| 1 | /* Todo |
| 2 | * - Add support for trigonometric functions |
| 3 | * - Remove all edge cases |
| 4 | */ |
| 5 | |
| 6 | import java.util.Stack; |
| 7 | import java.util.StringJoiner; |
| 8 | |
| 9 | public class Parser { |
| 10 | private String expr; |
| 11 | private String postfix; |
| 12 | private Stack<Character> operatorStack; |
| 13 | |
| 14 | public Parser(String infixExpr) { |
| 15 | expr = infixExpr.trim().replaceAll("\\s", ""); // remove whitespaces |
| 16 | postfix = toPostFix(); |
| 17 | } |
| 18 | |
| 19 | private boolean isOperand(char c) { |
| 20 | switch (c) { |
| 21 | case '0': |
| 22 | case '1': |
| 23 | case '2': |
| 24 | case '3': |
| 25 | case '4': |
| 26 | case '5': |
| 27 | case '6': |
| 28 | case '7': |
| 29 | case '8': |
| 30 | case '9': |
| 31 | return true; |
| 32 | default: |
| 33 | return false; |
| 34 | } |
| 35 | } |
| 36 | |
| 37 | private boolean isOperand(String c) { |
| 38 | return isOperand(c.charAt(0)); |
| 39 | } |
| 40 | |
| 41 | private int getPresedence(char c) { |
| 42 | switch (c) { |
| 43 | case '^': |
| 44 | return 3; |
| 45 | case '/': |
| 46 | return 2; |
| 47 | case '*': |
| 48 | return 2; |
| 49 | case '+': |
| 50 | return 1; |
| 51 | case '-': |
| 52 | return 1; |
| 53 | default: |
| 54 | return -1; |
| 55 | } |
| 56 | } |
| 57 | |
| 58 | private boolean hasLeftAssociativity(char c) { |
| 59 | if (c == '^') { |
| 60 | return false; |
| 61 | } else { |
| 62 | return true; |
| 63 | } |
| 64 | } |
| 65 | |
| 66 | private String toPostFix() { |
| 67 | operatorStack = new Stack<>(); |
| 68 | StringJoiner output = new StringJoiner(" "); |
| 69 | StringBuilder operand = new StringBuilder(); |
| 70 | char[] infix = expr.replaceAll(" ", "").toCharArray(); |
| 71 | |
| 72 | // Main Expression Iteration |
| 73 | for (int i = 0; i < infix.length; i++) { |
| 74 | char c = infix[i]; |
| 75 | |
| 76 | // Keep appending digits to the operand StringBuilder and skip iterations till |
| 77 | // we find a operator. |
| 78 | if (isOperand(c)) { |
| 79 | operand.append(infix[i]); |
| 80 | if (i < infix.length - 1) { |
| 81 | continue; |
| 82 | } |
| 83 | } |
| 84 | |
| 85 | if (operand.length() > 0) { |
| 86 | output.add(operand.toString()); |
| 87 | operand = new StringBuilder(); |
| 88 | } |
| 89 | |
| 90 | if (c == '(') { |
| 91 | operatorStack.push(c); |
| 92 | } else if (c == ')') { |
| 93 | while (!operatorStack.isEmpty() && operatorStack.peek() != '(') { |
| 94 | output.add(operatorStack.pop().toString()); |
| 95 | } |
| 96 | operatorStack.pop(); |
| 97 | } else { |
| 98 | // Manage precedence order of operatorStack operators |
| 99 | while (!operatorStack.isEmpty() && getPresedence(c) <= getPresedence((char) operatorStack.peek()) |
| 100 | && hasLeftAssociativity(c)) { |
| 101 | output.add(operatorStack.pop().toString()); |
| 102 | } |
| 103 | |
| 104 | // For whatever reason, the last digit gets added to the operatorStack, leading |
| 105 | // to a duplicate of last digit in the expression |
| 106 | // To fix this, we again make sure that the current char is not a operand. |
| 107 | if (!isOperand(c)) { |
| 108 | operatorStack.push(c); |
| 109 | } |
| 110 | } |
| 111 | } |
| 112 | |
| 113 | // pop all the operators left in the operatorStack after the loop is done. |
| 114 | while (!operatorStack.isEmpty()) { |
| 115 | if (operatorStack.peek() == '(') { |
| 116 | return "This expression is invalid"; |
| 117 | } |
| 118 | output.add(operatorStack.pop().toString()); |
| 119 | } |
| 120 | |
| 121 | return output.toString(); |
| 122 | } |
| 123 | |
| 124 | private Double evaluate(String operator, Double op1, Double op2) { |
| 125 | switch (operator.charAt(0)) { |
| 126 | case '+': |
| 127 | return op2 + op1; |
| 128 | case '-': |
| 129 | return op1 - op2; |
| 130 | case '*': |
| 131 | return op2 * op1; |
| 132 | case '/': |
| 133 | return op1 / op2; |
| 134 | case '^': |
| 135 | return (Math.pow(op2, op1)); |
| 136 | default: |
| 137 | return 0.0; |
| 138 | } |
| 139 | } |
| 140 | |
| 141 | public double eval() { |
| 142 | Stack<Double> stack = new Stack<Double>(); |
| 143 | for (String c : postfix.split(" ")) { |
| 144 | if (isOperand(c)) { |
| 145 | stack.push(Double.parseDouble(c)); |
| 146 | } else { |
| 147 | Double op1 = (Double) (stack.pop()); |
| 148 | Double op2 = (Double) (stack.pop()); |
| 149 | stack.push(evaluate(c, op2, op1)); |
| 150 | } |
| 151 | } |
| 152 | return stack.pop(); |
| 153 | } |
| 154 | |
| 155 | public String getPostfix() { |
| 156 | return postfix; |
| 157 | } |
| 158 | } |