[R] Format with n significant figures / digits

[wannymahoots at gmail.com]

I am not sure that format solves the problem (although again I may  
well be missing something)

# trailing zeros on large numbers
 >format(vec, digits=4, scientific=F)
[1] "      0.8000" "    123.4567" "      0.1235" "      7.6543"  
"7654321.0000"

# misses trailing zeros
 > format(vec[1], digits=4, scientific=F)
[1] "0.8"

I guess I could resort to writing a routine that called format with  
different arguments depending on the magnitude of each number in the  
vector, but I was hoping for a cleaner solution.  Any thoughts?

On 14 Aug 2008, at 14:57, Prof Brian Ripley wrote:

> 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