— Function: 1+ number-or-marker

This function returns number-or-marker plus 1. For example,

          (setq foo 4)
               ⇒ 4
          (1+ foo)
               ⇒ 5

This function is not analogous to the C operator ++—it does not increment a variable. It just computes a sum. Thus, if we continue,

          foo
               ⇒ 4

If you want to increment the variable, you must use setq, like this:

          (setq foo (1+ foo))
               ⇒ 5

— Function: 1- number-or-marker

This function returns number-or-marker minus 1.

— Function: + &rest numbers-or-markers

This function adds its arguments together. When given no arguments, + returns 0.

          (+)
               ⇒ 0
          (+ 1)
               ⇒ 1
          (+ 1 2 3 4)
               ⇒ 10

— Function: - &optional number-or-marker &rest more-numbers-or-markers

The - function serves two purposes: negation and subtraction. When - has a single argument, the value is the negative of the argument. When there are multiple arguments, - subtracts each of the more-numbers-or-markers from number-or-marker, cumulatively. If there are no arguments, the result is 0.

          (- 10 1 2 3 4)
               ⇒ 0
          (- 10)
               ⇒ -10
          (-)
               ⇒ 0

— Function: * &rest numbers-or-markers

This function multiplies its arguments together, and returns the product. When given no arguments, * returns 1.

          (*)
               ⇒ 1
          (* 1)
               ⇒ 1
          (* 1 2 3 4)
               ⇒ 24

— Function: / number &rest divisors

With one or more divisors, this function divides number by each divisor in divisors in turn, and returns the quotient. With no divisors, this function returns 1/number, i.e., the multiplicative inverse of number. Each argument may be a number or a marker.

If all the arguments are integers, the result is an integer, obtained by rounding the quotient towards zero after each division.

          (/ 6 2)
               ⇒ 3
          (/ 5 2)
               ⇒ 2
          (/ 5.0 2)
               ⇒ 2.5
          (/ 5 2.0)
               ⇒ 2.5
          (/ 5.0 2.0)
               ⇒ 2.5
          (/ 4.0)
               ⇒ 0.25
          (/ 4)
               ⇒ 0
          (/ 25 3 2)
               ⇒ 4
          (/ -17 6)
               ⇒ -2

If you divide an integer by the integer 0, Emacs signals an arith-error error (see Errors). Floating-point division of a nonzero number by zero yields either positive or negative infinity (see Float Basics).

— Function: % dividend divisor

This function returns the integer remainder after division of dividend by divisor. The arguments must be integers or markers.

For any two integers dividend and divisor,

          (+ (% dividend divisor)
             (* (/ dividend divisor) divisor))

always equals dividend if divisor is nonzero.

          (% 9 4)
               ⇒ 1
          (% -9 4)
               ⇒ -1
          (% 9 -4)
               ⇒ 1
          (% -9 -4)
               ⇒ -1

— Function: mod dividend divisor

This function returns the value of dividend modulo divisor; in other words, the remainder after division of dividend by divisor, but with the same sign as divisor. The arguments must be numbers or markers.

Unlike %, mod permits floating-point arguments; it rounds the quotient downward (towards minus infinity) to an integer, and uses that quotient to compute the remainder.

If divisor is zero, mod signals an arith-error error if both arguments are integers, and returns a NaN otherwise.

          (mod 9 4)
               ⇒ 1
          (mod -9 4)
               ⇒ 3
          (mod 9 -4)
               ⇒ -3
          (mod -9 -4)
               ⇒ -1
          (mod 5.5 2.5)
               ⇒ .5

For any two numbers dividend and divisor,

          (+ (mod dividend divisor)
             (* (floor dividend divisor) divisor))

always equals dividend, subject to rounding error if either argument is floating point and to an arith-error if dividend is an integer and divisor is 0. For floor, see Numeric Conversions.