User Defined Spreadsheet
Functions in Excel
Dermot Balson
Perth, Australia
Jerzy Tyszkiewicz
University of Warsaw, Poland
Why User Defined Functions?
• Method to control complexity
divide task into inpependent simpler pieces
• Write once, use often
makes sense to invest in quality
• Method to deliver high quality components
shape-independent integration
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Which one to choose?
Peyton Jones, Blackwell, Burnett
“A user-centered approach to
functions in Excel.
Proceedings ICFP 2003.
The same function in VBA
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vs.
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Less powerful 
Safe 
Same paradigm 
Easy start 
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More powerful 
Security concerns 
New paradigm 
Difficult start 
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User Defined Spreadsheet Functions
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Yes we can!*
Excel does have the capability to
declare and call User Defined
Spreadsheet Functions
• Immediately
• Out of the box
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*From "Bob the Builder" children’s animated TV series
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Plan of the talk
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how we do it
limitations and workarounds
auditing
side effects
speed of computation
construction of function libraries
how to popularize it
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Computation speed
seconds
150
100
50
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Number of calls
Adding 200 calls
Recomputing all calls
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How to popularize UDSFs?
• Difficult to create ...
• ... but not so difficult to use
• So let’s create "open source" libraries ...
• ... for everyone to use (and get attracted)
• And let EuSPRIG keep a repository 
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Summary
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UDSFs are possible in Excel
Serious limitations
Some workarounds are possible
Function libraries are possible
Can you help?
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Thank you!