R devel - Feb 2008 - Incorrect paste() output (PR#10801)

Hello,

I am writing this message because of an incorrect output by paste(). 
Please try the following script to see if the evidence I collected is 
reproducible:

x <- c(10152, 28177);
y <- c(9576, 26625);
d <- y - x;

d;
[1]  -576 -1552

paste(d, collapse = ", ");
[1] "-576, -1552"

x <- x / 1000;
y <- y / 1000;
d <- y - x;

d;
[1] -0.576 -1.552

paste(d, collapse = ", ");
[1] "-0.575999999999999, -1.552"

The output of the last paste() is incorrect. I am using R version 2.6.1.

Thank you.

-- 
Ant?nio Ferreira - http://www.di.fc.ul.pt/~asfe
Assistant Lecturer - Department of Informatics
Faculty of Sciences - University of Lisbon - PT

On 2/19/2008 9:50 AM, asfe at di.fc.ul.pt wrote:
> Hello,
> 
> I am writing this message because of an incorrect output by paste(). 
> Please try the following script to see if the evidence I collected is 
> reproducible:
> 
> x <- c(10152, 28177);
> y <- c(9576, 26625);
> d <- y - x;
> 
> d;
> [1]  -576 -1552
> 
> paste(d, collapse = ", ");
> [1] "-576, -1552"
> 
> x <- x / 1000;
> y <- y / 1000;
> d <- y - x;
> 
> d;
> [1] -0.576 -1.552
> 
> paste(d, collapse = ", ");
> [1] "-0.575999999999999, -1.552"
> 
> The output of the last paste() is incorrect. I am using R version 2.6.1.
It looks correct to me.  paste() doesn't use the same formatting rules 
as print() uses.  print() rounded the first number to 7 digits, but 
paste() uses as.character(), which gives

 > as.character(d)
[1] "-0.575999999999999" "-1.552"

If you want output formatted in a particular way, you should use sprintf 
or one of the format* functions.

Duncan Murdoch

This message is in MIME format.  The first part should be readable text,
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On Tue, 19 Feb 2008, asfe at di.fc.ul.pt wrote:
> Hello,
>
> I am writing this message because of an incorrect output by paste().
> Please try the following script to see if the evidence I collected is
> reproducible:
>
> x <- c(10152, 28177);
> y <- c(9576, 26625);
> d <- y - x;
>
> d;
> [1]  -576 -1552
>
> paste(d, collapse = ", ");
> [1] "-576, -1552"
>
> x <- x / 1000;
> y <- y / 1000;
> d <- y - x;
>
> d;
> [1] -0.576 -1.552
>
> paste(d, collapse = ", ");
> [1] "-0.575999999999999, -1.552"
>
> The output of the last paste() is incorrect. I am using R version 2.6.1.
Why do you think it is incorrect?

?paste says

      'paste' converts its arguments (_via_ 'as.character') to
character
      strings, and concatenates them (separating them by the string
      given by 'sep').

and
> as.character(d)
[1] "-0.575999999999999" "-1.552"

so this is correctly documented.  From as.character()

      'as.character' represents real and complex numbers to 15
      significant digits (technically the compiler's setting of the ISO
      C constant 'DBL_DIG', which will be 15 on machines supporting
      IEC60559 arithmetic according to the C99 standard).

and note
> print(d, digits=15)
[1] -0.575999999999999 -1.552000000000000

If you want a limited number of significant figures in paste(), use 
format() to convert the number yourself.

This is another manifestation of FAQ Q7.31: computer calculations on 
non-representable numbers are subject to rounding and representation 
errors.

> --
> Ant?nio Ferreira - http://www.di.fc.ul.pt/~asfe
> Assistant Lecturer - Department of Informatics
> Faculty of Sciences - University of Lisbon - PT
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
-- 
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
--27464147-598378112-1203442115=:3205--

Thank you for your kind answers and sorry for the noise I created.

-- 
Ant?nio Ferreira - http://www.di.fc.ul.pt/~asfe
Assistant Lecturer - Department of Informatics
Faculty of Sciences - University of Lisbon - PT