gam@astrovax.UUCP (Gary Mamon) (02/24/84)
If you thought you could get away using unoptimized f77 in UNIX 4.2,
be careful! There are a few bugs. Here is a pretty serious one.
I am sorry that I am too ignorant in compilers to be able to suggest
a fix for it.
In double precision, if you raise an expression x to a variable power y, and
if the expression contains a power and at least another operation, then
Fortran "may" return y**y instead of x**y.
Here is a simple example:
implicit real*8 (a-h,o-z)
c
111 continue
c
print *,' enter x and y'
read (5,*) x, y
c
z = (x**2.d0+1.d0)**(-y)
anum = x**2.d0+1.d0
power = -y
z2 = anum**power
print *,' x alph =',x,y
print *,' anum power =',anum,power
print *,' z z2 =',z,z2
c
go to 111
end
This program, compiled with no options, returns the wrong answer for z,
though z2 comes out correctly. Note that if the exponent is a constant
(for example through a parameter statement), the answers are correct.
Gary Mamon Princeton University, Astrophysics
{allegra,akgua,burl,cbosgd,decvax,ihnp4,kpno,princeton,vax135}!astrovax!gam