floating point with Shell script

[Tony Arnold]

Soo-Hyun Choi wrote:
> Hi,
> 
> This may be a little bit off-topic.
> 
> I would like to do some floating point arithmetic with shell script
> programming as the below.
> 
> if [ floating_number_01 = floating_number_02]; then
>    do-some-work
> else
>    do-some-other-work
> fi
> 
> 
> I know shell script cannot deal the floating point directly. Then, how
> do I deal with the above flow control statement?

You may want to consider using a scripting language such as PERL or
Python that understands floating point numbers. I don;t know of a way to
do this with bash.

However, it is never a good idea to compare two floating point numbers
for equality in any language! Represetnation of floating point numbers
on a computer is only an approximation and arithmetic can introduce
small errors in the least significant bits, enough to cause comparisons
of apparently equal values to return not equal!

If you must compare two floating point numbers, then the best way is to
subtract the two numbers and compare the absolute of the result against
a small number, e.g.,

	if (abs(a-b) < 0.0001)
		do some work.

Regards,
Tony.
-- 
Tony Arnold, IT Security Coordinator, University of Manchester,
IT Services Division, Kilburn Building, Oxford Road, Manchester M13 9PL.
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E: tony.arnold at manchester.ac.uk, H: http://www.man.ac.uk/Tony.Arnold