Go to the previous, next section.

Review

In the last few chapters we have introduced a fair number of functions and special forms. Here they are described in brief, along with a few similar functions that have not been mentioned yet.

eval-last-sexp
Evaluate the last symbolic expression before the current location of point. The value is printed in the echo area unless the function is invoked with an argument; in that case, the output is printed in the current buffer. This command is normally bound to C-x C-e.

defun
Define function. This special form has up to five parts: the name, a template for the arguments that will be passed the function, documentation, an optional interactive declaration, and the body of the definition.

For example:

(defun back-to-indentation ()
  "Point to first visible character on line."   
  (interactive)
  (beginning-of-line 1)
  (skip-chars-forward " \t"))

interactive
Declare to the interpreter that the function can be used interactively. This special form may be followed by a string with one or more parts that pass the information to the arguments of the function, in sequence. These parts may also tell the interpreter to prompt for information. Parts of the string are separated by newlines, `\n'.

Common code characters are:

b
The name of an existing buffer.

f
The name of an existing file.

p
The numeric prefix argument. (Note that this `p' is lower case.)
r
Point and the mark, as two numeric arguments, smallest first. This is the only code letter that specifies two successive arguments rather than one.

@xref{Interactive Codes, , Code Characters for elisp, The GNU Emacs Lisp Reference Manual}, for a complete list of code characters.

  • let Declare that a list of variables is for use within the body of the  let and give them an initial value, either  nil or a specified value; then evaluate the rest of the expressions in the body of the  let and return the value of the last one. Inside the body of the  let , the Lisp interpreter does not see the values of the variables of the same names that are bound outside of the  let .

    For example,

    (let ((foo (buffer-name))
          (bar (buffer-size)))
      (message
       "This buffer is %s and has %d characters."
       foo bar))
    

  • save-excursion Record the values of point and mark and the current buffer before evaluating the body of this special form. Restore the values of point and mark and buffer afterward.

    For example,

    (message "We are %d characters into this buffer."
             (- (point)
                (save-excursion
                  (goto-char (point-min)) (point))))
    

  • if Evaluate the first argument to the function; if it is true, evaluate the second argument; else evaluate the third argument, if there is one.

    The  if special form is called a conditional. There are other conditionals in Emacs Lisp, but  if is perhaps the most commonly used.

    For example,

    (if (string= (int-to-string 19)
                 (substring (emacs-version) 10 12))
        (message "This is version 19 Emacs")
      (message "This is not version 19 Emacs"))
    

  • equal
  • eq Test whether two objects are the same.  equal returns true if the two objects have a similar structure and contents. Another function,  eq , returns true if both arguments are actually the same object.

  • <
  • >
  • <=
  • >= The  < function tests whether the first argument is smaller than the second argument. A corresponding function,  > , tests whether the first argument is greater than the second. Likewise,  <= tests whether the first argument is less than or equal to the second and  >= tests whether the first argument is greater than or equal to the second. In all cases, both arguments must be numbers.

  • message Print a message in the echo area. The message can be only one line long. The first argument is a string that can contain `%s', `%d', or `%c' to print the value of arguments that follow the string. The argument used by `%s' must be a string or a symbol; the argument used by `%d' must be a number. The argument used by `%c' must be a number; it will be printed as the character with that ASCII code.

  • setq
  • set The  setq function sets the value of its first argument to the value of the second argument. The first argument is automatically quoted by  setq . It does the same for succeeding pairs of arguments. Another function,  set , takes only two arguments and evaluates both of them before setting the value returned by its first argument to the value returned by its second argument.

  • buffer-name Without an argument, return the name of the buffer, as a string.

  • buffer-file-name Without an argument, return the name of the file the buffer is visiting.

  • current-buffer Return the buffer in which Emacs is active; it may not be the buffer that is visible on the screen.

  • other-buffer Return the most recently selected buffer (other than the buffer passed to  other-buffer as an argument and other than the current buffer).

  • switch-to-buffer Select a buffer for Emacs to be active in and display it in the current window so users can look at it. Usually bound to C-x b.

  • set-buffer Switch Emacs's attention to a buffer on which programs will run. Don't alter what the window is showing.

  • buffer-size Return the number of characters in the current buffer.

  • point Return the value of the current position of the cursor, as an integer counting the number of characters from the beginning of the buffer.

  • point-min Return the minimum permissible value of point in the current buffer. This is 1, unless narrowing is in effect.

  • point-max Return the value of the maximum permissible value of point in the current buffer. This is the end of the buffer, unless narrowing is in effect.
  • Go to the previous, next section.