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message FunctionLike +, the message function takes a variable number of
arguments. It is used to send messages to the user and is so useful
that we will describe it here.
A message is printed in the echo area. For example, you can print a message in your echo area by evaluating the following list:
(message "This message appears in the echo area!")
The whole string between double quotation marks is a single argument
and is printed in toto. (Note that in this example, the message
itself will appear in the echo area within double quotes; that is
because you see the value returned by the message function. In
most uses of message in programs that you write, the text will
be printed in the echo area as a side-effect, without the quotes.
See multiply-by-seven in detail, for an example of this.)
However, if there is a %s in the quoted string of characters, the
message function does not print the %s as such, but looks
to the argument that follows the string. It evaluates the second
argument and prints the value at the location in the string where the
%s is.
You can see this by positioning the cursor after the following expression and typing C-x C-e:
(message "The name of this buffer is: %s." (buffer-name))
In Info, "The name of this buffer is: *info*." will appear in the
echo area. The function buffer-name returns the name of the
buffer as a string, which the message function inserts in place
of %s.
To print a value as an integer, use %d in the same way as
%s. For example, to print a message in the echo area that
states the value of the fill-column, evaluate the following:
(message "The value of fill-column is %d." fill-column)
On my system, when I evaluate this list, "The value of
fill-column is 72." appears in my echo area1.
If there is more than one %s in the quoted string, the value of
the first argument following the quoted string is printed at the
location of the first %s and the value of the second argument is
printed at the location of the second %s, and so on.
For example, if you evaluate the following,
(message "There are %d %s in the office!"
(- fill-column 14) "pink elephants")
a rather whimsical message will appear in your echo area. On my system
it says, "There are 58 pink elephants in the office!".
The expression (- fill-column 14) is evaluated and the resulting
number is inserted in place of the %d; and the string in double
quotes, "pink elephants", is treated as a single argument and
inserted in place of the %s. (That is to say, a string between
double quotes evaluates to itself, like a number.)
Finally, here is a somewhat complex example that not only illustrates
the computation of a number, but also shows how you can use an
expression within an expression to generate the text that is substituted
for %s:
(message "He saw %d %s"
(- fill-column 34)
(concat "red "
(substring
"The quick brown foxes jumped." 16 21)
" leaping."))
In this example, message has three arguments: the string,
"He saw %d %s", the expression, (- fill-column 32), and
the expression beginning with the function concat. The value
resulting from the evaluation of (- fill-column 32) is inserted
in place of the %d; and the value returned by the expression
beginning with concat is inserted in place of the %s.
When I evaluate the expression, the message "He saw 38 red
foxes leaping." appears in my echo area.
Actually, you
can use %s to print a number. It is non-specific. %d
prints only the part of a number left of a decimal point, and not
anything that is not a number.