Have you ever walked into a Chinese Takeaway and sold them three chicken curries with rice? Or gone to your local station and bought three tickets for yesterday? I'll be you haven't - there are certain things that don't work quite the same way when you go negative. I can't carry on once my car has run out of petrol, secure in the knowledge that I can then put 11 gallons into my 10 gallon tank when I reach the next filling station.

But with some things, negative values ARE acceptable. What is the temperature today? 6? 4? -4? The only difference is that you'll want gloves and need to scrape ice in the latter case.

When writing code to establish the maximum of a series of numbers, it's very tempting to assume that the largest_so_far is zero, then compare each of your series with largest_so_far and note the new value if it's higher. Which works very well if you've got positive numbers. But what were the temperatures in Sptitzbergen last week? -15, -20, -15, a hot -8, and -14. So my tempting algorithm wouldn't work - it would give me a scorching zero degrees.

Solution? Don't start at zero ... start at the first value in the series. Here's an example of a part of that code in Tcl:

if {$count} {
  if {$biggest < $input} {set biggest $input}
  if {$smallest > $input} {set smallest $input}
} else {
  set biggest $input
  set smallest $input
  }
(written 2008-03-31, updated 2008-04-01)

 G906 - Well House Consultants - Programming Standards
  [148] Programming in isolation - (2004-12-11)
  [272] More to programming than just programming - (2005-04-08)
  [343] Should I use structured or object oriented? - (2005-06-10)
  [356] Sudoku helper or sudoku cheat - (2005-06-23)
  [945] Code quality counts - (2006-11-26)
  [1679] PHP - Sanitised application principles for security and useability - (2008-06-16)
  [1852] Perl and Blackberries - (2008-10-23)
  [2322] Looking for a practical standards course - (2009-08-05)
  [2363] Alpaca Case or Camel Case - (2009-08-16)
  [2364] Getting it right from the start - new programmers - (2009-08-17)
  [3685] Programming Standards in Lua - (2012-04-06)

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