如何检查质数

Question

下面的代码列出了从1到10的所有质数：

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()

如何更改它，使其读取用户输入，然后检查它是否是质数？

Answer 1

创建返回true或false标志的函数。

这是我的模块，它生成随机数，然后检查素数：

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：

​

​

Answer 2

你就快到了。而不是循环，只需获取用户输入并将其放入该变量即可。然后在最后稍微修改一下你的信息。

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()

正如其他人所建议的那样，将逻辑放在一个方法中将是一个好主意。

Answer 3

这是我检查数字是否是质数的最快的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