Generating prime numbers in Go

Question

EDIT: The question essentially asks to generate prime numbers up to a certain limit. The original question follows.

I want my if statement to become true if only these two conditions are met:

for i := 2; i <= 10; i++ {

    if i%i == 0 && i%1 == 0 {

    } else {

    }
}

In this case every possible number gets past these conditions, however I want only the numbers 2, 3, 5, 7, 11... basically numbers that are divisible only with themselves and by 1 to get past, with the exception being the very first '2'. How can I do this?

Thanks

Answer 1

It seems you are looking for prime numbers. However the conditions you described are not sufficient. In fact you have to use an algorithm to generate them (up to a certain limit most probably).

This is an implementation of the Sieve of Atkin which is an optimized variation of the ancient Sieve of Eratosthenes.

Demo: http://play.golang.org/p/XXiTIpRBAu

For the sake of completeness:

package main

import (
    "fmt"
    "math"
)

// Only primes less than or equal to N will be generated
const N = 100

func main() {
    var x, y, n int
    nsqrt := math.Sqrt(N)

    is_prime := [N]bool{}

    for x = 1; float64(x) <= nsqrt; x++ {
        for y = 1; float64(y) <= nsqrt; y++ {
            n = 4*(x*x) + y*y
            if n <= N && (n%12 == 1 || n%12 == 5) {
                is_prime[n] = !is_prime[n]
            }
            n = 3*(x*x) + y*y
            if n <= N && n%12 == 7 {
                is_prime[n] = !is_prime[n]
            }
            n = 3*(x*x) - y*y
            if x > y && n <= N && n%12 == 11 {
                is_prime[n] = !is_prime[n]
            }
        }
    }

    for n = 5; float64(n) <= nsqrt; n++ {
        if is_prime[n] {
            for y = n * n; y < N; y += n * n {
                is_prime[y] = false
            }
        }
    }

    is_prime[2] = true
    is_prime[3] = true

    primes := make([]int, 0, 1270606)
    for x = 0; x < len(is_prime)-1; x++ {
        if is_prime[x] {
            primes = append(primes, x)
        }
    }

    // primes is now a slice that contains all primes numbers up to N
    // so let's print them
    for _, x := range primes {
        fmt.Println(x)
    }
}

Answer 2

Here's a golang sieve of Eratosthenes

package main
import "fmt"

// return list of primes less than N
func sieveOfEratosthenes(N int) (primes []int) {
    b := make([]bool, N)
    for i := 2; i < N; i++ {
        if b[i] == true { continue }
        primes = append(primes, i)
        for k := i * i; k < N; k += i {
            b[k] = true
        }
    }
    return
}

func main() {
    primes := sieveOfEratosthenes(100)
    for _, p := range primes {
        fmt.Println(p)
    }
}

Answer 3

The simplest method to get "numbers that are divisible only with themselves and by 1", which are also known as prime numbers is: http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

It's not a "simple if statement".

Answer 4

If you don't mind a very small chance (9.1e-13 in this case) of them not being primes you can use ProbablyPrime from math/big like this (play)

import (
    "fmt"
    "math/big"
)

func main() {
    for i := 2; i < 1000; i++ {
        if big.NewInt(int64(i)).ProbablyPrime(20) {
            fmt.Printf("%d is probably prime\n", i)
        } else {
            fmt.Printf("%d is definitely not prime\n", i)
        }
    }
}

Just change the constant 20 to be as sure as you like that they are primes.

Answer 5

Simple way(fixed):

package main

import "math"

const n = 100

func main() {
    print(1, " ", 2)

L:  for i := 3; i <= n; i += 2 {
        m := int(math.Floor(math.Sqrt(float64(i))))
        for j := 2; j <= m; j++ {
            if i%j == 0 {
                continue L
            }
        }
        print(" ", i)
    }
}

Answer 6

just change the 100 in the outer for loop to the limit of the prime number you want to find. cheers!!

    for i:=2; i<=100; i++{


        isPrime:=true

        for j:=2; j<i; j++{

            if i % j == 0 {

                isPrime = false
            }
        }

        if isPrime == true {

            fmt.Println(i)
        } 
    }


}

Answer 7

Here try this by checking all corner cases and optimised way to find you numbers and run the logic when the function returns true.

package main

import (
    "math"
    "time"
    "fmt"
)

func prime(n int) bool {

    if n < 1 {
        return false
    }

    if n == 2 {
        return true
    }

    if n % 2 == 0 && n > 2 {
        return false
    }

    var maxDivisor = int(math.Floor(math.Sqrt(float64 (n))))

    //d := 3
    for d:=3 ;d <= 1 + maxDivisor; d += 2 {

        if n%d == 0 {
            return false
        }
    }
    return true
}
//======Test Function=====

func main() {
    // var t0 = time.Time{}
    var t0= time.Second
    for i := 1; i <= 1000; i++ {
        fmt.Println(prime(i))
    }
    var t1= time.Second
    println(t1 - t0)
}

Answer 8

package main

import (
    "fmt"
)

func main() {
    //runtime.GOMAXPROCS(4)

    ch := make(chan int)
    go generate(ch)
    for {
        prime := <-ch
        fmt.Println(prime)
        ch1 := make(chan int)
        go filter(ch, ch1, prime)
        ch = ch1
    }
}

func generate(ch chan int) {
    for i := 2; ; i++ {
        ch <- i
    }
}

func filter(in, out chan int, prime int) {
    for {
        i := <-in
        if i%prime != 0 {
            out <- i
        }
    }
}

Answer 9

A C like logic (old school),

package main
import "fmt"

func main() {
    var num = 1000
    for j := 2; j < num  ; j++ {
        var flag = 0
        for i := 2; i <= j/2  ; i++ {   
            if j % i == 0 {
                flag = 1
                break
            }
        }
        if flag == 0 {
            fmt.Println(j)
        }
    }

}

Answer 10

A prime number is a positive integer that is divisible only by 1 and itself. For example: 2, 3, 5, 7, 11, 13, 17. What is Prime Number? A Prime Number is a whole number that cannot be made by multiplying other whole numbers

A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. Go Language Program to Check Whether a Number is Prime or Not https://www.golanguagehub.com/2021/01/primenumber.html

Answer 11

Simple solution for generating prime numbers up to a certain limit:

func findNthPrime(number int) int {

    if number < 1{
        fmt.Println("Please provide positive number")
        return number
    }

    var primeCounter, nthPrimeNumber int

    for i:=2; primeCounter < number; i++{
        isPrime := true
        for j:=2; j <= int(math.Sqrt(float64(i))) && i != 2  ; j++{
            if i % j == 0{
                isPrime = false
            }
        }
        if isPrime{
            primeCounter++
            nthPrimeNumber = i
            fmt.Println(primeCounter, "th prime number is ", nthPrimeNumber)
        }
    }
    fmt.Println("Nth prime number is ", nthPrimeNumber)
    return nthPrimeNumber
}