Data commit

Task/Unprimeable-numbers/Go/unprimeable-numbers.go

@@ -0,0 +1,87 @@
+package main
+
+import (
+    "fmt"
+    "strconv"
+)
+
+func isPrime(n int) bool {
+    switch {
+    case n < 2:
+        return false
+    case n%2 == 0:
+        return n == 2
+    case n%3 == 0:
+        return n == 3
+    default:
+        d := 5
+        for d*d <= n {
+            if n%d == 0 {
+                return false
+            }
+            d += 2
+            if n%d == 0 {
+                return false
+            }
+            d += 4
+        }
+        return true
+    }
+}
+
+func commatize(n int) string {
+    s := fmt.Sprintf("%d", n)
+    le := len(s)
+    for i := le - 3; i >= 1; i -= 3 {
+        s = s[0:i] + "," + s[i:]
+    }
+    return s
+}
+
+func main() {
+    fmt.Println("The first 35 unprimeable numbers are:")
+    count := 0           // counts all unprimeable numbers
+    var firstNum [10]int // stores the first unprimeable number ending with each digit
+outer:
+    for i, countFirst := 100, 0; countFirst < 10; i++ {
+        if isPrime(i) {
+            continue // unprimeable number must be composite
+        }
+        s := strconv.Itoa(i)
+        le := len(s)
+        b := []byte(s)
+        for j := 0; j < le; j++ {
+            for k := byte('0'); k <= '9'; k++ {
+                if s[j] == k {
+                    continue
+                }
+                b[j] = k
+                n, _ := strconv.Atoi(string(b))
+                if isPrime(n) {
+                    continue outer
+                }
+            }
+            b[j] = s[j] // restore j'th digit to what it was originally
+        }
+        lastDigit := s[le-1] - '0'
+        if firstNum[lastDigit] == 0 {
+            firstNum[lastDigit] = i
+            countFirst++
+        }
+        count++
+        if count <= 35 {
+            fmt.Printf("%d ", i)
+        }
+        if count == 35 {
+            fmt.Print("\n\nThe 600th unprimeable number is: ")
+        }
+        if count == 600 {
+            fmt.Printf("%s\n\n", commatize(i))
+        }
+    }
+
+    fmt.Println("The first unprimeable number that ends in:")
+    for i := 0; i < 10; i++ {
+        fmt.Printf("  %d is: %9s\n", i, commatize(firstNum[i]))
+    }
+}