add: x-tba/foundations

x-tba/foundations/03-if-switch-loop/01-for-crunch-the-primes/main.go

@@ -0,0 +1,55 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+// go run . {1..100}
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+)
+
+func main() {
+	// remember [1:] skips the first argument
+
+main:
+	for _, arg := range os.Args[1:] {
+		n, err := strconv.Atoi(arg)
+		if err != nil {
+			// skip non-numerics
+			continue
+		}
+
+		switch {
+		// prime
+		case n == 2 || n == 3:
+			fmt.Print(n, " ")
+			continue
+
+		// not a prime
+		case n <= 1 || n%2 == 0 || n%3 == 0:
+			continue
+		}
+
+		for i, w := 5, 2; i*i <= n; {
+			// not a prime
+			if n%i == 0 {
+				continue main
+			}
+
+			i += w
+			w = 6 - w
+		}
+
+		// all checks ok: it's a prime
+		fmt.Print(n, " ")
+	}
+
+	fmt.Println()
+}