减去与变量相关的循环

Question

我试图创建一个循环，从每个项目的初始数量减去，直到金额小于项目的价格。它还应该显示在金额低于物品价格之前购买的最后一件物品。汽车售价31万美元，戒指售价12,500美元，钻石售价15万美元，巧克力售价51美元。这些项目位于我的计算机上的文件中，以下是它应该是什么样子的示例。

样本输入：

350000
Car
Ring
Diamond
Car
Chocolate
Diamond

样本输出：

Ring
$27, 500 left

出于某种原因，当我减去时，我得到错误的值，但我无法弄清楚为什么。我已经宣布了每个项目的价格，并多次检查以确保它们是正确的，我已经检查了我的代码，但我仍然不知道为什么我得到了错误的输出。

Private Sub btnS_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnS.Click
    Dim inFile As StreamReader = New StreamReader("serena.txt")
    'Declare the varibles
    Dim variableName As String
    ' the current items from the file
    Dim amount As Integer
    Dim price As Integer
    amount = Val(inFile.ReadLine())
    Do
        'read in the words
        variableName = inFile.ReadLine()
        'determine each item's price
        If variableName = "car" Then price = 310000
        If variableName = "ring" Then price = 12500
        If variableName = "diamond" Then price = 150000
        If amount >= price Then amount = amount - price
    Loop Until amount < price

    'output the results
    Me.lblOutput.Text = variableName & _
        vbNewLine & "Serena has " & Format(amount, "currency") & " left"
End Sub

Answer 1

仔细看看我在下面的代码中添加的评论：

Private Sub btnS_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnS.Click
    Dim fileName As String = Path.Combine(My.Computer.FileSystem.SpecialDirectories.MyDocuments, "serena.txt")

    Dim inFile As New StreamReader(fileName)

    Dim itemToPurchase As String
    Dim lastThingBought As String = "{ Nothing Bought }" ' what if they can't afford anything?
    Dim balance As Integer
    Dim price As Integer
    Dim somethingBought As Boolean

    balance = Val(inFile.ReadLine())
    Do
        somethingBought = False ' we don't know yet if we can purchase the next item
        'read in the words
        itemToPurchase = inFile.ReadLine().ToLower().Trim() ' make sure it has no spaces and is lowercase to match below
        'determine each item's price
        Select Case itemToPurchase
            Case "car"
                price = 310000

            Case "ring"
                price = 12500

            Case "diamond"
                price = 150000

            Case "chocolate" ' FYI: you were missing chocolate in your code
                price = 51

            Case Else ' it could happen!
                MessageBox.Show(itemToPurchase, "Unknown Item!")
                lblOutput.Text = "Error in file"
                Exit Sub

        End Select
        If balance >= price Then
            somethingBought = True
            balance = balance - price
            ' we need to store the last thing purchased,
            ' because "itemToPurchase" gets replaced with the thing you couldn't afford
            lastThingBought = itemToPurchase
        End If
        ' Need to check for the end of file being reached...
        ' ...they may purchase everything in the file and still have money left!
    Loop While Not inFile.EndOfStream AndAlso somethingBought

    'output the results
    Me.lblOutput.Text = lastThingBought &
        vbNewLine & "Serena has " & Format(balance, "currency") & " left"
End Sub