8.10 Numeric Arguments

In mathematics and computer usage, argument means “data provided to a function or operation.” You can give any Emacs command a numeric argument (also called a prefix argument). Some commands interpret the argument as a repetition count. For example, C-f with an argument of ten moves forward ten characters instead of one. With these commands, no argument is equivalent to an argument of one. Negative arguments tell most such commands to move or act in the opposite direction.

If your terminal keyboard has a <META> key (labeled <ALT> on PC keyboards), the easiest way to specify a numeric argument is to type digits and/or a minus sign while holding down the <META> key. For example,

M-5 C-n

moves down five lines. The characters Meta-1, Meta-2, and so on, as well as Meta--, do this because they are keys bound to commands (digit-argument and negative-argument) that are defined to set up an argument for the next command. Meta-- without digits normally means -1. Digits and - modified with Control, or Control and Meta, also specify numeric arguments.

You can also specify a numeric argument by typing C-u (universal-argument) followed by the digits. The advantage of C-u is that you can type the digits without modifier keys; thus, C-u works on all terminals. For a negative argument, type a minus sign after C-u. A minus sign without digits normally means -1.

C-u alone has the special meaning of “four times”: it multiplies the argument for the next command by four. C-u C-u multiplies it by sixteen. Thus, C-u C-u C-f moves forward sixteen characters. This is a good way to move forward “fast,” since it moves about 1/5 of a line in the usual size screen. Other useful combinations are C-u C-n, C-u C-u C-n (move down a good fraction of a screen), C-u C-u C-o (make “a lot” of blank lines), and C-u C-k (kill four lines).

Some commands care whether there is an argument, but ignore its value. For example, the command M-q (fill-paragraph) fills text; with an argument, it justifies the text as well. (See section Filling Text, for more information on M-q.) Plain C-u is a handy way of providing an argument for such commands.

Some commands use the value of the argument as a repeat count, but do something peculiar when there is no argument. For example, the command C-k (kill-line) with argument n kills n lines, including their terminating newlines. But C-k with no argument is special: it kills the text up to the next newline, or, if point is right at the end of the line, it kills the newline itself. Thus, two C-k commands with no arguments can kill a nonblank line, just like C-k with an argument of one. (See section Killing and Moving Text, for more information on C-k.)

A few commands treat a plain C-u differently from an ordinary argument. A few others may treat an argument of just a minus sign differently from an argument of -1. These unusual cases are described when they come up; they exist to make an individual command more convenient, and they are documented in that command's documentation string.

You can use a numeric argument before a self-inserting character to insert multiple copies of it. This is straightforward when the character is not a digit; for example, C-u 6 4 a inserts 64 copies of the character ‘a’. But this does not work for inserting digits; C-u 6 4 1 specifies an argument of 641. You can separate the argument from the digit to insert with another C-u; for example, C-u 6 4 C-u 1 does insert 64 copies of the character ‘1’.

We use the term “prefix argument” as well as “numeric argument,” to emphasize that you type these argument before the command, and to distinguish them from minibuffer arguments that come after the command.