Tuesday, August 25, 2009

What is a number?

[This was originally posted at http://timstall.dotnetdevelopersjournal.com/what_is_a_number.htm]

Having an application collect a number from the user seems simple. Just whip out a textbox, maybe add an "integer validator", and ta da! Sounds great, but endless things can go wrong (things can always go wrong, like with states, zip codes, or even labels). I'm certainly not saying that every textbox in every line of business app should handle all of this, but here are some things to be aware of:

  • Do you allow for special characters, like dollar signs "$", percents "%", or financial negative numbers "( )"? [Seriously, try formatting a negative dollar amount in Excel]
  • Do you allow commas?
  • Do you allow decimals?
  • Do you handle globalization? Some countries use the "," for a decimal point, and the "." for a comma (the inverse of what America does).
  • Do you allow both "-" (for negative) and "+" for positive, or is the "+" just implied by default?
  • Do you allow pre and post zeros: "001.00". Keep in mind that the zeros on the left are mathematically redundant, but the zeros on the right may count as "significant digits", and denote a degree of precision. The extra zeros in "1.25000" implies a much more precise number than just "1.25". However, most numeric types in .programming languages will truncate the trailing zeros.
  • Do you allow exponents for large numbers, like "1E+12" (that's 1 with 12 zeros after it: "1,000,000,000,000")?
  • Do you allow abbreviations, like "10M" for "10,000,000"? (FWIW, I'm not sure of any app that actually does this.)
  • Do you have a clearly defined range? For example, a smallint (Int16) stores much smaller numbers than a long (Int64). Most business values can be stored with a double. However, if you're extending a legacy system that only provides a byte or smallint, but it expects occasionally huge values, then you could get into trouble.
  • Is your validation lenient? For example, if the user enters multiple commas "23,5,6" - do you simply remove all the commas and have "2346", or is that an error?
  • How do you handle nulls? Do you use a sentinel value (like Int32.MinValue), or use the new Nullable data types, or something else? If using a nullable value, does it "convert up" - i.e. Int32.MinValue is null if the type if Int32, but it would be a valid non-null value if the type was Int64. If you have multiple systems reading the same value, and one system uses Int32, and the other uses Int64, will your system still work (this can happen when you start doing things like letting the user dynamically create their own pages or reports).
  • Not to be smart-alecky about it, but I'd just assume that unless otherwise told, this accepts base-10 inputs. I.e. "A5" and "FF" are invalid values - although there may be apps where that is legit (like specify a color in HTML for some blog or content-management software).

Also, does your system handle transformations? That's where you collect the input, store the raw data as something else, and display a newly formatted value. Perhaps the most common example is with percents. A user may type "50" into a textbox (with the "%" in the label next to it), you may save it in the database as "0.5", and you may format it back as "50%" on a report. Or a user may enter "+0" in a textbox, and you simple store and display it as "0".

So, some numerical inputs to test with (besides the obvious invalid numeric inputs, like "xyz") :

  • -23,456.000
  • -23.456,00    //Globalization
  • $45,345.00
  • $(4.00)
  • The max/min values for each of the numeric types.
  • Fill the entire textbox with 9's.
  • 000000000
  • +23
  • 23     //try the simple case
  • -0
  • +0
  • 1.25E+12
  • 1.25 E + 12    //note the spaces

The things to look for are (1) will these prompt validation messages, (2) how will each value be stored in the database, and (3) how will the value be re-formatted when it's displayed.

Perhaps the best standard is to see how Excel handles it, as Excel is probably the most famous application in the world that handles numbers.

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