# Documentation Talk:Reference Section 2

`rand(I)`

Returns the next pseudo-random number from the stream
specified by the positive integer `I`

. You must call ```
seed()
```

to initialize a random stream before calling ```
rand()
```

. The numbers are uniformly distributed, and have values between
`0.0`

and `1.0`

, inclusively. The numbers generated by
separate streams are independent random variables.

` seed(I)`

Initializes a new pseudo-random stream with the
initial seed value `A`

. The number corresponding to this random
stream is returned. Any number of pseudo-random streams may be used as shown
in the example below:

#declare R1 = seed(0); #declare R2 = seed(12345); sphere { <rand(R1), rand(R1), rand(R1)>, rand(R2) }

Multiple random generators are very useful in situations where you use
`rand()`

to place a group of objects, and then decide to use
`rand()`

in another location earlier in the file to set some
colors or place another group of objects. Without separate ```
rand()
```

streams, all of your objects would move when you added more
calls to `rand()`

. This is very annoying.

I think the text

In addition to the above built-in functions, you may also define your own functions
using the #macro directive. See the section "User Defined Macros" for more details.

should be changed to

In addition to the above built-in functions, you may also define your own functions
using the function keyword. See the section "User Defined Functions" for more details.

Considering that #macro is different from a function and clicking on the function
keyword in the overview bring you here.Froesccn 22:42, 31 August 2010 (UTC)

- done --jholsenback 11:01, 1 September 2010 (UTC)

- Adding bitwise_and, bitwise_or & bitwise_xor: syntax description (only the FLOAT_FUNCTION of the PRE is updated) & functions description (further down, in atanh and ceil). the warning apply to all 3 functions.

--Le Forgeron 09:12, 4 December 2010 (UTC)

- tis done: --jholsenback 13:28, 4 December 2010 (UTC)

#### Float Expressions

Many parts of the POV-Ray language require you to specify one or more floating point numbers. A floating point number is a number with a decimal point. Floats may be specified using literals, identifiers or functions which return float values. You may also create very complex float expressions from combinations of any of these using various familiar operators.

Where POV-Ray needs an integer value it allows you to specify a float value
and it truncates it to an integer. When POV-Ray needs a logical or boolean
value it interprets any non-zero float as true and zero as false. Because
float comparisons are subject to rounding errors POV-Ray accepts values
extremely close to zero as being false when doing boolean functions.
Typically values whose absolute values are less than a preset value *
epsilon* are considered false for logical expressions. The value of *
epsilon* is system dependent but is generally about 1.0e-10. Two floats
*a* and *b* are considered to be equal if *abs(a-b) <
epsilon.*

The full syntax for float expressions is given below. Detailed explanations are given in the following sub-sections.

FLOAT: NUMERIC_TERM [SIGN NUMERIC_TERM]... SIGN: + | - NUMERIC_TERM: NUMERIC_FACTOR [MULT NUMERIC_FACTOR]... MULT: * | / NUMERIC_FACTOR: FLOAT_LITERAL | FLOAT_IDENTIFIER | SIGN NUMERIC_FACTOR | FLOAT_FUNCTION | FLOAT_BUILT-IN_IDENT | ( FULL_EXPRESSION ) | ! NUMERIC_FACTOR VECTOR DECIMAL_POINT DOT_ITEM FLOAT_LITERAL: [DIGIT...] [DECIMAL_POINT] DIGIT... [EXP [SIGN] DIGIT...] DIGIT: 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 DECIMAL_POINT: . EXP: e | E DOT_ITEM: x | y | z | t | u | v | red | blue | green | filter | transmit | gray FLOAT_FUNCTION: abs( FLOAT ) | acos( FLOAT ) | acosh( FLOAT ) | asc( STRING ) | asin( FLOAT ) | asinh( FLOAT ) | atan( FLOAT) | atanh( FLOAT) | atan2( FLOAT , FLOAT ) | bitwise_and( FLOAT, FLOAT, ...) | bitwise_or (FLOAT, FLOAT, ...) | bitwise_xor ( FLOAT, FLOAT, ...) | ceil( FLOAT ) | cos( FLOAT ) | cosh( FLOAT ) | defined(IDENTIFIER ) | degrees( FLOAT ) | dimensions( ARRAY_IDENTIFIER ) | dimension_size( ARRAY_IDENTIFIER , FLOAT ) | div( FLOAT , FLOAT ) | exp( FLOAT ) | file_exists( STRING ) | floor( FLOAT ) | int( FLOAT ) | ln(Float | log( FLOAT ) | max( FLOAT , FLOAT, ... ) | min( FLOAT , FLOAT, ... ) | mod( FLOAT , FLOAT ) | pow( FLOAT , FLOAT ) | radians( FLOAT ) | rand( FLOAT ) | seed( FLOAT ) | select( FLOAT, FLOAT, FLOAT [,FLOAT]) | sin( FLOAT ) | sinh( FLOAT ) | sqrt( FLOAT ) | strcmp( STRING , STRING ) | strlen( STRING ) | tan( FLOAT ) | tanh( FLOAT ) | val( STRING ) | vdot( VECTOR , VECTOR ) | vlength( VECTOR ) | FLOAT_BUILT-IN_IDENT: clock | clock_delta | clock_on | false | final_clock | final_frame | frame_number | initial_clock | initial_frame | image_width | image_height | no | off | on | pi | true | version | yes | FULL_EXPRESSION: LOGICAL_EXPRESSION [? FULL_EXPRESSION : FULL_EXPRESSION] LOGICAL_EXPRESSION: REL_TERM [LOGICAL_OPERATOR REL_TERM]... LOGICAL_OPERATOR: & | | (note: this means an ampersand or a vertical bar is a logical operator) REL_TERM: FLOAT [REL_OPERATOR FLOAT]... REL_OPERATOR: < | <= | = | >= | > | != INT: FLOAT (note: any syntax which requires a integer INT will accept a FLOAT and it will be truncated to an integer internally by POV-Ray).

` bitwise_and(A,B,...)`

Bitwise And of two or more float values considered as integers.

` bitwise_or(A,B,...)`

Bitwise Or of two or more float values considered as integers.

` bitwise_xor(A,B,...)`

Bitwise Xor of two or more float values considered as integers.

**Note:** Each parameter is first converted to integer (so fractional part is ignored), and the computation is performed on a classical int (do not expect to much precision, as convertion from a double precision number (usually with 52 bits mantissa) to an integer and back to a double precision number is not the best way to keep track of every bits).