GNU Emacs Lisp Reference Manual

3.4: Comparison of Numbers

To test numbers for numerical equality, you should normally use =, not eq. There can be many distinct floating point number objects with the same numeric value. If you use eq to compare them, then you test whether two values are the same object. By contrast, = compares only the numeric values of the objects.

At present, each integer value has a unique Lisp object in Emacs Lisp. Therefore, eq is equivalent = where integers are concerned. It is sometimes convenient to use eq for comparing an unknown value with an integer, because eq does not report an error if the unknown value is not a number---it accepts arguments of any type. By contrast, = signals an error if the arguments are not numbers or markers. However, it is a good idea to use = if you can, even for comparing integers, just in case we change the representation of integers in a future Emacs version.

There is another wrinkle: because floating point arithmetic is not exact, it is often a bad idea to check for equality of two floating point values. Usually it is better to test for approximate equality. Here's a function to do this:

(defvar fuzz-factor 1.0e-6)
(defun approx-equal (x y)
  (or (and (= x 0) (= y 0))
      (< (/ (abs (- x y))
            (max (abs x) (abs y)))
         fuzz-factor)))

Common Lisp note: Comparing numbers in Common Lisp always requires = because Common Lisp implements multi-word integers, and two distinct integer objects can have the same numeric value. Emacs Lisp can have just one integer object for any given value because it has a limited range of integer values.

Function: = number-or-marker1 number-or-marker2

This function tests whether its arguments are numerically equal, and returns t if so, nil otherwise.

Function: /= number-or-marker1 number-or-marker2

This function tests whether its arguments are numerically equal, and returns t if they are not, and nil if they are.

Function: < number-or-marker1 number-or-marker2

This function tests whether its first argument is strictly less than its second argument. It returns t if so, nil otherwise.

Function: <= number-or-marker1 number-or-marker2

This function tests whether its first argument is less than or equal to its second argument. It returns t if so, nil otherwise.

Function: > number-or-marker1 number-or-marker2

This function tests whether its first argument is strictly greater than its second argument. It returns t if so, nil otherwise.

Function: >= number-or-marker1 number-or-marker2

This function tests whether its first argument is greater than or equal to its second argument. It returns t if so, nil otherwise.

Function: max number-or-marker &rest numbers-or-markers

This function returns the largest of its arguments.

(max 20)
     => 20
(max 1 2.5)
     => 2.5
(max 1 3 2.5)
     => 3

Function: min number-or-marker &rest numbers-or-markers

This function returns the smallest of its arguments.

(min -4 1)
     => -4