#### 4.27.2.4 IEEE 754 floating point arithmetic

The Prolog ISO standard defines that floating point arithmetic
returns a valid floating point number or raises an exception. IEEE
floating point arithmetic defines two modes: raising exceptions and
propagating the special float values `NaN`

, `Inf`

, `-Inf`

and
`-0.0`

. SWI-Prolog implements a part of the
ECLiPSe
proposal to support non-exception based processing of floating point
numbers. There are four flags that define handling the four exceptional
events in floating point arithmetic, providing the choice between
`error`

and returning the IEEE special value. Note that these
flags *only* apply for floating point arithmetic. For example
rational division by zero always raises an exception.

Flag | Default | Alternative |

float_overflow | error | infinity |

float_zero_div | error | infinity |

float_undefined | error | nan |

float_underflow | ignore | error |

The Prolog flag float_rounding
and the function
roundtoward/2
control the rounding mode for floating point arithmetic. The default
rounding is `to_nearest`

and the following alternatives are
provided: `to_positive`

, `to_negative`

and
`to_zero`

.

- [det]
**float_class**(`+Float, -Class`) - Wraps C99
**fpclassify()**to access the class of a floating point number. Raises a type error if`Float`is not a float. Defined classes are below.**nan**`Float`is “Not a number” . See nan/0. May be produced if the Prolog flag float_undefined is set to`nan`

. Although IEEE 754 allows NaN to carry a*payload*and have a sign, SWI-Prolog has only a single NaN values. Note that two NaN*terms*compare equal in the standard order of terms (==/2, etc.), they compare non-equal for arithmetic (=:=/2, etc.).**infinite**`Float`is positive or negative infinity. See inf/0. May be produced if the Prolog flag float_overflow or the flag float_zero_div is set to`infinity`

.**zero**`Float`is zero (0.0 or -0.0)**subnormal**`Float`is too small to be represented in normalized format. May**not**be produced if the Prolog flag float_underflow is set to`error`

.**normal**`Float`is a normal floating point number.

- [det]
**float_parts**(`+Float, -Mantissa, -Base, -Exponent`) - True when
`Mantissa`is the normalized fraction of`Float`,`Base`is the*radix*and`Exponent`is the exponent. This uses the C function**frexp()**. If`Float`is NaN or`±`Inf`Mantissa`has the same value and`Exponent`is 0 (zero). In the current implementation`Base`is always 2. The following relation is always true:`Float =:= Mantissa × Base^Exponent` - [det]
**bounded_number**(`?Low, ?High, +Num`) - True if
`Low`<`Num`<`High`. Raises a type error if`Num`is not a number. This predicate can be used both to check and generate bounds across the various numeric types. Note that a number cannot be bounded by itself and`NaN`

,`Inf`

, and`-Inf`

are not bounded numbers.If

`Low`and/or`High`are variables they will be unified with*tightest*values that still meet the bounds criteria. The generated bounds will be integers if`Num`is an integer; otherwise they will be floats (also see nexttoward/2 for generating float bounds). Some examples:?- bounded_number(0,10,1). true. ?- bounded_number(0.0,1.0,1r2). true. ?- bounded_number(L,H,1.0). L = 0.9999999999999999, H = 1.0000000000000002. ?- bounded_number(L,H,-1). L = -2, H = 0. ?- bounded_number(0,1r2,1). false. ?- bounded_number(L,H,1.0Inf). false.