[R] Number of digits of a value for problem 7.31 RFAQ

[Richard.Cotton at hsl.gov.uk]

> I need it to solve the problem (see RFAQ 7.31)that 0.2==0.2+0.1-0.1 
FALSE
> The solution suggested in RFAQ is to use 
isTRUE(all.equal(0.2,0.2+0.1-0.1))
> 
> But if I want to compare inequality:
> 0.2<=0.2 +0.1-0.1 TRUE
> but 0.2<=0.2 +0.1-0.1 FALSE
> bad!
> but in this case all.equal does not work I think... Unless to write a 
> double condition like
> 0.2>0.2 +0.1-0.1 | isTRUE(all.equal(0.2,0.2 +0.1-0.1))
> 
> The solution I found is to round the values, because 
> 0.2==round(0.2+0.1-0.1,2) TRUE
> However, one has to know the number of digits of the target value. How 
> can you do when it is unknown?

You can do this by using all.equal to check for cases close to the 
boundary:

if(isTRUE(all.equal(0.2, 0.2 + 0.1 - 0.1))) 
{
  message("Possible rounding problems.") 
  #investigate further
} else if(0.2 <= 0.2 + 0.1 - 0.1)
{
  #your code here
}

all.equal has a tolerance parameter that you can set to see how strict you 
want the equality to be; you may want to make this value smaller.

> What I mean is if R shows 2.3456 I want to obtain the info that digits=4 

> even if in facts the value has more (internal) digits.

Try:
x = 1.23456789
format(x, nsmall=20)
# [1] "1.23456788999999989009"

Regards,
Richie.

Mathematical Sciences Unit
HSL



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