top == -1);
}
public boolean isFull() {
return (top == maxSize - 1);
}
public int size() {
return top + 1;
}
public int peekN(int n) {
return stackArray[n];
}
public void displayStack(String s) {
System.out.print(s);
System.out.print("Stack (bottom>top): ");
for (int j = 0; j < size(); j++) {
System.out.print(peekN(j));
System.out.print(' ');
}
System.out.println("");
}
}
}
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