Data commit

Task/Strong-and-weak-primes/Go/strong-and-weak-primes.go

@@ -0,0 +1,86 @@
+package main
+
+import "fmt"
+
+func sieve(limit int) []bool {
+    limit++
+    // True denotes composite, false denotes prime.
+    // Don't bother marking even numbers >= 4 as composite.
+    c := make([]bool, limit)
+    c[0] = true
+    c[1] = true
+
+    p := 3 // start from 3
+    for {
+        p2 := p * p
+        if p2 >= limit {
+            break
+        }
+        for i := p2; i < limit; i += 2 * p {
+            c[i] = true
+        }
+        for {
+            p += 2
+            if !c[p] {
+                break
+            }
+        }
+    }
+    return c
+}
+
+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() {
+    // sieve up to 10,000,019 - the first prime after 10 million
+    const limit = 1e7 + 19
+    sieved := sieve(limit)
+    // extract primes
+    var primes = []int{2}
+    for i := 3; i <= limit; i += 2 {
+        if !sieved[i] {
+            primes = append(primes, i)
+        }
+    }
+    // extract strong and weak primes
+    var strong []int
+    var weak = []int{3}                  // so can use integer division for rest
+    for i := 2; i < len(primes)-1; i++ { // start from 5
+        if primes[i] > (primes[i-1]+primes[i+1])/2 {
+            strong = append(strong, primes[i])
+        } else if primes[i] < (primes[i-1]+primes[i+1])/2 {
+            weak = append(weak, primes[i])
+        }
+    }
+
+    fmt.Println("The first 36 strong primes are:")
+    fmt.Println(strong[:36])
+    count := 0
+    for _, p := range strong {
+        if p >= 1e6 {
+            break
+        }
+        count++
+    }
+    fmt.Println("\nThe number of strong primes below 1,000,000 is", commatize(count))
+    fmt.Println("\nThe number of strong primes below 10,000,000 is", commatize(len(strong)))
+
+    fmt.Println("\nThe first 37 weak primes are:")
+    fmt.Println(weak[:37])
+    count = 0
+    for _, p := range weak {
+        if p >= 1e6 {
+            break
+        }
+        count++
+    }
+    fmt.Println("\nThe number of weak primes below 1,000,000 is", commatize(count))
+    fmt.Println("\nThe number of weak primes below 10,000,000 is", commatize(len(weak)))
+}