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## AST_MATHMAP - Create a MathMap

Description:
This function creates a new MathMap and optionally initialises its attributes.

A MathMap is a Mapping which allows you to specify a set of forward and/or inverse transformation functions using arithmetic operations and mathematical functions similar to those available in Fortran. The MathMap interprets these functions at run-time, whenever its forward or inverse transformation is required. Because the functions are not compiled in the normal sense (unlike an IntraMap), they may be used to describe coordinate transformations in a transportable manner. A MathMap therefore provides a flexible way of defining new types of Mapping whose descriptions may be stored as part of a dataset and interpreted by other programs.

Invocation:
RESULT = AST_MATHMAP( NIN, NOUT, NFWD, FWD, NINV, INV, OPTIONS, STATUS )
Arguments:
NIN = INTEGER
Number of input variables for the MathMap. This determines the value of its Nin attribute.
NOUT = INTEGER
Number of output variables for the MathMap. This determines the value of its Nout attribute.
NFWD = INTEGER
The number of forward transformation functions being supplied. This must be at least equal to NOUT, but may be increased to accommodate any additional expressions which define intermediate variables for the forward transformation (see the "Calculating Intermediate Values" section below).
FWD = CHARACTER ( )( NFWD )
An array which contains the expressions defining the forward transformation. The syntax of these expressions is described below.
NINV = INTEGER
The number of inverse transformation functions being supplied. This must be at least equal to NIN, but may be increased to accommodate any additional expressions which define intermediate variables for the inverse transformation (see the "Calculating Intermediate Values" section below).
INV = CHARACTER ( )( NINV )
An array which contains the expressions defining the inverse transformation. The syntax of these expressions is described below.
OPTIONS = CHARACTER ( ) (Given)
A character string containing an optional comma-separated list of attribute assignments to be used for initialising the new MathMap. The syntax used is identical to that for the AST_SET routine. If no initialisation is required, a blank value may be supplied.
STATUS = INTEGER (Given and Returned)
The global status.
Returned Value:
AST_MATHMAP = INTEGER
A pointer to the new MathMap.
Notes:
• The sequence of numbers produced by the random number functions available within a MathMap is normally unpredictable and different for each MathMap. However, this behaviour may be controlled by means of the MathMap's Seed attribute.

• Normally, compound Mappings (CmpMaps) which involve MathMaps will not be subject to simplification (e.g. using AST_SIMPLIFY) because AST cannot know how different MathMaps will interact. However, in the special case where a MathMap occurs in series with its own inverse, then simplification may be possible. Whether simplification does, in fact, occur under these circumstances is controlled by the MathMap's SimpFI and SimpIF attributes.

• A null Object pointer (AST__NULL) will be returned if this function is invoked with STATUS set to an error value, or if it should fail for any reason.
Defining Transformation Functions
A MathMap's transformation functions are supplied as a set of expressions in an array of character strings. Normally you would supply the same number of expressions for the forward transformation, via the FWD argument, as there are output variables (given by the MathMap's Nout attribute). For instance, if Nout is 2 you might use:
• 'R = SQRT( X X Y Y )'

• 'THETA = ATAN2( Y, X )'

which defines a transformation from Cartesian to polar coordinates. Here, the variables that appear on the left of each expression (R and THETA) provide names for the output variables and those that appear on the right (X and Y) are references to input variables.

To complement this, you must also supply expressions for the inverse transformation via the INV argument. In this case, the number of expressions given would normally match the number of MathMap input coordinates (given by the Nin attribute). If Nin is 2, you might use:

• 'X = R COS( THETA )'

• 'Y = R SIN( THETA )'

which expresses the transformation from polar to Cartesian coordinates. Note that here the input variables (X and Y) are named on the left of each expression, and the output variables (R and THETA) are referenced on the right.

