Elisp: Format String

By Xah Lee. Date: .

How a lisp object is converted to string for printing is done by the format function. It takes a input string, and several other arguments of lisp objects, and output a string. Alt+x describe-function to lookup its doc string. Here's a excerpt:

(format STRING &rest OBJECTS)

Format a string out of a format-string and arguments.
The first argument is a format control string.
The other arguments are substituted into it to make the result, a string.

The format control string may contain %-sequences meaning to substitute
the next available argument:

%s means print a string argument.  Actually, prints any object, with `princ'.
%d means print as number in decimal (%o octal, %x hex).
%X is like %x, but uses upper case.
%e means print a number in exponential notation.
%f means print a number in decimal-point notation.
%g means print a number in exponential notation
  or decimal-point notation, whichever uses fewer characters.
%c means print a number as a single character.
%S means print any object as an s-expression (using `prin1').

…

For example, if you want to print a date format in yyyy-mm-dd, padding with leading “0”, you can do:

;; format yyyy-mm-dd, ISO 8601 format
(print (format "%04d-%02d-%02d" 2012 4 10))

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