[Rd] a little bug for the function 'sprintf' (PR#14161)

[(Ted Harding)]

On 21-Dec-09 17:36:12, Peter Dalgaard wrote:
> baoliqin at gmail.com wrote:
>> Dear R-ers,
>> I am a gratuate student from South China University of Technology.
>> I fond the function 'sprintf' in R2.10.1 have a little bug(?):
>> 
>> When you type in the example codes:
>> 
>>> sprintf("%s is %f feet tall\n", "Sven", 7.1)
>> 
>> and R returns:
>> 
>> [1] "Sven is 7.100000 feet tall\n"
>> 
>> this is very different from the 'sprintf' function in C/C++,
>> for in C/C++, the format string "\n" usually represents a new line,
>> but here, just the plain text "\n"!
> 
> No, this is exactly the same as in C/C++. If you compare the result
> of sprintf to "Sven is 7.100000 feet tall\n" with strcmp() in C,
> they will compare equal.
> 
>  > s <- sprintf("%s is %f feet tall\n", "Sven", 7.1)
>  > s
> [1] "Sven is 7.100000 feet tall\n"
>  > nchar(s)
> [1] 27
>  > substr(s,27,27)
> [1] "\n"
> 
> The thing that is confusing you is that strings are DISPLAYED
> using the same escape-character mechanisms as used for input.
> Compare
> 
>  > cat(s)
> Sven is 7.100000 feet tall
>  >
> 
>> 
>> Is it a bug, or a deliberate design?
> 
> Design, not bug (and please don't file as bug when you are in doubt.)

And another confusion is that the C/C++ function sprintf() indeed
creates a string AND ASSIGNS IT TO A NAMED VARIABLE, according to
the syntax

  int sprintf(char *str, const char *format, ...);

as in

  char *X ;
  sprintf(X,"%s is %f feet tall\n", "Sven", 7.1) ;

as a result of which the string X will have the value
  "Sven is 7.100000 feet tall\n"

R's sprintf does not provide for the parameter "Char *str", here X,
and so RETURNS the string as the value of the function.

This is NOT TO BE CONFUSED with the behaviour of the C/C++ functions
printf() and fprintf(), both of which create the string and then
send it to either stdout or to a file:

  int printf(const char *format, ...);
  int fprintf(FILE *stream, const char *format, ...);

Therefore, if you programmed

  printf("%s is %f feet tall\n", "Sven", 7.1) ;

you would see on-screen (stdout) the string 

  "Sven is 7.100000 feet tall"

(followed by a line-break due to the "\n"), while

  mystream = fopen("myoutput.txt",a) ;
  fprintf(mystream, "%s is %f feet tall\n", "Sven", 7.1) ;

would append

  "Sven is 7.100000 feet tall"

(followed by a line-break) to myoutput.txt

Hoping this helps!
Ted.

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 21-Dec-09                                       Time: 18:14:57
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