[R] Format with n significant figures / digits

[Prof Brian Ripley]

I think you missed the function called 'format'.  R's internal print 
routines (used by format) calculate the format passed to (C level) sprintf 
based on the input, including 'digits'.  Just make sure you pass one 
number at a time to format() if you don't want a common layout for all the 
numbers.

On Thu, 14 Aug 2008, wannymahoots at gmail.com wrote:

> Hi everyone,
>
> I can't figure out how to format numbers to have a certain number of 
> significant figures (as opposed to decimal places) without using scientific 
> notation and including trailing zeros if necessary.
>
> e.g. I would like to achieve the following:
> 0.800001  --->  0.8000
> 123.4567    --->    123.4
> 0.1234567  --->    0.1234
> 7.654321    --->    7.654
> 7654321     --->    7654000
>
> It seems like it should be simple, but I can't seem to do this using sprintf. 
> Specifically, I get the following outputs:
>
>> vec = c(0.800001, 123.4567, 0.1234567, 7.654321, 7654321)
>
>> #  this specifies precision after decimal point, rather than significant 
>> figures
>> sprintf("%.4f", vec) 
> [1] "0.8000"       "123.4567"     "0.1235"       "7.6543" 
> "7654321.0000"
>
>> #  uses scientific notation, but significant figures are correct
>> sprintf("%.3e", vec)
> [1] "8.000e-01" "1.235e+02" "1.235e-01" "7.654e+00" "7.654e+06"
>
>> #  correct number of significant figures, and without scientific notation, 
>> but has trailing zeros on large numbers
>> sprintf("%.4f", sprintf("%.3e", vec))
> [1] "0.8000"       "123.5000"     "0.1235"       "7.6540" 
> "7654000.0000"
>
> Am I missing something obvious and can someone help me?
>
> Many thanks
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595