Excel Linear Interpolation VBA

Question

this function interpolates\/extrapolates a table of known x,y For example,

x y
1 10
2 15
3 20

Answer 1

one very simple way is having the function accepting two ranges in input, one for X values (say rX) and one for Y ones (say rY), and then changing every occurrence of r(foo,1) to rX(foo) and r(foo,2) to rY(foo)

like follows

Option Explicit

Function Linterp2(rX As Range, rY As Range, x As Double) As Double
     ' linear interpolator / extrapolator
     ' R is a two-column range containing known x, known y
    Dim lR As Long, l1 As Long, l2 As Long
    Dim nR As Long
     'If x = 1.5 Then Stop

    nR = rX.Rows.Count
    If nR < 2 Then Exit Function

    If x < rX(1) Then ' x < xmin, extrapolate
        l1 = 1: l2 = 2: GoTo Interp

    ElseIf x > rX(nR) Then ' x > xmax, extrapolate
        l1 = nR - 1: l2 = nR: GoTo Interp

    Else
         ' a binary search would be better here
        For lR = 1 To nR
            If rX(lR) = x Then ' x is exact from table
                Linterp2 = rY(lR)
                Exit Function

            ElseIf rX(lR) > x Then ' x is between tabulated values, interpolate
                l1 = lR: l2 = lR - 1: GoTo Interp

            End If
        Next
    End If

Interp:
    Linterp2 = rY(l1) _
    + (rY(l2) - rY(l1)) _
    * (x - rX(l1)) _
    / (rX(l2) - rX(l1))

End Function

but you must implement code to check for consistency of the two ranges, like being both of one column each and with the same number of rows

Answer 2

use this function :

Public Function lineare_iterpolation(x As Variant, x1 As Variant, x2 As Variant, y1 As Variant, y2 As Variant) As Variant
    If x = x1 Then
        lineare_iterpolation = y1
        Exit Function
    End If
    If x = x2 Then
        lineare_iterpolation = y2
        Exit Function
    End If
    lineare_iterpolation = y1 + (x - x1) * (y2 - y1) / (x2 - x1)
    Exit Function
End Function