Incorrect comparison of two identical values rounded to 2dp

[kepston]

Hi
I have a report which splits invoice costs to various departments and shows a summary.
The various divisors and multipliers used in deriving the values for each department, are such that the final figures will not be to 2 decimal places.
I decided to use the Round() function so that the calculated figure would equal the formatted figure. I then Summed the rounded figures to reach a total.
Occasionally the sum will not equal the actual invoice total, because of rounding errors.

I also print the actual invoice total on the report in a subreport.
To indicate that the values are different I have a conditionally visible label, warning of the error. However, more times than not, the label is visible even when the invoice value and report sum are identical.

Using the VB debug tracing/watch facility I can see that the values are indeed identical but the comparison evaluates to false.

Does anyone have any ideas how to get around this problem?

Report Detail Text box:
Split value rationalised=Ro und([Split value],2)

Report Footer Text box:
Total=Sum(Round ([Split value],2))

Footer OnFormat:

    If Me.Total <> Me.Invoice.Report![total cost] Then
        Me.Warning.Visible = True
    Else
        Me.Warning.Visible = False
    End If
 

[ADezii]

Hi
I have a report which splits invoice costs to various departments and shows a summary.
The various divisors and multipliers used in deriving the values for each department, are such that the final figures will not be to 2 decimal places.
I decided to use the Round() function so that the calculated figure would equal the formatted figure. I then Summed the rounded figures to reach a total.
Occasionally the sum will not equal the actual invoice total, because of rounding errors.

I also print the actual invoice total on the report in a subreport.
To indicate that the values are different I have a conditionally visible label, warning of the error. However, more times than not, the label is visible even when the invoice value and report sum are identical.

Using the VB debug tracing/watch facility I can see that the values are indeed identical but the comparison evaluates to false.

Does anyone have any ideas how to get around this problem?

Report Detail Text box:
Split value rationalised=Ro und([Split value],2)

Report Footer Text box:
Total=Sum(Round ([Split value],2))

Footer OnFormat:

    If Me.Total <> Me.Invoice.Report![total cost] Then
        Me.Warning.Visible = True
    Else
        Me.Warning.Visible = False
    End If
 

Why not Round at all. How about truncating from the 3rd decimal position on such as:

Fix(((2987.367894)*100))/100 ==> produces 2987.36

[kepston]

Thanks for that.
Unfortunately for me, that produced inaccuracies everytime.
But it did give me an idea

Function Precise(num As Double, dp As Integer) As Double
    ' This function takes a number (num) with an undefined number of decimal places
    ' and returns the value with dp decimal places 
    ' It is used for checking, by removing tiny errors eg 1.4e-14
 
    Dim pw As Integer
 
    pw = 10 ^ dp
    Precise = int(num * pw)/pw
 
End Function

Initially this worked, but after testing with several data sets I saw that

 Precise(136.46,2)

returned 136.45!

Any form of division introduces errors.
So what I've come up with that works so far is this less than perfect routine

Function Precise(num As Double, dp As Integer) As Long
    ' This function takes a number (num) with an undefined number of decimal places
    ' and returns the value without decimal places left shifted to retain the required
    ' accuracy
    ' It is used for checking only, by removing tiny errors eg 1.4e-14
 
Dim pw As Integer
 
    pw = 10 ^ dp
    Precise = num * pw
 
End Function
 

And there was me, thinking that computers were good at numbers!!

[ADezii]

Thanks for that.
Unfortunately for me, that produced inaccuracies everytime.
But it did give me an idea

Function Precise(num As Double, dp As Integer) As Double
    ' This function takes a number (num) with an undefined number of decimal places
    ' and returns the value with dp decimal places 
    ' It is used for checking, by removing tiny errors eg 1.4e-14
 
    Dim pw As Integer
 
    pw = 10 ^ dp
    Precise = int(num * pw)/pw
 
End Function

Initially this worked, but after testing with several data sets I saw that

 Precise(136.46,2)

returned 136.45!

Any form of division introduces errors.
So what I've come up with that works so far is this less than perfect routine

Function Precise(num As Double, dp As Integer) As Long
    ' This function takes a number (num) with an undefined number of decimal places
    ' and returns the value without decimal places left shifted to retain the required
    ' accuracy
    ' It is used for checking only, by removing tiny errors eg 1.4e-14
 
Dim pw As Integer
 
    pw = 10 ^ dp
    Precise = num * pw
 
End Function
 

And there was me, thinking that computers were good at numbers!!

You stated that your data involved various costs. Have you tried using the Currency Data type? The Currency data type is useful for calculations involving money and for fixed-point calculations in which accuracy is particularly important.

[kepston]

You stated that your data involved various costs. Have you tried using the Currency Data type? The Currency data type is useful for calculations involving money and for fixed-point calculations in which accuracy is particularly important.

Thank you once again.
When I tried it, I found that it made no difference, until I changed the multiplying percentage field to Currency as well - not quite so intuitive!

The important fact to remember, then, is fixed point rather than floating point arithmetic.
Is this only available with the Currency data type?
I can see that in VB a variable can be declared as Currency, but can Currency data type be forced, for example, in QBE grid queries? Or is this unnecessary?

[ADezii]

Thank you once again.
When I tried it, I found that it made no difference, until I changed the multiplying percentage field to Currency as well - not quite so intuitive!

The important fact to remember, then, is fixed point rather than floating point arithmetic.
Is this only available with the Currency data type?
I can see that in VB a variable can be declared as Currency, but can Currency data type be forced, for example, in QBE grid queries? Or is this unnecessary?

CCur(<expressio n>) will coerce an Expression to the Currency Data Type. Several examples of CCur() can be seen in a few Northwind.mdb Queries, namely: Invoice Query, Order Details Extended, and Order Subtotals. I do believe, however, that the logic for using this Function is not the same as yours.

[kepston]

Thanks, I don't know how I missed CCur() when I searched Help, before my last post - unless I put CCurr.
I don't have the Northwind sample installed, I will check it out when I find my CD.