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From: Xah Lee on 11 Aug 2010 09:39
• Emacs Lisp's print, princ, prin1, format, message
http://xahlee.org/emacs/elisp_printing.html

plain text version follows.
--------------------------------------------------

This is a short tutorial on printing in emacs lisp. If you don't know
elisp, first take a look at Emacs Lisp Basics.

Simple Printing with “message”

The most basic printing function is “message”. Here's a example:

; printing
(message "hi")

; printing variable values
(message "Her age is: %d " 16) ; %d is for number
(message "Her name is: %s " "Vicky") ; %s is for string
(message "Her mid init is: %c " 86) ; %c is for character in ascii
code
Note: You can see all past output from “message” in the buffer named
“*Messages*”. You can switch to it by “Alt+x switch-to-buffer”.

The “message” function prints to the special buffer “*Messages*”. That
buffer is special, because it is the general output destination for
any messages from emacs, designed to be read by human.

For example, it automatically truncate the top entries when the buffer
reaches some size. Also, when a message is repeated many times, it
automatically condense the repeated lines. And if the a message is a
huge line, the line is truncated automatically.

;; print a line many times
(setq xx 1)
(while (< xx 20)
(message "yay")
(setq xx (1+ xx))
)
(switch-to-buffer "*Messages*")
In the above example, you'll just see: “yay [19 times]”.

(info "(elisp)Displaying Messages")

Print to Your Own Buffer

When writing a elisp script that does batch processing, it's best to
print to your own buffer.

For example, suppose you have a elisp batch script that do find and
replace on all files in a dir. For each file visited, it prints out
the file path. If you use “(message ...)”, it prints to the
“*Messages*” buffer, which automatically roll off the top if you have
more than a hundred lines. Also, it may intermix your script's output
with output from other emacs activities.

Here's a example of printing to your own buffer:

(with-output-to-temp-buffer "*my output*"
(mapc 'my-process-file (find-lisp-find-files inputDir "\\.html$"))
(princ "Done.\n")
(switch-to-buffer "*my output*")
)
“print” function

Elisp provides the “print” function. The basic syntax is this:

(print OBJECT)
The “OBJECT” is any elisp object you want to print. It can be any lisp
datatype, such as string, number, list, ...

“format” function

How a lisp object is converted to string for printing is done by the
“format” function. Use describe-function to lookup its docs..

For example, if you want better control on how your numbers are
formatted, you can do:

(print (format ...))
“princ” and “prin1”

Elisp provide several other convenient functions to control printing.
Here's a summary of their differences:

Function Name Purpose Comment
print print with newline. output can be read back by “read”..
prin1 like print, but does not add newline. output can be read back by
“read”.
princ print without newline nor delimiters. For human reading. Output
can not be read back
(info "(elisp) Output Functions")

Xah
∑ http://xahlee.org/

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