Question about rounding the decimal

[ray]

Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I should
get the result "48.21". However, the program finally give "48.20". Is there
any mistake for my expression? Please help.
Thanks a lot,
Ray

[Teemu]

"ray" <so*****@micros oft.comkirjoitt i
viestissä:ud*** ***********@TK2 MSFTNGP02.phx.g bl...

Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I should
get the result "48.21". However, the program finally give "48.20". Is
there any mistake for my expression? Please help.
Thanks a lot,
Ray

Quite interesting.

I can't say why it works like that but you could try
Format(31*1.555 ,"0.00").

-Teemu

[rowe_newsgroups]

Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero)

Thanks,

Seth Rowe
ray wrote:

Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I should
get the result "48.21". However, the program finally give "48.20". Is there
any mistake for my expression? Please help.
Thanks a lot,
Ray

[ray]

Dear all,
I have tried Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero) and
the problem is solved.
However, when I tried Math.Round(81.4 45,
2,MidpointRound ing.AwayFromZer o), I suppose that the program should give me
81.45 but actually it give me 81.44. Is there any mistake for my expression?
Please help.
Thanks a lot,
Ray

"rowe_newsgroup s" <ro********@yah oo.com>
???????:11***** *************** **@f16g2000cwb. googlegroups.co m...

Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero)

Thanks,

Seth Rowe
ray wrote:

>Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I
should
get the result "48.21". However, the program finally give "48.20". Is
there
any mistake for my expression? Please help.
Thanks a lot,
Ray

[rowe_newsgroups]

That is very odd! It seems to not like .445 as the following all round
up:

..345 = .35
..545 = .55
..455 = .46
..435 = .44

I have no idea why it's rounding down .445. The documentation states
"The behavior of this method follows IEEE Standard 754, section 4"
perhaps something in the IEEE documentation will explain it. I'll do
some more research and let you know what I find. (Unless someone else
wants to share :-) )

Thanks,

Seth Rowe
ray wrote:

Dear all,
I have tried Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero) and
the problem is solved.
However, when I tried Math.Round(81.4 45,
2,MidpointRound ing.AwayFromZer o), I suppose that the program should give me
81.45 but actually it give me 81.44. Is there any mistake for my expression?
Please help.
Thanks a lot,
Ray

"rowe_newsgroup s" <ro********@yah oo.com>
???????:11***** *************** **@f16g2000cwb. googlegroups.co m...

Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero)

Thanks,

Seth Rowe
ray wrote:

Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I
should
get the result "48.21". However, the program finally give "48.20". Is
there
any mistake for my expression? Please help.
Thanks a lot,
Ray

[Chris Dunaway]

rowe_newsgroups wrote:

That is very odd! It seems to not like .445 as the following all round
up:

.345 = .35
.545 = .55
.455 = .46
.435 = .44

I have no idea why it's rounding down .445. The documentation states
"The behavior of this method follows IEEE Standard 754, section 4"
perhaps something in the IEEE documentation will explain it. I'll do
some more research and let you know what I find. (Unless someone else
wants to share :-) )

It must be a issue with using doubles. When I use this code:

decimal d = 81.445m;
MessageBox.Show (Math.Round(d,2 ,MidpointRoundi ng.AwayFromZero ).ToString());

It shows 81.45.

But using a double shows 81.44

[Göran Andersson]

As always with floating point math, there are almost no values that can
be represented exactly. The closest value to 81.445 that can be
represented is probably something like 81.444999999, which of course
rounds to 81.44.

ray wrote:

Dear all,
I have tried Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero) and
the problem is solved.
However, when I tried Math.Round(81.4 45,
2,MidpointRound ing.AwayFromZer o), I suppose that the program should give me
81.45 but actually it give me 81.44. Is there any mistake for my expression?
Please help.
Thanks a lot,
Ray

"rowe_newsgroup s" <ro********@yah oo.com>
???????:11***** *************** **@f16g2000cwb. googlegroups.co m...