Normally, you cannot refer to a variable on the right of an expression unless it is named on the left of an expression in the complementary set of functions. Therefore both sets of functions (forward and inverse) must be formulated using the same consistent set of variable names. This means that if you wish to leave one of the transformations undefined, you must supply dummy expressions which simply name each of the output (or input) variables. For example, you might use:

• 'X'

• 'Y'

for the inverse transformation above, which serves to name the input variables but without defining an inverse transformation.
Calculating Intermediate Values
It is sometimes useful to calculate intermediate values and then to use these in the final expressions for the output (or input) variables. This may be done by supplying additional expressions for the forward (or inverse) transformation functions. For instance, the following array of five expressions describes 2-dimensional pin-cushion distortion:
• 'R = SQRT( XIN XIN YIN YIN )'

• 'ROUT = R ( 1 0.1 R R )'

• 'THETA = ATAN2( YIN, XIN )',

• 'XOUT = ROUT COS( THETA )'

• 'YOUT = ROUT SIN( THETA )'

Here, we first calculate three intermediate results (R, ROUT and THETA) and then use these to calculate the final results (XOUT and YOUT). The MathMap knows that only the final two results constitute values for the output variables because its Nout attribute is set to 2. You may define as many intermediate variables in this way as you choose. Having defined a variable, you may then refer to it on the right of any subsequent expressions.

Note that when defining the inverse transformation you may only refer to the output variables XOUT and YOUT. The intermediate variables R, ROUT and THETA (above) are private to the forward transformation and may not be referenced by the inverse transformation. The inverse transformation may, however, define its own private intermediate variables.

Expression Syntax
The expressions given for the forward and inverse transformations closely follow the syntax of Fortran (with some extensions for compatibility with the C language). They may contain references to variables and literal constants, together with arithmetic, logical, relational and bitwise operators, and function invocations. A set of symbolic constants is also available. Each of these is described in detail below. Parentheses may be used to over-ride the normal order of evaluation. There is no built-in limit to the length of expressions and they are insensitive to case or the presence of additional white space.
Variables
Variable names must begin with an alphabetic character and may contain only alphabetic characters, digits, and the underscore character "_". There is no built-in limit to the length of variable names.
Literal Constants
Literal constants, such as "0", "1", "0.007" or "2.505E-16" may appear in expressions, with the decimal point and exponent being optional (a "D" may also be used as an exponent character). A unary minus "-" may be used as a prefix.
Arithmetic Precision
All arithmetic is floating point, performed in double precision.
Propagation of Missing Data
Unless indicated otherwise, if any argument of a function or operator has the value AST__BAD (indicating missing data), then the result of that function or operation is also AST__BAD, so that such values are propagated automatically through all operations performed by MathMap transformations. The special value AST__BAD can be represented in expressions by the symbolic constant "bad".

A bad result (i.e. equal to AST__BAD) is also produced in response to any numerical error (such as division by zero or numerical overflow), or if an invalid argument value is provided to a function or operator.

Arithmetic Operators
The following arithmetic operators are available:
• X1 X2: Sum of X1 and X2.

• X1 - X2: Difference of X1 and X2.

• X1 X2: Product of X1 and X2.

• X1 / X2: Ratio of X1 and X2.

• X1 X2: X1 raised to the power of X2.

• X: Unary plus, has no effect on its argument.

• - X: Unary minus, negates its argument.
Logical Operators
Logical values are represented using zero to indicate .FALSE. and non-zero to indicate .TRUE.. In addition, the value AST__BAD is taken to mean "unknown". The values returned by logical operators may therefore be 0, 1 or AST__BAD. Where appropriate, "tri-state" logic is implemented. For example, A.OR.B may evaluate to 1 if A is non-zero, even if B has the value AST__BAD. This is because the result of the operation would not be affected by the value of B, so long as A is non-zero.

The following logical operators are available:

• X1 .AND. X2: Logical AND between X1 and X2, returning 1 if both X1 and X2 are non-zero, and 0 otherwise. This operator implements tri-state logic. (The synonym "&&" is also provided for compatibility with C.)

• X1 .OR. X2: Logical OR between X1 and X2, returning 1 if either X1 or X2 are non-zero, and 0 otherwise. This operator implements tri-state logic. (The synonym "" is also provided for compatibility with C.)

• X1 .NEQV. X2: Logical exclusive OR (XOR) between X1 and X2, returning 1 if exactly one of X1 and X2 is non-zero, and 0 otherwise. Tri-state logic is not used with this operator. (The synonym ".XOR." is also provided, although this is not standard Fortran. In addition, the C-like synonym "" may be used, although this is also not standard.)

• X1 .EQV. X2: Tests whether the logical states of X1 and X2 (i.e. .TRUE./.FALSE.) are equal. It is the negative of the exclusive OR (XOR) function. Tri-state logic is not used with this operator.

