initial commit

Calculator.java

@@ -0,0 +1,8 @@
+public class Calculator {
+    public static void main(String[] args) {
+        String expr = "(36 * 3 + 9 * 7) * (84 / 12) * 5";
+        Parser p = new Parser(expr);
+        String postfix = p.toPostFix();
+        System.out.println(p.evalExpr(postfix));
+    }
+}

Parser.java

@@ -0,0 +1,143 @@
+/* Todo
+* - Fix Substraction errors
+* - Add support for trigonometric functions
+* - Remove all edge cases
+*/
+
+import java.util.Stack;
+import java.util.StringJoiner;
+
+public class Parser {
+    private String expr;
+    private Stack<Character> operatorStack;
+
+    public Parser(String infixExpr) {
+        expr = infixExpr.trim().replaceAll("\\s", ""); // remove whitespaces
+    }
+
+    public static boolean isOperand(char c) {
+        switch (c) {
+            case '1':
+            case '2':
+            case '3':
+            case '4':
+            case '5':
+            case '6':
+            case '7':
+            case '8':
+            case '9':
+                return true;
+            default:
+                return false;
+        }
+    }
+
+    public static boolean isOperand(String c) {
+        return isOperand(c.charAt(0));
+    }
+
+    public int getPresedence(char c) {
+        switch (c) {
+            case '^':
+                return 3;
+            case '/':
+                return 2;
+            case '*':
+                return 2;
+            case '+':
+                return 1;
+            case '-':
+                return 1;
+            default:
+                return -1;
+        }
+    }
+
+    public boolean hasLeftAssociativity(char c) {
+        if (c == '^') {
+            return false;
+        } else {
+            return true;
+        }
+    }
+
+    public String toPostFix() {
+        operatorStack = new Stack<>();
+        StringJoiner output = new StringJoiner(" ");
+        StringBuilder operand = new StringBuilder();
+        char[] infix = expr.replaceAll(" ", "").toCharArray();
+
+        // Main Expression Iteration
+        for (int i = 0; i < infix.length; i++) {
+            char c = infix[i];
+            if (isOperand(c)) {
+                operand.append(infix[i]);
+                if (i < infix.length - 1) {
+                    continue;
+                }
+            }
+
+            if (operand.length() > 0) {
+                output.add(operand.toString());
+                operand = new StringBuilder();
+            }
+
+            if (c == '(') {
+                operatorStack.push(c);
+            } else if (c == ')') {
+                while (!operatorStack.isEmpty() && operatorStack.peek() != '(') {
+                    output.add(operatorStack.pop().toString());
+                }
+                operatorStack.pop();
+            } else {
+                while (!operatorStack.isEmpty() && getPresedence(c) <= getPresedence((char) operatorStack.peek())
+                        && hasLeftAssociativity(c)) {
+                    output.add(operatorStack.pop().toString());
+                }
+                operatorStack.push(c);
+            }
+        }
+
+        while (!operatorStack.isEmpty()) {
+            if (operatorStack.peek() == '(') {
+                return "This expression is invalid";
+            }
+            output.add(operatorStack.pop().toString());
+        }
+
+        // last digit gets duplicate entry for some reason, so have to remove it.
+        String op = output.toString();
+        return op.substring(0, op.length() -2);
+    }
+
+    private Double evaluate(String operator, Double op1, Double op2) {
+        switch (operator.charAt(0)) {
+            case '+':
+                return op2 + op1;
+            case '-':
+                return op2 - op1;
+            case '*':
+                return op2 * op1;
+            case '/':
+                return op1 / op2;
+            case '^':
+                return (Math.pow(op2, op1));
+            default:
+                return 0.0;
+        }
+    }
+
+    public double evalExpr(String postfix) {
+        Stack<Double> stack = new Stack<Double>();
+        for (String c : postfix.split(" ")) {
+            if (isOperand(c)) {
+                stack.push(Double.parseDouble(c));
+            } else {
+                Double op1 = (Double) (stack.pop());
+                Double op2 = (Double) (stack.pop());
+                stack.push(evaluate(c, op2, op1));
+            }
+        }
+        return stack.pop();
+    }
+}