Archive for July, 2010

Multi-Area Ranges

We tend to think of a Range as a contiguous set of cells: a single cell, part (or all) of a column, part (or all) of a row, or a rectangular area. However, a Range is one or more or these. The contiguous areas of a Range are held in its Areas collection. If Areas.Count > 1 then we have a multi-area Range.

To construct a multi-area Range from several single-area ranges, you can use the Union operation, which is provided by the Application (so, effectively, global).

I came across an example of this in customizing a chart to show only the most recent N records in the source data. Here’s an accumulating table of monthly records:

I want a clustered column chart, with Months as the category (X) axis, and In/Out as the series.

If I wanted data from all records, I would set the chart’s source data range to be the entire table, including the headers, which Excel interprets to give the series names (that is, In and Out).

If I want my chart to show only the most recent 3 months (say), then I need to specify the source data range as the union of A1:C1 and A6:C8. Here’s the Source Data Range as it appears in the chart configuration:

=’Data 2010’!$A$1:$C$1,’Data 2010’!$A$6:$C$8

Here’s a procedure that constructs and sets the range for a given number of months:

Sub LastNMonths(nmonths As Integer)
'pre: 0 < nmonths
'  <= Range("A1").CurrentRegion.Rows.Count - 1

    Dim ws As Worksheet
    Dim firstrow As Integer, lastrow As Integer

    Set ws = Worksheets("Data 2010")

    lastrow = ws.Range("A1").CurrentRegion.Rows.Count
    firstrow = lastrow - nmonths + 1

    Dim headers As Range
    Set headers = ws.Range("A1:C1")

    Dim rng As Range
    Set rng = ws.Range("A" & firstrow & ":C" & lastrow)

    Set rng = Union(headers, rng)

    Dim cht As Chart
    Set cht = Charts("Chart 2010")
    With cht
        .SetSourceData rng
        .ChartTitle.Text = _
               MonthsTitle(ws, firstrow, lastrow)
    End With
End Sub

MonthsTitle simply constructs the chart title from the first and last months (from column A) – for example, “Report May-10 to Jul-10″.

As I did this initially in v2003, I have not made use of the ‘table-ness’ of the data – hence the Range(“A1″).CurrentRegion.

The procedure was called from a ‘Refresh Chart’ command button on a worksheet, with the nmonths value provided via an InputBox.

Before anyone points this out, let me say that you can do a certain amount of this kind of thing using formulas in the Chart configuration (using OFFSET, etc) – see Walkenbach’s ‘Formulas’ book. However, I think it’s clearer, and certainly more flexible to do it in VBA.

Formula Builder

Suppose that we have a large number of numerical data sets, which we want to examine using aggregation functions, such as SUM, AVERAGE, STDEV, and possibly some custom functions of our own devising. Each of these functions takes an entire dataset and retuns a single result.

Here’s the data:

where ‘data1′, ‘data2′, etc, name the dataset ranges; the column headers are themselves named ‘datasets’. We could, of course, have larger and more numerous datasets.

Now, we could have another worksheet with formulas such as:

=SUM(data1), =SUM(data2), …

= STDEV(data1), = STDEV(data2), …

and so on, for each combination of function and dataset. However, that’s rather a sledgehammer approach.

Alternatively, we could select a function from a drop-down list, select a dataset from another list, and so build a particular formula of interest:

The list of available functions is defined elsewhere, and picked up by data validation in cell B1:

(SUMDIFFS is a sample custom function). Similarly, the data validation on cell D1 picks up the ‘datasets’ list (the data column headers).

The result cell (F1) contains the formula:

=CALC()

where CALC is a public VBA function:

Public Function CALC() As Variant

  Dim func As String
  Dim dataset As String
  Dim formula As String

  func = Range("function").Value
  dataset = Range("dataset").Value
  formula = func & "(" & dataset & ")"

  CALC = Evaluate(formula)
End Function

The formula String does not need an initial ‘=’.

As CALC does not reference its input cells explicitly, it does not get recalculated automatically if the input cells change. So we need to handle Change events that emanate from the two input cells. In the ‘Analysis’ Sheet module:

Private Sub Worksheet_Change(ByVal Target As Range)

  If Target.Address = Range("function").Address Or _
     Target.Address = Range("dataset").Address Then
       Range("result").Dirty
  End If
End Sub

I’ve always wanted use the ‘Dirty’ method … This is equivalent to re-entering the =CALC() formula, triggering recalculation.

A constraint here is that the functions must form a family that have the same argument signature: here, a single array of values. Other statistical functions take more arguments, and might form other families. Alternatively, a multi-argument built-in function could be wrapped in a custom function that defaulted all but the main ‘data’ argument.


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