Holy cow, I wrote a book!
In a previous life, I wrote database software.
A customer complained that one of their reports was taking an
unacceptably long amount of time to generate, and I was asked
to take a look at it even though it wasn't my account.
The report was a vacation-days report, listing the number of
vacation days taken and available for each employee. Vacation
days accrued at a fixed rate but were granted only in
quarter-day increments. For example, if you earned 15 vacation
days per year and the year was 32% complete, then you had
accrued 32% × 15 = 4.8 vacation days, of which 4.75 were
available to use.
The existing code to round the number of accrued days down to
the nearest quarter-day went something like this:
* assume that at this point, ACCRUED is the number
* of accrued days.
* STR(ACCRUED,6,2) converts ACCRUED to a 6-character
* string: 3 integer digits, a decimal point, and two
* fractional digits. Excess fractional digits are rounded.
STORE STR(ACCRUED,6,2) TO S
STORE RIGHT(S,2) TO F && extract digits after decimal
IF F < "25"
F = "00" && 00 to 24 becomes 00
IF F < "50"
F = "25" && 25 to 49 becomes 25
IF F < "75"
F = "50" && 50 to 74 becomes 50
F = "75" && 75 to 99 becomes 75
ROUNDED = VAL(LEFT(S,4) + F) && reconstruct value and convert
In other words, the code converted the number to a string,
extracted the digits after the decimal point, did string comparisons
to figure out which quartile the fraction resided in, then
created a new string with the replacement fraction and converted
that string back to a number.
And all this in an interpreted language.
This code fragment was repeated each time rounding-down was
needed because the language supported only 32 subroutines,
and this procedure wasn't important enough to be worth kicking
out one of the other existing subroutines.
I replaced this seventeen-line monstrosity with the one-line
equivalent each time it occurred, and the report ran much faster.
(This is nowhere near the strangest way of implementing rounding.
There are far worse examples.)
Exercise: What is the one-line equivalent?
Exercise: What is the double-rounding bug in the original