The following example RhinoScript subroutine demonstrates how to set the overall length of a selected curve object. The subroutine prompts the user to pick a curve and to enter a new overall length. From this, the script determines which end of the curve to modify and trims it accordingly.
Option Explicit
Sub SetCrvLength
Dim crv, lold, lnew, dom, pts, t
crv = Rhino.GetCurveObject("Select curve to set length", 4, True)
If Not IsArray(crv) Then Exit Sub
If Rhino.IsCurveClosed(crv(0)) Then
Rhino.Print "Cannot set the length of closed curves."
Exit Sub
End If
lold = Rhino.CurveLength(crv(0))
lnew = Rhino.GetReal("New curve length", lold, 0.0, lold)
If Not IsNumeric(lnew) Then Exit Sub
If (lnew <= 0) Or (lnew >= lold) Then Exit Sub
dom = Rhino.CurveDomain(crv(0))
If (dom(1)-crv(4)) < (crv(4)-dom(0)) Then
Rhino.ReverseCurve crv(0)
dom = Rhino.CurveDomain(crv(0))
End If
pts = Rhino.DivideCurveLength(crv(0), lnew)
t = Rhino.CurveClosestPoint(crv(0), pts(1))
Rhino.TrimCurve crv(0), Array(dom(0), t), True
End Sub