Excel Add-in Development in C/C++: Applications in ... - F9

22 Excel Add-in Development in C/C++The all-or-nothing edit feature of array formulae makes them useful for helping toprotect calculations from being accidentally overwritten. The worksheet protection featureof Excel is stronger. It allows precise control over what can be modified with passwordprotection. However, it also disables other features that you might want to be accessible,such as the collapse and expansion of grouped rows and columns. Array formulae providea half-way house alternative.Functions and operators that usually take single cell references can also be passedrange arguments in array formulae. How Excel deals with these is covered above insection 2.6.8.2.9.3 Required, optional and missing arguments and variable argument listsSome functions take a fixed number of arguments, all of which need to be suppliedotherwise an error will be returned, for example DATE(). Some take required and optionalarguments, for example, VLOOKUP(). Some take a variable number such as SUM(). Afewfunctions have more than one form of argument-list, such as INDEX(), equivalent to theconcept of overloading in C++.With C/C++ DLL functions, Excel handles variable length argument lists by alwayspassing an argument, regardless of whether the user provided one. A special missing datatype is passed. If the argument can take different types, say, a string or a number, thefunction can be declared in such a way that Excel will pass a general data type. It is thenup to the function’s code whether to execute or fail with the arguments as provided. Thisand related subjects are covered in detail in Chapter 6 Passing Data between Excel andthe DLL on page 105.2.10.1 Data Tables2.10 COMPLEX FUNCTIONS AND COMMANDSData Tables provide a very useful way of creating dynamic tables without having toreplicate the calculations for each cell in the table. Once the calculation has been setup for a single result cell (not in the table), a table of results for a range of inputs isproduced. Excel plugs your inputs in one-by-one and then places the resulting value inthe Data Table. Data Tables can be based on one input to produce a single row or columnof results, or on two inputs to produce a 2-dimensional table.Tables are set up with the Data/Table...command, invoking a simple wizard that promptsyou to specify the input row and/or column for the table. This book doesn’t go into anydetail (refer to Excel’s help to find out more), but it is worth considering what they are.If you look at the formula that Excel puts in part of the table where the results are placed,you will see that there is an array formula {=TABLE(...)}. On the face of it, therefore, itlooks like a Data Table is just another function entered as an array formula. It gives theappearance of being recalculated like a function, except that Excel enables you to turnthe automatic recalculation of tables off using Tools/Options.../Calculation.However: you can’t edit and re-enter the cells under the TABLE() function, even if youhave changed nothing; the Paste Function dialog does not recognise TABLE() as a validfunction; you can’t move the cells that are immediately above or to the left of the cells