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Decimal Precision - short code, strange problem


Could anyone help me to understand why in the following code sum1 is
different from sum2?
Basically it's the same calculation done. Keep in mind the x is 28
digits so it's within the range of decimal.

decimal x = (-0.0084682975822 291150192357277 M);
decimal sum1 , sum2;

sum2 = (3500.91M * x) + (x * 3500M + 822m);
sum1 = (3500.91M * x) + (x * 3500M) + 822m;

Console.WriteLi ne (sum1);
Console.WriteLi ne (sum2);

Thank you,
Saar Carmi

Feb 6 '06 #1
2 1602
<sa*******@gmai l.com> wrote:
Could anyone help me to understand why in the following code sum1 is
different from sum2?
Basically it's the same calculation done. Keep in mind the x is 28
digits so it's within the range of decimal.

decimal x = (-0.0084682975822 291150192357277 M);
decimal sum1 , sum2;

sum2 = (3500.91M * x) + (x * 3500M + 822m);
sum1 = (3500.91M * x) + (x * 3500M) + 822m;

Console.WriteLi ne (sum1);
Console.WriteLi ne (sum2);

I believe I understand the reason for the difference. Suppose you have
a type which is accurate to 3 significant digits. Now consider the
difference between:

100 + 1.49 + 0.03
100 + (1.49 + 0.03)

The first would be evaluated as:

(100 + 1.49) + 0.03
=> 101 + 0.03
=> 101

The second is:

100 + (1.49 + 0.03)
=> 100 + 1.52
=> 102

Do you follow why the order is important?

I haven't checked whether or not that's what's happening in your case,
but I suspect it is. You could print out each part of the sum and do it
manually to verify it.

Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too
Feb 6 '06 #2
Seems you're right.
Thank you.

Feb 7 '06 #3

This thread has been closed and replies have been disabled. Please start a new discussion.

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