Trial and error process to find a variable that its function value is known

Hello friends
I have a code uses a module (IAPWS) and uses its function (enthalpySatVapTW).
this code must calculate the the temprature "T_dis", when "h_dis = enthalpySatVapTW @ T_dis" is known. I've witten the code but i dont know why it doesnt work properly! if someone can help me,i would be grateful of him.
thanks.
In the first box a number between "2600 - 2700" must be intered.
the file is edited an d corrected.

It appears that a Module is missing from your attachment.

Quote:

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Originally Posted by Doogle

It appears that a Module is missing from your attachment.

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the file has been corrected

There are two issues I can see

Code:

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Private Sub OK_Click()

Call Tdiscalcs
h_dis = Val(Text1.Text)


Text2.Text = T_dis
Text3.Text = enthalpySatVapTW(T_dis + 273.15)

End Sub

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you're calling the Subroutine before assigning a value to h_dis

Code:

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Private Sub OK_Click()
h_dis = Val(Text1.Text)

Call Tdiscalcs


Text2.Text = T_dis
Text3.Text = enthalpySatVapTW(T_dis + 273.15)

End Sub

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would be better !

In your Calculations Module, the method used to define the Public variables is not doing what you expect. Only T_dis will be defined as Single all the others will default to Variant
You should be using something like

Code:

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Public T1 As Single, T2 As Single, T3 As Single, h1 As Single, h2 As Single
Public h3 As Single, h_dis As Single, T_dis As Single

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Having 'fixed' those, the program seems to get into an endless loop so I suspect there's something wrong with the calculations (h3 never seems to change value)

Thanks for your helps.
so what can i do to make program converge now?
what other ways do u suggest?

Using '2605' in the first TextBox I used a 'Debug.Print Abs(h3 - h_dis)' statement in Subroutine Tdiscalcs in the 'Do While Abs(h3 - h_dis) > 1' loop and it is constantly showing as 67.53052, so it's not converging.

Quote:

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Originally Posted by Doogle

Using '2605' in the first TextBox I used a 'Debug.Print Abs(h3 - h_dis)' statement in Subroutine Tdiscalcs in the 'Do While Abs(h3 - h_dis) > 1' loop and it is constantly showing as 67.53052, so it's not converging.

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So is there any way to find a function's (enthalpySatVapTW) variable (T) when the value of function is known and the value of variable is unknown and must be found like this?

Quote:

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Originally Posted by hossein8862

so what can i do to make program converge now?
what other ways do u suggest?

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I'm afraid I don't understand the Science so I'm not going to be much help there. I think you're going to have to look at the sequence of calculations.

On second thoughts, perhaps you should be looking at this

Code:

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    Do While Abs(h3 - h_dis) > 1
        T2 = T3
        T3 = (T1 + T2) / 2
        h3 = enthalpySatVapTW(T3 + 273.15)
    Loop

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when T1 = T2 (and it does eventually), T3 will remain constant (20) so enthalpySatVapTW will always return the same value. Perhaps the loop should be based on T2 - T1 = 0 ?? (or Abs(T2 - T1) > 0.000001)

Since T1 starts off as 20 and T2 as 220, T2 will equal T1 after about 28 iterations.

Again, talking off the top of my head and with total ignorance, the fact the h3 and h_dis don't converge is because the T1 -> T2 Range is too small or is of the wrong magnitude, perhaps widening the range may help.

I tested with T1 = 20 and T2 = 320 and h3 and h_dis do converge, when h_dis is 2605, after 2 iterations. Whether that's scientifically 'allowed' you'll have to decide. (T1 = 20, T2 = 95, T3 = 57.5 at the point of convergence)

EDIT: I suppose you could put 57.5 into the 'known' Function and see if you get 2605 out

OK, there are a lot of problems with this code. The first of which is that calling this trial and error is a little misleading. Trial and error is for middle school math... what you are attempting to do is much more sophisticated, and has a much more sophisticated name: binary search.

