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Floating-point numbers are the computer equivalent of scientific
notation; you can think of a floating-point number as a fraction
together with a power of ten. The precise number of significant
figures and the range of possible exponents is machine-specific; Emacs
uses the C data type `double`

to store the value, and internally
this records a power of 2 rather than a power of 10.

The printed representation for floating-point numbers requires either
a decimal point (with at least one digit following), an exponent, or
both. For example, ‘`1500.0`’, ‘`+15e2`’, ‘`15.0e+2`’,
‘`+1500000e-3`’, and ‘`.15e4`’ are five ways of writing a floating-point
number whose value is 1500. They are all equivalent.

See Numbers, for more information.