Is this a bug?

ruby-talk
1

for following version:
ruby 1.9.1p129 (2009-05-12 revision 23412) [i386-darwin9.7.0]

irb(main):002:0> sprintf("%.1f",-1.05)
=> "-1.1"
irb(main):003:0> sprintf("%.1f",-32.05)
=> "-32.0"

note the inconsistent rounding.

-1.05 rounds to -1.1
and
-32.50 rounds to -32.0

from sprintf

···

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2

note the inconsistent rounding.

-1.05 rounds to -1.1
and
-32.50 rounds to -32.0

from sprintf

For me it looks like you are rounding -32.05, not -32.50.

irb(main):003:0> sprintf("%.1f",-32.05)

Regards, Rimantas

···

--
http://rimantas.com/

3

Rimantas Liubertas wrote:

note the inconsistent rounding.

-1.05 rounds to -1.1
and
-32.50 rounds to -32.0

from sprintf

For me it looks like you are rounding -32.05, not -32.50.

irb(main):003:0> sprintf("%.1f",-32.05)

Regards, Rimantas

Yes, I intend to round -32.05 to the nearest 1 decimal place not to a
whole number. sprintf is suppose to round to the precision you specify.

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4

Gerry Jenkins wrote:

Rimantas Liubertas wrote:

note the inconsistent rounding.

-1.05 rounds to -1.1
and
-32.50 rounds to -32.0

from sprintf

For me it looks like you are rounding -32.05, not -32.50.

irb(main):003:0> sprintf("%.1f",-32.05)

Regards, Rimantas

Yes, I intend to round -32.05 to the nearest 1 decimal place not to a
whole number. sprintf is suppose to round to the precision you specify.

OK also inconsistent:

irb(main):007:0> sprintf("%.0f",-1.5)
=> "-2"
irb(main):008:0> sprintf("%.0f",-10.5)
=> "-10"

-1.5 rounds down to -2
-10.5 rounds up to -10

···

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5

Gerry Jenkins wrote:

Gerry Jenkins wrote:

Rimantas Liubertas wrote:

note the inconsistent rounding.

-1.05 rounds to -1.1
and
-32.50 rounds to -32.0

from sprintf

For me it looks like you are rounding -32.05, not -32.50.

irb(main):003:0> sprintf("%.1f",-32.05)

Regards, Rimantas

Yes, I intend to round -32.05 to the nearest 1 decimal place not to a
whole number. sprintf is suppose to round to the precision you specify.

OK also inconsistent:

irb(main):007:0> sprintf("%.0f",-1.5)
=> "-2"
irb(main):008:0> sprintf("%.0f",-10.5)
=> "-10"

-1.5 rounds down to -2
-10.5 rounds up to -10

fails even positive numbers:

sprintf("%.0f",10.5)
=> "10"

irb(main):013:0> sprintf("%.0f",11.5)
=> "12"

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6

"Rounding is used when the exact result of a floating-point operation (or a conversion to floating-point format) would need more digits than there are digits in the significand. There are several different rounding schemes (or rounding modes). Historically, truncation was the typical approach. Since the introduction of IEEE 754, the default method (round to nearest, ties to even, sometimes called Banker's Rounding) is more commonly used."

As for the original issue, how numbers ending in 0.05 are rounded for display with 1 digit after the decimal:

It does seem "inconsistent", but it isn't wrong. Both 32.1 and 32.0 are equally distant from 32.05. Internally, floating point numbers are represented as binary digits, so 32.05 isn't *exactly* 32.05, but the closest value that can be represented in binary, which will be ever so slightly higher or lower.

Try looking at the numbers with more decimal points and you'll see what I mean:

irb(main):043:0> sprintf("%.1f",-29.05)
=> "-29.1"
irb(main):044:0> sprintf("%.1f",-30.05)
=> "-30.1"
irb(main):045:0> sprintf("%.1f",-31.05)
=> "-31.1"
irb(main):046:0> sprintf("%.1f",-32.05)
=> "-32.0"
irb(main):047:0> sprintf("%.1f",-33.05)
=> "-33.0"
irb(main):048:0> sprintf("%.1f",-34.05)
=> "-34.0"
irb(main):049:0> sprintf("%.20f",-29.05)
=> "-29.05000000000000071054"
irb(main):050:0> sprintf("%.20f",-30.05)
=> "-30.05000000000000071054"
irb(main):051:0> sprintf("%.20f",-31.05)
=> "-31.05000000000000071054"
irb(main):052:0> sprintf("%.20f",-32.05)
=> "-32.04999999999999715783"
irb(main):053:0> sprintf("%.20f",-33.05)
=> "-33.04999999999999715783"
irb(main):054:0> sprintf("%.20f",-34.05)
=> "-34.04999999999999715783"

So what it's rounding isn't the exact number you're typing in, but the internal representation of that number in binary floating point format.

