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text box data  covert to number and calculate NOT WORKING please help

I am trying to allow users of my application to enter numeric data in a textbox then add up to 5 amounts against 1 to verify they are the same. so I need to add convert text to numbers then subtract. the code has been working for months, no issue until someone tried entering 89.63 in tbxTicketAmount, 32.00 in tbxAmount1 and 57.63 in tbxAmount2. these amounts should equal 0 but it does not work. other decimals and/or whole numbers work fine.
Dim x1 As Double
Dim amt1 As Double
Dim amt2 As Double
Dim amt3 As Double
Dim amt4 As Double
Dim amt5 As Double
Dim bal As Double
'set values for each result
x1 = Val(tbxTicketAmount.Text)
amt1 = Val(tbxAmount1.Text)
amt2 = Val(tbxAmount2.Text)
amt3 = Val(tbxAmount3.Text)
amt4 = Val(tbxAmount4.Text)
amt5 = Val(tbxAmount5.Text)
'subtract values, must answer 0
bal = x1  amt1  amt2  amt3  amt4  amt5
If bal <> 0 Then
lblBalance.Text = "[!] Transaction does not balance"
lblSUBMIT.Text = "[!] Transaction does not balance"
Else
Question
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In a computer a single or double precision number has a certain rounding that occurs (or does not). So if you look a the value of what you think is an integer it can be 3.999999999(..) instead of 4. I suspect you dont get exactly zero you get 0.000000000000012 or something. If you view the value of bal it is not zero.
One work around is just use a range of values for zero like this:
Dim tolerance As Double = 0.000001 If bal > tolerance AndAlso bal < tolerance Then End If
PS Using a decimal type may help if you just have hundreths.
 Edited by tommytwotrain Friday, April 21, 2017 9:43 PM add neg

Hi
This sort of calculation can be tricky. When using Doubles in a calculation, results may (usually) have some trailing 'remainders' that can not easily be noticed until trying to do a comparison  such as you are doing.
The Double value of the subtractions you do, even though you can see the result should be zero, actually have a very small fraction remaining (due to the way a computer does these sort of calculations. So, when you try to compare to zero, there is no match. Without doing this sort of thing, your result as computed is 7.1054273576010019E15 quite a small number, but does not equal zero.
Here is some crude code to show one way to overcome the limitations. (there are many ways to do this).
Option Strict On Option Explicit On Public Class Form1 Dim tb1, tb2, tb3 As New TextBox Private Sub Form1_Load(sender As System.Object, e As EventArgs) Handles MyBase.Load tb1.Text = "89.63" tb2.Text = "32.00" tb3.Text = " 57.63" Dim result As Double = (CInt(GetDouble(tb1.Text) * 100)  CInt(GetDouble(tb3.Text) * 100)  CInt(GetDouble(tb2.Text) * 100)) / 100 End Sub Function GetDouble(s As String) As Double Dim v As Double = 0.0 If Double.TryParse(s, v) Then Return v Return 0.0 End Function End Class
Regards Les, Livingston, Scotland
 Edited by leshay Friday, May 05, 2017 11:34 PM


Hi iJams6,
If you format the output, you can achieve the expected results.
Private Sub TextBoxControl_Load(sender As Object, e As EventArgs) Handles MyBase.Load Dim x1 As Double Dim amt1 As Double Dim amt2 As Double Dim amt3 As Double Dim amt4 As Double Dim amt5 As Double Dim bal As Double TextBox1.Text = Convert.ToDouble(89.63) TextBox2.Text = Convert.ToDouble(32.0) TextBox3.Text = Convert.ToDouble(57.63) TextBox4.Text = Convert.ToDouble(0) TextBox5.Text = Convert.ToDouble(0) TextBox6.Text = Convert.ToDouble(0) 'set values for each result x1 = Val(TextBox1.Text) amt1 = Val(TextBox2.Text) amt2 = Val(TextBox3.Text) amt3 = Val(TextBox4.Text) amt4 = Val(TextBox5.Text) amt5 = Val(TextBox6.Text) 'subtract values, must answer 0 bal = x1  amt1  amt2  amt3  amt4  amt5 bal = Format(bal, 0.0) If bal <> 0 Then TextBox7.Text = "[!] Transaction does not balance" TextBox8.Text = "[!] Transaction does not balance" ElseIf bal = 0 Then MessageBox.Show("Value=0") End If MessageBox.Show(TextBox7.Text + TextBox8.Text) End Sub
Hope it is helpful to you.
Best regards,
Cherry Bu
MSDN Community Support
Please remember to click "Mark as Answer" the responses that resolved your issue, and to click "Unmark as Answer" if not. This can be beneficial to other community members reading this thread. If you have any compliments or complaints to MSDN Support, feel free to contact MSDNFSF@microsoft.com. Proposed as answer by Cherry BuMicrosoft contingent staff, Moderator Thursday, April 27, 2017 9:39 AM