[R] what is th diffence among "cat","print"and"format"

[Gabor Grothendieck]

ronggui wong <0034058 <at> fudan.edu.cn> writes:

> what is the differnces?
> and when to use each?


I think the main difference is that cat does not know about
objects.  It just unclasses the input (someone can correct
me on this if this is not true), converts each of the
unclassed inputs to character and concatenates and displays
the contatenated result on the standard output or to a file,
if specified. For example, 

	x <- 3
	class(x) <- "myclass"
	cat(x, "\n")
	cat(unclass(x), "\n")  # same

In contrast, print, format and as.character are S3 generics
which look at the class of their first argument and dispatch
a different method depending on which class is.  e.g.

	# using x from above
	print.myclass <- function(x) cat("***", x, "***\n")
	print(x)

The dispatched method will understand how to represent the
object as a character string so that it can display it in
the case of print or return it as a character string in the
case of format and as.character.  as.character is often just
a wrapper around format, e.g. the source for
as.character.Date is:

	as.character.Date <- function(x, ...) format(x, ...)

You can find out which methods are actually available using:

	methods(print)
	methods(format)
	methods(as.character)

The default method is used, e.g. print.default, if there is no 
available method for the class of the object.

If one wants to display a character string with control over
newlines then one typically uses cat.  If one wants to display
an object one uses print or else converts it to a character string
using format or as.character and then display it using cat.

If you just type the name of an object into R then it invokes
the print method for that object.  e.g.

	x   # same as print(x)