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GNU Emacs Lisp Reference Manual

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3.2 Floating Point Basics

Floating point numbers are useful for representing numbers that are not integral. The precise range of floating point numbers is machine-specific; it is the same as the range of the C data type double on the machine you are using.

The read-syntax for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, `1500.0', `15e2', `15.0e2', `1.5e3', and `.15e4' are five ways of writing a floating point number whose value is 1500. They are all equivalent. You can also use a minus sign to write negative floating point numbers, as in `-1.0'.

Most modern computers support the IEEE floating point standard, which provides for positive infinity and negative infinity as floating point values. It also provides for a class of values called NaN or "not-a-number"; numerical functions return such values in cases where there is no correct answer. For example, (sqrt -1.0) returns a NaN. For practical purposes, there's no significant difference between different NaN values in Emacs Lisp, and there's no rule for precisely which NaN value should be used in a particular case, so Emacs Lisp doesn't try to distinguish them. Here are the read syntaxes for these special floating point values:

positive infinity
`1.0e+INF'
negative infinity
`-1.0e+INF'
Not-a-number
`0.0e+NaN'.

In addition, the value -0.0 is distinguishable from ordinary zero in IEEE floating point (although equal and = consider them equal values).

You can use logb to extract the binary exponent of a floating point number (or estimate the logarithm of an integer):

Function: logb number
This function returns the binary exponent of number. More precisely, the value is the logarithm of number base 2, rounded down to an integer.

 
(logb 10)
     => 3
(logb 10.0e20)
     => 69


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