CVMG(3I) Last changed: 1-6-98
NAME
CVMGM, CVMGN, CVMGP, CVMGT, CVMGZ - Conditional vector merge functions
SYNOPSIS
CVMGM ([I=]i, [J=]j, [K=]k)
CVMGN ([I=]i, [J=]j, [K=]k)
CVMGP ([I=]i, [J=]j, [K=]k)
CVMGT ([I=]i, [J=]j, [K=]k)
CVMGZ ([I=]i, [J=]j, [K=]k)
IMPLEMENTATION
UNICOS, UNICOS/mk, and IRIX systems
STANDARDS
CF90 and MIPSpro 7 Fortran 90 compiler extension to Fortran 90
DESCRIPTION
At run time, k is tested. You can use the conditional vector merge
(CVMG) functions when an IF statement involving arrays prevents
vectorization of a loop. The compiler can vectorize almost all such
loops, but these functions can be used in older codes. Scalar
arguments can also be used with these functions.
CVMG functions cannot be passed as arguments. They are elemental
functions.
These functions test for the following:
* CVMGM tests for minus (negative). i is returned if k < 0. j is
returned if k >= 0.
* CVMGN tests for nonzero. i is returned if k is not equal to 0. j
is returned if k = 0.
* CVMGP tests for positive or zero. i is returned if k >= 0. j is
returned if k < 0.
* CVMGT tests for true. i is returned if k is true. j is returned if
k is false.
* CVMGZ tests for zero. i is returned if k = 0. j is returned if k
is not equal to 0.
These functions accept the following arguments:
i Can be of type logical, Boolean, integer, real, or Cray pointer.
See the RETURN VALUES section of this man page for more
information on how the type of i affects the return value.
j Can be of type logical, Boolean, integer, real, or Cray pointer.
See the RETURN VALUES section of this man page for more
information on how the type of j affects the return value.
k Can be of type logical, Boolean, integer, real, or Cray pointer.
See the RETURN VALUES section of this man page for more
information on how the type of k affects the return value.
If k satisfies the condition tested by the function (for example, in
CVMGP, if k is positive), the first argument (i) is returned as the
function result. If k does not satisfy the condition tested by the
function, j is returned as the function result.
For operands other than type logical, the vector merge functions are
not generic with respect to data typing. They accept arithmetic
values of different types but interpret them as Boolean type (that is,
as bit patterns). The returned function value is also Boolean.
Therefore, the function reference CVMGT(1.0,2.0,.TRUE.) is not type
real, with a value of 1.0, but a Boolean value that acts the same as
1.0 in a floating-point operation.
A problem can arise if you assume that the function reference is type
real and use it in an expression or assignment that causes automatic
type conversion. For example, if you use the function reference in a
context where an integer is needed, the result is not valid because
1.0 and 1 have different bit patterns.
Because CVMG function values are Boolean, a binary operation involving
two CVMG functions uses integer arithmetic. This can produce
unexpected results in assignments such as the following in which A, B,
C, and D are real:
X = CVMGT(A,B,LOGIC1) + CVMGT(C,D,LOCIC2)
! Integer arithmetic, invalid results
However, when used in an expression with another operand, a CVMG
function value takes on the type of the other operand, without any
explicit type conversion. For example, the following expression uses
real arithmetic:
X = 1.0 + CVMGT(2.0,3.0,LEXP) ! Valid (types agree)
The following suggestions explain how to prevent bugs when using these
functions:
* Use only one CVMG function in a given expression. If you use more
than one CVMG function in an expression, use explicit type
conversion. This does not generate any additional code. For
example:
X = REAL(CVMGT(1.0,2.0,LTEST)) + REAL(CVMGT(X,Y,LTEST2))
* Ensure that the assignment type matches the function argument.
Example:
X = CVMGT(1.0,2.0,LTEST) ! Valid
X = REAL(CVMGT(1,2,LTEST)) ! Valid (uses explicit type conversion)
X = CVMGT(1,2,LTEST) ! Invalid (type mismatch)
* Never use mixed typing in the first two arguments of a CVMGT
function. Example:
CVMGT(1,2.0,LTEST) ! DO NOT DO THIS
The names of these intrinsics cannot be passed as arguments.
NOTES
The CVMG intrinsic functions are outmoded. Refer to the Fortran
Language Reference Manual, Volume 3, publication SR-3905, for
information on outmoded features and their preferred standard
alternatives.
RETURN VALUES
For CVMGM, CVMGN, CVMGP, CVMGT, and CVMGZ, the return value is either
type Boolean, logical, integer, or real, depending on the type of i
and j.
On UNICOS and UNICOS/mk systems, type logical is returned if i is of
type logical; otherwise it is of type Boolean.
On IRIX systems, if i and j are both of type real, the result type is
also real. If i is of type logical, type logical is returned. In all
other cases, the return value is of type integer.
EXAMPLES
Consider the following code:
DO I = N,M
X(I) = A(I)
IF (B(I) .GT. C(I)) X(I) = D(I)
END DO
This could be rewritten as follows:
DO I = N,M
X(I) = CVMGT(D(I), A(I), B(I).GT.C(I))
END DO
The following rewrite would also be valid:
DO I = N,M
X(I) = CVMGP( D(I), A(I), B(I) - C(I))
END DO
SEE ALSO
Intrinsic Procedures Reference Manual, publication SR-2138, for the
printed version of this man page.
