Maybe a Ruby question: Strange math!

Rails
1

irb(main):004:0> (19.9 * 100).to_i
=> 1989
irb(main):005:0> (19.9 * 100)
=> 1990.0
irb(main):006:0> 1990.0.to_i
=> 1990

Can someone explain this?
Regards
Till V.

2

Floating point number aren’t completely accurate.

On 04/09/06, [email protected] [email protected]

3

crazy stuff indeed.

I understand (vaguely) the issue with FP numbers not being accurate,
but I can’t wrap my head around this.

I can’t seem to find any real discussion of ways to deal with this. Is
the only workable solution to this issue to work exclusively with
Integers?

Eric

4

etlund wrote:

crazy stuff indeed.

I understand (vaguely) the issue with FP numbers not being accurate,
but I can’t wrap my head around this.

I can’t seem to find any real discussion of ways to deal with this. Is
the only workable solution to this issue to work exclusively with
Integers?

Eric

Yeah, for the simplicity you can use one integer for the integer part
and one integer for the decimal part. Maybe there is a class around here
on internet for this.

Good luck for all of us

5

On 9/5/06, etlund [email protected] wrote:

crazy stuff indeed.

I understand (vaguely) the issue with FP numbers not being accurate,
but I can’t wrap my head around this.

I can’t seem to find any real discussion of ways to deal with this. Is
the only workable solution to this issue to work exclusively with
Integers?

Yup. It’s standard to deal with integers. If you look at the banking
system then everything there is in whole numbers. On the computer
level $55 is always represented as 5500.
However that method may not work for your needs.

BigDecimal seems to do a bunch. Never used it thought.

001:0> require ‘bigdecimal’
002:0> require ‘bigdecimal/util’
003:0> (19.9.to_d * 100).to_i
=> 1990

http://www.ruby-doc.org/stdlib/libdoc/bigdecimal/rdoc/index.html

–

Jon Gretar B.
http://www.jongretar.net/

6

etlund wrote:

crazy stuff indeed.

I understand (vaguely) the issue with FP numbers not being accurate,
but I can’t wrap my head around this.

I can’t seem to find any real discussion of ways to deal with this. Is
the only workable solution to this issue to work exclusively with
Integers?

Eric

Floating point numbers can only approximate some integers.
In your case, the final product of 19.9 * 100 is probably something
like

1989.99999999999999999999998, which just displays as 1990.0 in most
cases.

Now, .to_i truncates off the decimal part pretty ruthlessly.

To be safe, you should probably use a function like .round instead of
.to_i unless you are absolutely certain you can do that safely.

.round will round the number to the nearest integer, which is really
what you want.

_Kevin

7

_Kevin wrote:

etlund wrote:

crazy stuff indeed.

I understand (vaguely) the issue with FP numbers not being accurate,
but I can’t wrap my head around this.

I can’t seem to find any real discussion of ways to deal with this. Is
the only workable solution to this issue to work exclusively with
Integers?

Eric

Floating point numbers can only approximate some integers.
In your case, the final product of 19.9 * 100 is probably something
like

1989.99999999999999999999998, which just displays as 1990.0 in most
cases.

Now, .to_i truncates off the decimal part pretty ruthlessly.

To be safe, you should probably use a function like .round instead of
.to_i unless you are absolutely certain you can do that safely.

.round will round the number to the nearest integer, which is really
what you want.

_Kevin

I think holding the floating data by two distinct integers is the way to
go. In a practical sense, a class can also make complexity invisible.

8

On Sep 4, 2006, at 5:32 PM, [email protected] wrote:

irb(main):004:0> (19.9 * 100).to_i
=> 1989
irb(main):005:0> (19.9 * 100)
=> 1990.0
irb(main):006:0> 1990.0.to_i
=> 1990

Can someone explain this?

to_i truncates, so

irb(main):001:0> 99.9.to_i
=> 99

You are probably expecting rounding rather than truncation:

irb(main):002:0> 99.9.round
=> 100

Cheers,
Bob

Regards
Till V.

Bob H. – blogs at <http://www.recursive.ca/
hutch/>
Recursive Design Inc. – http://www.recursive.ca/
Raconteur – http://www.raconteur.info/
xampl for Ruby – http://rubyforge.org/projects/xampl/