Re: Pure and internal procedures

On 9-12-2011 4:12, Michael Trover wrote:
Think of it this way. This is an example using a function whose
interface we know and a code snippet.

Example 1:

interface
function f(x)
real, intent(in):: x
end function
end interface

x = 3.0
y = f(x)
z = f(x)

In this example the function invocations for y and z have to be done.
That is the code has to invoke the function twice. The second invocation
of f may not return the same value as the first invocation.

Example 2:

interface
pure function f(x)
real, intent(in):: x
end function
end interface

x = 3.0
y = f(x)
z = f(x)

Here the compiler knows, just from the interface, that it only needs to
invoke the function once and copy the result to both y and z. That's the
power of PURE.


Unless the variable y is host accessible by the pure function, and the function uses its value to calculate the result.

The following program prints

y = 3.0
z = 6.0

program pure_p
implicit none

real :: x, y, z

y = 0.0

x = 3.0
y = pure_f(x)
z = pure_f(x)

write(*, '("y = ", f0.1)') y
write(*, '("z = ", f0.1)') z
contains
pure function pure_f(x)
real, intent(in) :: x
real :: pure_f

pure_f = x + y
end function pure_f
end program pure_p

Erik.
.

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