VBA how to call function inside another function with partial arguments

Question

So I want to translate the following Python code into VBA (just toy example to illustrate the issue, details omitted).

def numerical_integration(fun, a, b):
    """

    :param fun: callable 
    :param a, b: float
    :return: float
    """
    # do something based on both fun and a, b
    res = ...
    return res


def h(x, k):
    """
    helper func
    :param x: float, main arg
    :param k: float, side param
    :return: float
    """
    # calc based on both x and k
    res = ...
    return res


def main_function(k):
    """

    :param k: float 
    :return: float
    """
    a, b = 0.0, 1.0
    res = numerical_integration(fun=lambda x: h(x, k), a=a, b=b)
    return res

The problem is that I don't know how to properly pass something like a "partial" function as argument in VBA. I found how to pass "entire" func in VBA in this post. The idea is to pass the function as a string and then let VBA evaluate the string (something like a Python eval equivalent I guess?). But I don't know how this is gonna help if my function is partial i.e. I want it to be a func on only the first param like lambda x: f(x, k) where the second param is in the context in the main func.

Thanks for any sugguestions in advance.

Possible duplicate of excel vba: Special Types - Functions as Arguments of Functions — Christopher Peisert
– Christopher Peisert, Commented Aug 1, 2019 at 4:10
@ChristopherPeisert that's where I came from. It doesn't solve the "partial" func problem, though. I cannot simply call the whole function using the string. — Vim
– Vim, Commented Aug 1, 2019 at 4:32

Rubik · Accepted Answer · 2019-08-23 13:24:55Z

I don't know Python and "partial" func, but I've try to use an object to replace it. I've created a class module trying to mimic the "partial" func, having optional parameters and default value.

Public defaultX As Double
Public defaultY As Double

Public Function h(Optional x As Variant, Optional y As Variant) As Double

    ' The IsMissing() function below only works with the Variant datatype.

    If (IsMissing(x)) Then
        x = defaultX
    End If
    If (IsMissing(y)) Then
        y = defaultY
    End If

    h = x + y

End Function

Then I've decided to use the "CallByName" function in numerical_integration. And here's the main module code :

Public Function numerical_integration(fun As clsWhatEver, funName As String, a As Double, b As Double) As Integer

    Dim r1 As Double, r2 As Double

    ' CallByName on the "fun" object, calling the "funName" function.
    ' Right after "vbMethod", the parameter is left empty
    ' (assuming default value for it was already set).
    r1 = CallByName(fun, funName, VbMethod, , a)
    r2 = CallByName(fun, funName, VbMethod, , b)

    numerical_integration = r1 + r2

End Function

Public Function main_function(k As Double) As Double
    Dim res As Double

    Dim a As Double, b As Double
    a = 0#
    b = 1#

    Dim oWE As New clsWhatEver
    ' Sets the default value for the first parameter of the "h" function.
    oWE.defaultX = k

    res = numerical_integration(oWE, "h", a, b)

    main_function = res

End Function

user11509084 · Accepted Answer · 2019-08-01 08:15:30Z

0

I'm really unsure what you are asking but if you are trying to pass the result of the first function as an argument for the second then...

Function numerical_integration(fun, a, b)
    numerical_integration = 1
End Function

Function h(x, k)
    h = 0
End Function

Sub main()
    Debug.Print numerical_integration(h(1, 2), 3, 4)
End Sub

(And yes, these should all be strongly-typed variables)

answered Aug 1, 2019 at 8:15

user11509084

2 Comments

Vim Over a year ago

No, not the result. But rather passing lambda x: h(x,k), the mapping per se. The numerical_integration function accepts a function/mapping as input, and returns the integral value, i.e. f -> int f(x)dx from a to b.

user11509084 Over a year ago

I still don't know if I understand the question, but have you checked the CallByName function? learn.microsoft.com/en-us/office/vba/Language/Reference/…

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