>Math.Round((31 *1.555),2, MidpointRoundin g.AwayFromZero)

Thanks,

Seth Rowe
ray wrote:

>>Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I
should
get the result "48.21". However, the program finally give "48.20". Is
there
any mistake for my expression? Please help.
Thanks a lot,
Ray

[Göran Andersson]

ray wrote:

Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I should
get the result "48.21". However, the program finally give "48.20". Is there
any mistake for my expression? Please help.
Thanks a lot,
Ray

The precision is lost either in the calculation or the conversion. First
you multiply two Double values, which most likely results in a value
that is not exact. Then you convert the Double value into Decimal, which
also can introduce rounding errors.

[rowe_newsgroups]

I just wish I knew how it calculated the value, and why it only rounds
down with .445. Any other double that ends with 45 (.345, .545, etc)
rounds up. Any ideas on the math that goes on behind the scenes?

Thanks,

Seth Rowe
Göran Andersson wrote:

As always with floating point math, there are almost no values that can
be represented exactly. The closest value to 81.445 that can be
represented is probably something like 81.444999999, which of course
rounds to 81.44.

ray wrote:

Dear all,
I have tried Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero) and
the problem is solved.
However, when I tried Math.Round(81.4 45,
2,MidpointRound ing.AwayFromZer o), I suppose that the program should give me
81.45 but actually it give me 81.44. Is there any mistake for my expression?
Please help.
Thanks a lot,
Ray

"rowe_newsgroup s" <ro********@yah oo.com>
???????:11***** *************** **@f16g2000cwb. googlegroups.co m...

Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero)

Thanks,

Seth Rowe
ray wrote:
Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I
should
get the result "48.21". However, the program finally give "48.20". Is
there
any mistake for my expression? Please help.
Thanks a lot,
Ray

[Göran Andersson]

Not exactly, but something like this:

The value 31 is represented as a floating point number as closely to
0.96875*2^5 as possible. The mantissa and exponent are stored binary,
which means that 0.96875 is stored as 1/4 + 1/32 + 1/64 + 1/128 + 1/256
+ 1/1024 + ... etc.

3.875 * 2^3

(I'm not sure if that is totally accurate, but you see how it works.)

A Double value is accurate to 15 digits, so usually any value is off by
something like 0.0000000000000 5%.

So, even though 31 can easily be represented with total accuracy as an
integer, it's very unlikely that it's stored as exactly 31 as a floating
point number.

rowe_newsgroups wrote:

I just wish I knew how it calculated the value, and why it only rounds
down with .445. Any other double that ends with 45 (.345, .545, etc)
rounds up. Any ideas on the math that goes on behind the scenes?

Thanks,

Seth Rowe
Göran Andersson wrote:

>As always with floating point math, there are almost no values that can
be represented exactly. The closest value to 81.445 that can be
represented is probably something like 81.444999999, which of course
rounds to 81.44.

ray wrote:

>>Dear all,
I have tried Math.Round((31* 1.555),2, MidpointRoundin g.AwayFromZero) and
the problem is solved.
However, when I tried Math.Round(81.4 45,
2,MidpointRou nding.AwayFromZ ero), I suppose that the program should give me
81.45 but actually it give me 81.44. Is there any mistake for my expression?
Please help.
Thanks a lot,
Ray

"rowe_newsgro ups" <ro********@yah oo.com>
???????:11*** *************** ****@f16g2000cw b.googlegroups. com...
Math.Round(( 31*1.555),2, MidpointRoundin g.AwayFromZero)

Thanks,

Seth Rowe
ray wrote:
Dear all,
In my vb.net program, I have to round a decimal number to two decimal
place.
For example, for the expression "Decimal.Round( 31* 1.555, 2)", I
should
get the result "48.21". However, the program finally give "48.20". Is
there
any mistake for my expression? Please help.
Thanks a lot,
Ray