tasks a-s

Task/Parsing-Shunting-yard-algorithm/Go/parsing-shunting-yard-algorithm.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+    "fmt"
+    "strings"
+)
+
+var input = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3"
+
+var opa = map[string]struct {
+    prec   int
+    rAssoc bool
+}{
+    "^": {4, true},
+    "*": {3, false},
+    "/": {3, false},
+    "+": {2, false},
+    "-": {2, false},
+}
+
+func main() {
+    fmt.Println("infix:  ", input)
+    fmt.Println("postfix:", parseInfix(input))
+}
+
+func parseInfix(e string) (rpn string) {
+    var stack []string // holds operators and left parenthesis
+    for _, tok := range strings.Fields(e) {
+        switch tok {
+        case "(":
+            stack = append(stack, tok) // push "(" to stack
+        case ")":
+            var op string
+            for {
+                // pop item ("(" or operator) from stack
+                op, stack = stack[len(stack)-1], stack[:len(stack)-1]
+                if op == "(" {
+                    break // discard "("
+                }
+                rpn += " " + op // add operator to result
+            }
+        default:
+            if o1, isOp := opa[tok]; isOp {
+                // token is an operator
+                for len(stack) > 0 {
+                    // consider top item on stack
+                    op := stack[len(stack)-1]
+                    if o2, isOp := opa[op]; !isOp || o1.prec > o2.prec ||
+                        o1.prec == o2.prec && o1.rAssoc {
+                        break
+                    }
+                    // top item is an operator that needs to come off
+                    stack = stack[:len(stack)-1] // pop it
+                    rpn += " " + op              // add it to result
+                }
+                // push operator (the new one) to stack
+                stack = append(stack, tok)
+            } else { // token is an operand
+                if rpn > "" {
+                    rpn += " "
+                }
+                rpn += tok // add operand to result
+            }
+        }
+    }
+    // drain stack to result
+    for len(stack) > 0 {
+        rpn += " " + stack[len(stack)-1]
+        stack = stack[:len(stack)-1]
+    }
+    return
+}