如何检查素数答案

【问题标题】：How to check for prime numbers如何检查素数
【发布时间】：2014-03-16 14:53:38
【问题描述】：

以下代码列出了从 1 到 10 的所有素数：

Dim primeN As Integer 
Dim primeI As Integer
Dim primeFlag As Boolean

For primeN = 1 To 10
    primeFlag = True
    For primeI = 2 To primeN / 2
       If primeN Mod primeI = 0 Then
         primeFlag = False
       End If
    Next
    If primeFlag Then
       Console.WriteLine(primeN)
    End If
Next
Console.ReadLine()

我怎样才能改变它，让它读取用户输入，然后检查它是否是质数？

【问题讨论】：

标签： vb.net primes

【解决方案1】：

你快到了。而不是循环，只需获取用户输入并将其放入该变量中。然后稍微改变一下最后的信息。

代码：

Dim primeN As Integer
Dim primeI As Integer
Dim primeFlag As Boolean

primeN = Int32.Parse(Console.ReadLine())

primeFlag = True
For primeI = 2 To primeN / 2
    If primeN Mod primeI = 0 Then
        primeFlag = False
    End If
Next
If primeFlag Then
    Console.WriteLine("Is prime")
Else
    Console.WriteLine("Is not prime")
End If

Console.ReadLine()

就像其他人建议的那样，将逻辑放入方法中是个好主意。

【讨论】：

【解决方案2】：

创建一个返回真或假标志的函数。

这是我生成随机数然后检查素数的模块：

Module PrimeNumber

    Sub Main()
        Dim numbers As New ArrayList
        For i As Integer = 1 To 20
            numbers.Add(CInt(Math.Ceiling(Rnd() * 1000000)))
        Next

        For Each int As Integer In numbers
            Console.WriteLine("{0} -> {1}", int, PrimeNumberCheck(int))
        Next

    End Sub

    Public Function PrimeNumberCheck(ByVal number As Integer) As Boolean

        Dim primeI As Integer
        Dim primeFlag As Boolean

        primeFlag = True
        For primeI = 2 To number / 2
            If number Mod primeI = 0 Then
                Return False
            End If
        Next

        Return primeFlag

    End Function

End Module

输出：

【讨论】：

【解决方案3】：

这是我检查数字是否为素数的最快 VBA 代码。

Sub ISPRIME()
Dim number_to_be_checked As Long
Dim c As Long
Dim b As Long
number_to_be_checked = 2000000000 'input value here
c = Round(Sqr(number_to_be_checked)) + 1
If number_to_be_checked = 1 Then
    MsgBox "Not Prime"
Else
    For b = 2 To c
        If number_to_be_checked Mod b = 0 And c <> b Then
            MsgBox "Not Prime. Divisible by " & b
            Exit Sub
        Else
            If b = c Then
                MsgBox "Prime"
                Exit Sub
            End If
        End If
    Next b
End If
End Sub

【讨论】：

【解决方案4】：

Sub generateprimenumbersbetween()
starting_number = 99990 'input value here 
last_number = 99999 'input value here 
primenumbers = "" 
For a = starting_number To last_number 
    For b = 2 To a 
        If a - b * Int(a / b) = 0 And a <> b Then 
            Exit For 
        Else 
            If a = b Then 
                primenumbers = primenumbers & " " & a 
            End If 
        End If 
    Next b 
Next a 
MsgBox primenumbers 
End Sub 

Function primenumbersbetween(starting_number, last_number) 
primenumbers = "" 
For a = starting_number To last_number 
    For b = 2 To a 
        If a - b * Int(a / b) = 0 And a <> b Then 
            Exit For 
        Else 
            If a = b Then 
                primenumbers = primenumbers & " " & a 
            End If 
        End If 
    Next b 
Next a 
primenumbersbetween = primenumbers 
End Function 

'to check if a number is prime 
Sub ISPRIME() 
number_to_be_checked = 102 'input value here 
For b = 2 To number_to_be_checked 
    If number_to_be_checked - b * Int(number_to_be_checked / b) = 0 And _ 
    number_to_be_checked <> b Then 
        MsgBox "Not Prime. Divisible by " & b 
        Exit Sub 
    Else 
        If number_to_be_checked = b Then 
            MsgBox "Prime" 
        End If 
    End If 
Next b 
End Sub 

'to check if a number is prime 
Function ISPRIME2(number_to_be_checked) 
For b = 2 To number_to_be_checked 
    If number_to_be_checked - b * Int(number_to_be_checked / b) = 0 And _ 
    number_to_be_checked <> b Then 
        ISPRIME2 = "Not Prime. Divisible by " & b 
        Exit Function 
    Else 
        If number_to_be_checked = b Then 
            ISPRIME2 = "Prime" 
        End If 
    End If 
Next b 
End Function

【讨论】：