• .NOT. X: Logical unary NOT operation, returning 1 if X is zero, and 0 otherwise. (The synonym "!" is also provided for compatibility with C.)
Relational Operators
Relational operators return the logical result (0 or 1) of comparing the values of two floating point values for equality or inequality. The value AST__BAD may also be returned if either argument is bad.

The following relational operators are available:

• X1 .EQ. X2: Tests whether X1 equals X2. (The synonym "==" is also provided for compatibility with C.)

• X1 .NE. X2: Tests whether X1 is unequal to X2. (The synonym "!=" is also provided for compatibility with C.)

• X1 .GT. X2: Tests whether X1 is greater than X2. (The synonym "" is also provided for compatibility with C.)

• X1 .GE. X2: Tests whether X1 is greater than or equal to X2. (The synonym "=" is also provided for compatibility with C.)

• X1 .LT. X2: Tests whether X1 is less than X2. (The synonym "" is also provided for compatibility with C.)

• X1 .LE. X2: Tests whether X1 is less than or equal to X2. (The synonym "=" is also provided for compatibility with C.)

Note that relational operators cannot usefully be used to compare values with the bad value (representing missing data), because the result is always bad. The ISBAD() function should be used instead.

Note, also, that because logical operators can operate on floating point values, care must be taken to use parentheses in some cases where they would not normally be required in Fortran. For example, the expresssion:

• .NOT. A .EQ. B

must be written:
• .NOT. ( A .EQ. B )

to prevent the .NOT. operator from associating with the variable A.
Bitwise Operators
Bitwise operators are often useful when operating on raw data (e.g. from instruments), so they are provided for use in MathMap expressions. In this case, however, the values on which they operate are floating point values rather than the more usual pure integers. In order to produce results which match the pure integer case, the operands are regarded as fixed point binary numbers (i.e. with the binary equivalent of a decimal point) with negative numbers represented using twos-complement notation. For integer values, the resulting bit pattern corresponds to that of the equivalent signed integer (digits to the right of the point being zero). Operations on the bits representing the fractional part are also possible, however.

The following bitwise operators are available:

• X1 X2: Rightward bit shift. The integer value of X2 is taken (rounding towards zero) and the bits representing X1 are then shifted this number of places to the right (or to the left if the number of places is negative). This is equivalent to dividing X1 by the corresponding power of 2.

• X1 X2: Leftward bit shift. The integer value of X2 is taken (rounding towards zero), and the bits representing X1 are then shifted this number of places to the left (or to the right if the number of places is negative). This is equivalent to multiplying X1 by the corresponding power of 2.

• X1 & X2: Bitwise AND between the bits of X1 and those of X2 (equivalent to a logical AND applied at each bit position in turn).

• X1 X2: Bitwise OR between the bits of X1 and those of X2 (equivalent to a logical OR applied at each bit position in turn).

• X1 X2: Bitwise exclusive OR (XOR) between the bits of X1 and those of X2 (equivalent to a logical XOR applied at each bit position in turn).

Note that no bit inversion operator is provided. This is because inverting the bits of a twos-complement fixed point binary number is equivalent to simply negating it. This differs from the pure integer case because bits to the right of the binary point are also inverted. To invert only those bits to the left of the binary point, use a bitwise exclusive OR with the value -1 (i.e. X-1).
Functions
The following functions are available:
• ABS(X): Absolute value of X (sign removal), same as FABS(X).

• ACOS(X): Inverse cosine of X, in radians.

• ACOSD(X): Inverse cosine of X, in degrees.

• ACOSH(X): Inverse hyperbolic cosine of X.

• ACOTH(X): Inverse hyperbolic cotangent of X.

• ACSCH(X): Inverse hyperbolic cosecant of X.

• AINT(X): Integer part of X (round towards zero), same as INT(X).

• ASECH(X): Inverse hyperbolic secant of X.

• ASIN(X): Inverse sine of X, in radians.

• ASIND(X): Inverse sine of X, in degrees.

• ASINH(X): Inverse hyperbolic sine of X.

• ATAN(X): Inverse tangent of X, in radians.

• ATAND(X): Inverse tangent of X, in degrees.

• ATANH(X): Inverse hyperbolic tangent of X.

• ATAN2(X1, X2): Inverse tangent of X1/X2, in radians.

