[R] unexpected result from format(x, digits)

[(Ted Harding)]

Follow-up: You can strip the quotation marks from the output,
if you want, by wrapping the formatC() command in cat().
Example:

  formatC(X,3,format="f")
  # [1] "1.865" "0.405" "0.147" "1.731" "0.090"
  cat(formatC(X,3,format="f"),"\n")
  # 1.865 0.405 0.147 1.731 0.090

Ted.

On 28-Apr-10 22:12:56, Ted Harding wrote:
> On 28-Apr-10 21:45:12, Steve Taylor wrote:
>>  
>> Is this a bug somewhere?  The format function, using a specific
>> number of digits, doesn't give sensible results:
>>  
>> R> set.seed(2);print(x<-rexp(5))
>> [1] .
>> R> format(x,digits=1)
>> [1] "1.87" "0.40" "0.15" "1.73" "0.09"
>> R> format(x,digits=2)
>> [1] "1.87" "0.40" "0.15" "1.73" "0.09"
>> R> format(x,digits=3)
>> [1] "1.8654" "0.4047" "0.1467" "1.7307" "0.0895"
> 
> Not a bug (unless you consider the documented behaviour to amount
> to a bug)!
> 
> From: ?format
> 
>   digits: how many significant digits are to be used for
>     numeric and complex 'x'. The default, 'NUL'?, uses
>     'getOption(digits)'. This is a suggestion: enough
>     decimal places will be used so that the smallest
>     (in magnitude) number has this many significant digits, 
> 
> And that is exactly what is happening.
> 
> In your first example, the smallest number 0.08953 rounds
> (to 1 significant digit) to 0.09 and therefore prints as 0.09.
> 
> In the second, 0.08953 rounds (to 2 significant digits) to 0.090,
> which can be printed without further loss of precision as 0.09
> while giving the other numbers 2 significant digits.
> 
> In the third, 0.08953 rounds (to 3 significant digits) to 0895
> The point about the four significant digits given for the
> other (larger numbers) is that withg format() *all the numbers
> are printed to the same number of decimal places*. Hence the
> smallest number gets 3 signficicant digits, hence 4 decimal
> places. The other larger numbers will then also get 4.
> 
> It looks as though, lurking behind your question, is a secret
> wish for a fixed number of *decimal places* (1, or 2 or 3 in
> your examples). For this, try formatC() (but be careful):
> 
>   X <- c(1.86535, 0.40475, 0.14665, 1.73071, 0.08953)
>   formatC(X,2)
>   # [1] "1.9"  "0.4"  "0.15" "1.7"  "0.09"
>   formatC(X,2,format="f")
>   # [1] "1.87" "0.40" "0.15" "1.73" "0.09"
>   formatC(X,3)
>   # [1] "1.87"   "0.405"  "0.147"  "1.73"   "0.0895"
>   formatC(X,3,format="f")
>   # [1] "1.865" "0.405" "0.147" "1.731" "0.090"
> 
> Hoping this helps!
> Ted.
> 
> --------------------------------------------------------------------
> E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
> Fax-to-email: +44 (0)870 094 0861
> Date: 28-Apr-10                                       Time: 23:12:53
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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 28-Apr-10                                       Time: 23:20:31
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