There are ways of tweaking the math so that the rounding seems more consistent, but no matter what, the values you enter won't be precisely equal to that value when you're using floating point numbers.

If you're not careful about rounding floating point digits, you can end up with something like this:


Ben

···

On Sep 20, 2010, at 18:00 , Gerry Jenkins wrote:

OK also inconsistent:

irb(main):007:0> sprintf("%.0f",-1.5)
=> "-2"
irb(main):008:0> sprintf("%.0f",-10.5)
=> "-10"

-1.5 rounds down to -2
-10.5 rounds up to -10

fails even positive numbers:

sprintf("%.0f",10.5)
=> "10"

irb(main):013:0> sprintf("%.0f",11.5)
=> "12"

7

Thank you Ben,

I did discover that there are various ways of rounding just before your
reply at Rounding - Wikipedia

I am writing a ruby web based unit testing for my java students to
submit their programs to.

My real problem is that sprintf in ruby behaves differently in this
regard to how java rounds.

Oh well, I fixed it by creating unit test data that was within the
precision of the final java output, so that rounding was not used in the
unit testing.

Ben Giddings wrote:

···

On Sep 20, 2010, at 18:00 , Gerry Jenkins wrote:

fails even positive numbers:

sprintf("%.0f",10.5)
=> "10"

irb(main):013:0> sprintf("%.0f",11.5)
=> "12"

Floating-point arithmetic - Wikipedia

"Rounding is used when the exact result of a floating-point operation
(or a conversion to floating-point format) would need more digits than
there are digits in the significand. There are several different
rounding schemes (or rounding modes). Historically, truncation was the
typical approach. Since the introduction of IEEE 754, the default method
(round to nearest, ties to even, sometimes called Banker's Rounding) is
more commonly used."

As for the original issue, how numbers ending in 0.05 are rounded for
display with 1 digit after the decimal:

It does seem "inconsistent", but it isn't wrong. Both 32.1 and 32.0 are
equally distant from 32.05. Internally, floating point numbers are
represented as binary digits, so 32.05 isn't *exactly* 32.05, but the
closest value that can be represented in binary, which will be ever so
slightly higher or lower.

Try looking at the numbers with more decimal points and you'll see what
I mean:

irb(main):043:0> sprintf("%.1f",-29.05)
=> "-29.1"
irb(main):044:0> sprintf("%.1f",-30.05)
=> "-30.1"
irb(main):045:0> sprintf("%.1f",-31.05)
=> "-31.1"
irb(main):046:0> sprintf("%.1f",-32.05)
=> "-32.0"
irb(main):047:0> sprintf("%.1f",-33.05)
=> "-33.0"
irb(main):048:0> sprintf("%.1f",-34.05)
=> "-34.0"
irb(main):049:0> sprintf("%.20f",-29.05)
=> "-29.05000000000000071054"
irb(main):050:0> sprintf("%.20f",-30.05)
=> "-30.05000000000000071054"
irb(main):051:0> sprintf("%.20f",-31.05)
=> "-31.05000000000000071054"
irb(main):052:0> sprintf("%.20f",-32.05)
=> "-32.04999999999999715783"
irb(main):053:0> sprintf("%.20f",-33.05)
=> "-33.04999999999999715783"
irb(main):054:0> sprintf("%.20f",-34.05)
=> "-34.04999999999999715783"

So what it's rounding isn't the exact number you're typing in, but the
internal representation of that number in binary floating point format.

There are ways of tweaking the math so that the rounding seems more
consistent, but no matter what, the values you enter won't be precisely
equal to that value when you're using floating point numbers.

If you're not careful about rounding floating point digits, you can end
up with something like this:

http://img.thedailywtf.com/images/200902/errord/DSC00669.JPG

Ben

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8

I did discover that there are various ways of rounding just before your
reply at Rounding - Wikipedia

I am writing a ruby web based unit testing for my java students to
submit their programs to.

My real problem is that sprintf in ruby behaves differently in this
regard to how java rounds.

Since you are coming from a Java background I guess you are aware that JUnit's TestCase uses a delta for equals comparisons of float and double:

http://www.junit.org/junit/javadoc/3.8.1/junit/framework/Assert.html

Oh well, I fixed it by creating unit test data that was within the
precision of the final java output, so that rounding was not used in the
unit testing.

The proper solution IMHO would be a comparison of numeric values (i.e. not of strings) that takes the delta into account much the same way as it's done in JUnit because there is also named mismatch between the binary internal representation and the decimal external representation that we humans love to use.

Cheers

  robert

···

On 21.09.2010 00:39, Gerry Jenkins wrote:

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