• ATAN2D(X1, X2): Inverse tangent of X1/X2, in degrees.

• CEIL(X): Smallest integer value not less then X (round towards plus infinity).

• COS(X): Cosine of X in radians.

• COSD(X): Cosine of X in degrees.

• COSH(X): Hyperbolic cosine of X.

• COTH(X): Hyperbolic cotangent of X.

• CSCH(X): Hyperbolic cosecant of X.

• DIM(X1, X2): Returns X1-X2 if X1 is greater than X2, otherwise 0.

• EXP(X): Exponential function of X.

• FABS(X): Absolute value of X (sign removal), same as ABS(X).

• FLOOR(X): Largest integer not greater than X (round towards minus infinity).

• FMOD(X1, X2): Remainder when X1 is divided by X2, same as MOD(X1, X2).

• GAUSS(X1, X2): Random sample from a Gaussian distribution with mean X1 and standard deviation X2.

• INT(X): Integer part of X (round towards zero), same as AINT(X).

• LOG(X): Natural logarithm of X.

• LOG10(X): Logarithm of X to base 10.

• MAX(X1, X2, ...): Maximum of two or more values.

• MIN(X1, X2, ...): Minimum of two or more values.

• MOD(X1, X2): Remainder when X1 is divided by X2, same as FMOD(X1, X2).

• NINT(X): Nearest integer to X (round to nearest).

• POISSON(X): Random integer-valued sample from a Poisson distribution with mean X.

• POW(X1, X2): X1 raised to the power of X2.

• QIF(x1, x2, x3): Returns X2 if X1 is true, and X3 otherwise.

• RAND(X1, X2): Random sample from a uniform distribution in the range X1 to X2 inclusive.

• SECH(X): Hyperbolic secant of X.

• SIGN(X1, X2): Absolute value of X1 with the sign of X2 (transfer of sign).

• SIN(X): Sine of X in radians.

• SINC(X): Sinc function of X [= SIN(X)/X].

• SIND(X): Sine of X in degrees.

• SINH(X): Hyperbolic sine of X.

• SQR(X): Square of X (= XX).

• SQRT(X): Square root of X.

• TAN(X): Tangent of X in radians.

• TAND(X): Tangent of X in degrees.

• TANH(X): Hyperbolic tangent of X.
Symbolic Constants
The following symbolic constants are available (the enclosing "" brackets must be included):
• bad: The "bad" value (AST__BAD) used to flag missing data. Note that you cannot usefully compare values with this constant because the result is always bad. The ISBAD() function should be used instead.

• dig: Number of decimal digits of precision available in a floating point (double precision) value.

• e: Base of natural logarithms.

• epsilon: Smallest positive number such that 1.0epsilon is distinguishable from unity.

• mant_dig: The number of base radix digits stored in the mantissa of a floating point (double precision) value.

• max: Maximum representable floating point (double precision) value.

• max_10_exp: Maximum integer such that 10 raised to that power can be represented as a floating point (double precision) value.

• max_exp: Maximum integer such that radix raised to that power minus 1 can be represented as a floating point (double precision) value.

• min: Smallest positive number which can be represented as a normalised floating point (double precision) value.

• min_10_exp: Minimum negative integer such that 10 raised to that power can be represented as a normalised floating point (double precision) value.

• min_exp: Minimum negative integer such that radix raised to that power minus 1 can be represented as a normalised floating point (double precision) value.

• pi: Ratio of the circumference of a circle to its diameter.

• radix: The radix (number base) used to represent the mantissa of floating point (double precision) values.

• rounds: The mode used for rounding floating point results after addition. Possible values include: -1 (indeterminate), 0 (toward zero), 1 (to nearest), 2 (toward plus infinity) and 3 (toward minus infinity). Other values indicate machine-dependent behaviour.
Evaluation Precedence and Associativity
Items appearing in expressions are evaluated in the following order (highest precedence first):
• Constants and variables

• Function arguments and parenthesised expressions

• Function invocations

• Unary - ! .not.

• /

• -

• .lt. = .le. .gt. = .ge.

• == .eq. != .ne.

• &

• && .and.

• .or

• .eqv. .neqv. .xor.

All operators associate from left-to-right, except for unary , unary -, !, .not. and which associate from right-to-left.

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AST A Library for Handling World Coordinate Systems in Astronomy