Now that we have a name for what you are doing, let's compare your code to standard binary search pseudocode (taken directly from Wikipedia):

Binary Search Pseudocode:

Code:

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int binary_search(int A[], int key, int imin, int imax)
{
  // continue searching while [imin,imax] is not empty
  while (imax >= imin)
    {
      /* calculate the midpoint for roughly equal partition */
      int imid = midpoint(imin, imax);
 
      // determine which subarray to search
      if (A[imid] < key)
        // change min index to search upper subarray
        imin = imid + 1;
      else if (A[imid] > key)
        // change max index to search lower subarray
        imax = imid - 1;
      else
        // key found at index imid
        return imid;
    }
  // key not found
  return KEY_NOT_FOUND;
}

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Your code:

Code:

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Public T1, T2, T3, h1, h2, h3, h_dis, T_dis As Single

Public Sub Tdiscalcs()

    ' h is incremental between 0 < T < 220 Celcius

    T1 = 20                              'Celcius
    T2 = 220
    T3 = (T1 + T2) / 2

    h3 = enthalpySatVapTW(T3 + 273.15)
    h1 = enthalpySatVapTW(T1 + 273.15)  'kJ/kg
    h2 = enthalpySatVapTW(T2 + 273.15)

    If (h_dis < h1) Or (h_dis > h2) Then
        Call MsgBox("Out of range outlet Enthalpy", vbOKCancel + vbExclamation, "Range Error !")
    End If
   
    If (h_dis > h1) And (h_dis < h3) Then
        Do While Abs(h3 - h_dis) > 1
            T2 = T3
            T3 = (T1 + T2) / 2
            h3 = enthalpySatVapTW(T3 + 273.15)
        Loop
           
    ElseIf (h_dis > h3) And (h_dis < h2) Then
        Do While Abs(h3 - h_dis) > 1
            T1 = T3
            T3 = (T1 + T2) / 2
            h3 = enthalpySatVapTW(T3 + 273.15)
        Loop
    End If

    T_dis = T3

End Sub

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So there are numerous problems with your code. Let's start with some VB6/programming basics first:
- Always declare Option Explicit at the start of a module. It's just good practice
- use Doubles instead of Singles, which gets you better precision, especially since this is a scientific application
- This is a single subroutine. Get rid of all the global variables! Instead of "Public T1, T2, T3, h1, h2, h3, h_dis, T_dis As Single" outside of Tdiscalcs(), use "Dim T1, T2, T3, h1, h2, h3, h_dis, T_dis As Single" inside of Tdiscalcs()
- But not so fast... when you declare variables in line like "Dim T1, T2, T3, h1, h2, h3, h_dis, T_dis As Single", T_dis is a Single, but all the other ones are Variants, which is clearly not your intention, so instead declare them like this (and remember to use Double, not Single): "Dim T1 As Double, T2 As Double,... h_dis As Double, T_dis As Double". Better yet, declare each variable on its own line.
- This routine accepts a single value as input and calculates a single value as output. Why is this a Subroutine? This should be a Function, just like enthalpySatVapTW() is a Function: Public Function TDiscalcs(h_dis As Double) As Double
- It would be helpful to use more descriptive names. For example, instead of T1, T2, and T3, use something like dblLowTemp, dblHighTemp and dblMidTemp
- Constant values, such as the difference between absolute zero in Kelvin and Celsius (273.15), should be stored as a constant: Private Const KELVIN_CONVERSION As Double = 273.15

Now let's compare the algorithms:
- You have just one problem, but it's pretty major: In the pseudocode, there is a single loop, with an If-Else statement inside. In your code, there is a single If-Else statement with loops inside each branch. That's your problem. You need to change those loops into a single outer loop and have your If-Else inside it. That's it!
- Also, a little helpful tip. I find it easier, in scientific applications like this in VB6, to use square powers instead of absolute values. Squaring a number gets rid of negative values, and in VB6, lets you avoid Type conversion issues with the Abs() function, which uses Variants.

So now that you know where your mistake is, it's your job to fix it. Good luck!

I probably shouldn't but I'm going to rise to the bait.....

Quote:

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Originally Posted by Lenggries

In the pseudocode, there is a single loop, with an If-Else statement inside. In your code, there is a single If-Else statement with loops inside each branch. That's your problem. You need to change those loops into a single outer loop and have your If-Else inside it. That's it!

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Just out of interest, how does quoting some generalised 'C' code and re-naming the problem as 'binary search', help ?

OP's problem is with convergence. Have you actually read all the Posts in this thread?

Quote:

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Originally Posted by Lenggries

So now that you know where your mistake is, it's your job to fix it.

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The 'issues' have already been well defined - Using Option Explicit, defining variables as Double rather than Single, using a Constant rather than an assigned variable, and using descriptive variable names may be good ideas but none of those will actually help the problem of non-convergence.

I think OP is asking for help to resolve the problem, rather than inviting criticism