# How to truncate float number after dot?

Discussion in 'Ruby' started by Marcin Tyman, Jul 26, 2007.

1. ### Marcin TymanGuest

Hi,

How to truncate float number after dot? I suppose it would be easy.

MT

Marcin Tyman, Jul 26, 2007

2. ### hemantGuest

19.84.round #=> big brother is watching you

hemant, Jul 26, 2007

irb(main):001:0> 12.68.floor
=> 12
irb(main):002:0> 12.68.ceil
=> 13

Pat

Gah, I forgot the all important round!

irb(main):003:0> 12.68.round
=> 13
irb(main):004:0> 12.38.round
=> 12

Pat

5. ### Robert KlemmeGuest

12.38.to_i

robert

Robert Klemme, Jul 26, 2007
6. ### Marcin TymanGuest

I didn't mean to truncate all. But i.e. to round to several places after
dot.

Marcin Tyman, Jul 26, 2007
7. ### Robert KlemmeGuest

Usually it's better to calculate with full precision and round when
printing, e.g. using printf of

irb(main):022:0> "%4.1f" % 1.0
=> " 1.0"
irb(main):023:0> "%4.1f" % 1.04
=> " 1.0"
irb(main):024:0> "%4.1f" % 1.05
=> " 1.1"

Kind regards

robert

Robert Klemme, Jul 26, 2007
8. ### Josef 'Jupp' SchugtGuest

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Rounding to a certain number of digits works as follows:

Let n be the number of digits following the decimal point you want to
obtain. Then

1. Multiply the number by 10 to the power n (i.e. by 10.0 ** n)
2. Round the number (i.e. use Float#round)
3. Divide the result by 10 to the power n

The latter can either be done by division by 10.0 ** n or by
multiplication by 0.1 ** n.

But as Robert Klemme already noted: It rarely makes sense to actually
round numbers. It usually makes more sense to compute with the available
precision and limit the output to the wanted number of digits.

Anyway, sometimes it makes sense. For example if you are required by law
to round intermediate calculation results to a certain amount of digits.
But in such cases one should not at all use floating point numbers for
the computations. One would rather use integers, fixed-point arithmetics
(as e.g. supported by COBOL) or the Decimal data type from Microsoft's
Common Type System (e.g. supported by C#).

Josef 'Jupp' Schugt
- --
Blog available at http://www.mynetcologne.de/~nc-schugtjo/blog/
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Josef 'Jupp' Schugt, Jul 26, 2007