Tag: abstraction

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Maintainability

Good software must be maintainable. It is very rarely the case that a piece of software is developed once and then never nedds to be changed again during its lifetime. This is especially true of user-developed software such as spreadsheets, whose big advantage is that they are easily modified.

Even if the requirements do not change, any reasonably sized piece of software is most unlikely to work properly at first; changes will have to be made in order to remove bugs. In fact, the whole process of development can be seen as one of maintenance, starting from a situation in which nothing works at all.

In the worst case, a simple change requires the whole piece of software to be rewritten from scratch. In the best case, really quite significant changes to the functionality require only small changes in the code. The secret is to write good code and to document it effectively. In this context, good code is code that uses techniques of abstraction and modularity.

Resources

The following external links are relevant:

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Defining Names

In Excel, you can give a range a name. You can do this in two ways, using the Name Box or by using the Define Name dialog. You can change the range that a name refers to, or delete a name, using the Define Name dialog.

Using the Name Box is quicker and easier for straightforward name definitions, but the Define Name dialog provides a more flexible interface.

Using the Name Box to define names

Picture of Name Box

Select the range that you wish to give a name to, and type the name into the Name Box, which is the box just above the top left of the spreadsheet area. In the example, we are defining the name Total to refer to the single cell
A2. Multi-celled ranges canbe given names in exactly the same way.

The Name Box shows all the names that are currently defined, through the drop-down just to its right. Select any name in the drop down to select that range in the workbook.

If you happen to select a range that has a name defined for it, the name will appear in the Name Box.

Using the Define Name dialog to define names

Open the Define Name dialog from the menu, using Insert\Name\Define... Type the name that you wish to define in the top box (Excel will often suggest one for you) and then use the Refers to box to enter the range. You can type the range in, or select it in the usual way. If you type it in, make sure it is an absolute range (with dollar signs) unless you know what you are doing (see related note on using names for formulae). Finally, use the Add button to add the new name to the list.

To delete a name, select it in the list and use the Delete button.

To change the definition of an existing name, select it in the list and change its definition in the Refers To box, then use the Add button.

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Names can be local to worksheets

Many names in Excel are global in scope: the same name means the same thing wherever it is used. For example, the name
Inflation might refer to cell B2 on worksheet Costs.

Often, though, you may have many similar sheets; for example, an Costs sheet for each year of a 5 year business plan. In such cases you might want to use a different inflation rate for each year, but you would still want to use a named range. The solution is to use a name that is local to the worksheet in question.

Names local to worksheets

You can define a name that is local to a specific worksheet by prefixing the name with the worksheet name followed by an exclamation mark. So, for example, you might define CostsYear1!Inflation to refer to B2 on sheet CostsYear1, CostsYear2!Inflation to refer to B2 on sheet CostsYear2, and so on.

If you use the name Inflation in a formula on sheet CostsYear1, it will refer to the value in cell B2 on that sheet. Similarly, the name Inflation when used in a formula on sheet CostsYear2 will refer to the value for year 2.

You can use a name that is local to a specific worksheet on another sheet by using the full name with the sheet prefix, for example CostsYear1!Inflation.

If you have a local and a global name with the same name, the global name is not visible on the worksheet in which the local name is defined.

The Define Name dialog shows only global names and those that are local to the currently active worksheet. Similarly, the Name Box also omits names that are local to other worksheets.

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Using Names for formulae

The usual way to use Names in Excel is to use them to refer to absolute ranges. Typically, the name VATRate might refer to the range $D$2, for example.

However, it is possible to define names to refer to formulae that use relative ranges.

Names that refer to formulae can be useful when they are used to define dynamic ranges. On the whole, however, they should be used with caution: make sure they are documented somewhere, so that a future maintainer will find it easy to understand what is going on. One source of confusion is that their definition may depend on the currently active cell.

Names for formulae

When you define a name using the Define Name dialog, you can actually type any formula you like in the Refers to box. The formula is interpreted as being relative to the currently active cell in the workbook.

For example, if you have cell B9 selected, so that it is currently active, you could define a name AboveAndLeft as referring to =B8+A9. You could then use this name in cell D12, say, to add the
values in D11 and C12.

Picture of formula name

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Abstraction

The notion of abstraction is extremely important for writing good programs or designing good systems. Basically, the idea is that you want to avoid getting hung up on the details: you want to separate the things that change from those that
don’t.

In Excel spreadsheets the easiest example of abstraction is the use of names. You can define a name to refer to a particular range: for example, you could define VATrate to refer to cell D3 on sheet TaxCalcs. By using a name for this, you have abstracted away two levels of detail:

  • The actual VAT rate to use; instead you use the value in cell D3 on sheet TaxCalcs.
  • The actual cell that contains the value; instead you use the name VATrate.

Using abstraction usually has advantages for maintainability, and we can see several of them in this example.

  1. A formula that reads =B26*(1+VATrate) is much easier to understand than either =B26*(1+$D$3) or =B26*1.175
  2. If you want to change the rate of VAT that you are assuming, it is much easier to change the value of a single cell than it is to go through all the formulae in the workbook looking for places where 17.5% is used, and changing them all individually.
  3. If you want to change the layout of your sheet, it is much easier to change the definition of the name VATrate than it is to change all references to cell D3, or to make sure that you drag and drop the cell or cut and paste it in such as way as to preserve all the references.

There are other ways of making use of the advantages of abstraction, but the use of names is the most common in Excel.