MapBasic For Loop ignoring every other row

Question

I have made a loop in MapBasic but is it only updating every other row in the table:

My code works from the bottom of the table upwards to generate start and end coordinates for polyline features:

    Dim nrows as integer

Dim USX, USY, UEX, UEY as float

For nrows = TableInfo(WorTab, TAB_INFO_NROWS) to 1 Step - 1 
    Select * from WorTab where rowid = nrows
    Dim ObjTYPE as integer
    ObjType = ObjectInfo(Selection.obj, OBJ_INFO_TYPE)
    If ObjType = 4 then
    nPoints = ObjectInfo(Selection.obj, OBJ_INFO_NPNTS)
    USX = ObjectNodeX(Selection.Obj,1,1) 
    USY = ObjectNodeY(Selection.Obj,1,1)
    UEX = ObjectNodeX(Selection.Obj,1,nPoints)
    UEY = ObjectNodeY(Selection.Obj,1,nPoints)
    Update Selection Set SX = USX, SY = USY, EX = UEX, EY = UEY
    nrows = nrows -1
    Close Table Selection
    Else
    End if

I'd be very grateful if somebody could point out what I'm doing wrong and why nrows -1 is the wrong choice.

Answer 1

A For Loop automatically adds to or subtracts from the counter variable.

You are subtracting one from it and then the loop itself subtracts one from the counter. That's why it skips every second records in your table.

If you are sure all your records are polylines and that noe of them have more than one segment, you can do this in a single update statement:

Update Selection 
   Set SX = ObjectNodeX(OBJ, 1, 1)
     , SY = ObjectNodeY(OBJ, 1, 1)
     , EX = ObjectNodeX(OBJ, 1, ObjectInfo(OBJ, OBJ_INFO_NPNTS))
     , EY = ObjectNodeY(OBJ, 1, ObjectInfo(OBJ, OBJ_INFO_NPNTS))

This should also be quicker than doing the loop and querying