June 2018 Update

Task/Parsing-Shunting-yard-algorithm/Kotlin/parsing-shunting-yard-algorithm.kotlin

@@ -0,0 +1,61 @@
+// version 1.2.0
+
+import java.util.Stack
+
+/* To find out the precedence, we take the index of the
+   token in the OPS string and divide by 2 (rounding down).
+   This will give us: 0, 0, 1, 1, 2 */
+const val OPS = "-+/*^"
+
+fun infixToPostfix(infix: String): String {
+    val sb = StringBuilder()
+    val s = Stack<Int>()
+    val rx = Regex("""\s""")
+    for (token in infix.split(rx)) {
+        if (token.isEmpty()) continue
+        val c = token[0]
+        val idx = OPS.indexOf(c)
+
+        // check for operator
+        if (idx != - 1) {
+            if (s.isEmpty()) {
+                s.push(idx)
+            }
+            else {
+                while (!s.isEmpty()) {
+                    val prec2 = s.peek() / 2
+                    val prec1 = idx / 2
+                    if (prec2 > prec1 || (prec2 == prec1 && c != '^')) {
+                        sb.append(OPS[s.pop()]).append(' ')
+                    }
+                    else break
+                }
+                s.push(idx)
+            }
+        }
+        else if (c == '(') {
+            s.push(-2)  // -2 stands for '('
+        }
+        else if (c == ')') {
+            // until '(' on stack, pop operators.
+            while (s.peek() != -2) sb.append(OPS[s.pop()]).append(' ')
+            s.pop()
+        }
+        else {
+            sb.append(token).append(' ')
+        }
+    }
+    while (!s.isEmpty()) sb.append(OPS[s.pop()]).append(' ')
+    return sb.toString()
+}
+
+fun main(args: Array<String>) {
+    val es = listOf(
+        "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3",
+        "( ( 1 + 2 ) ^ ( 3 + 4 ) ) ^ ( 5 + 6 )"
+    )
+    for (e in es) {
+        println("Infix : $e")
+        println("Postfix : ${infixToPostfix(e)}\n")
+    }
+}