Rounding for number "Single" filed problem

D

Daryl

Dear All,

I have a lots of trouble of rounding the number (single). Can somebody help.
I have declared a "Single" field and input some numbers for example 12.23,
1.99, 0.99 ...etc
when I try to use those fileds for calculation, for example 1.99 + 0.99, it
will show 2.97 not 2.98 (just for example). Then I checked for the field,
the number I keyed in 0.99 will become 0.988888888888889, so I might get
problem for calculation. Does anyone know what the problem is?


Regards,

Daryl
 
A

Allen Browne

Floating point numbers such as Single and Double are imprecise, as you find
when you perform further math on them, or you discover that 2 numbers that
look the same don't actually match.

Using a Double will give you more digits of precision, but ultimately the
issue is still there. This is true in all computer languages.

If you need no more than 4 places after the decimal point, you can solve the
problem by using the Currency type.

There is also a Decimal type in Access 2000 and later which is scalable to
more than 4 places, but it is too buggy to use:
http://allenbrowne.com/bug-08.html
 
J

John Vinson

Dear All,

I have a lots of trouble of rounding the number (single). Can somebody help.
I have declared a "Single" field and input some numbers for example 12.23,
1.99, 0.99 ...etc
when I try to use those fileds for calculation, for example 1.99 + 0.99, it
will show 2.97 not 2.98 (just for example). Then I checked for the field,
the number I keyed in 0.99 will become 0.988888888888889, so I might get
problem for calculation. Does anyone know what the problem is?


Regards,

Daryl
Click to expand...

Single numbers are *approximations* - as are Double. They're stored as
a binary fraction; just as the number 1/7 cannot be accurately
represented as a finite decimal expansion (0.142856142856142856...),
0.99 cannot be represented exactly as a finite binary fraction. I bet
it's actually Double and 0.98999999999999 or so - Doubles are accurate
to about 14 decimal places, while Singles are accurate to about 7.

I'd suggest that if you need no more than four decimal places, that
you not use *any* kind of Number datatype. Instead use the Currency
datatype. It's a scaled huge integer with exactly four decimals, and
*no* roundoff error.

John W. Vinson[MVP]
 
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