arrays - Why are these two calculations which are exactly same giving different results in Fortran using gfortran? - 


real, dimension(3), parameter :: boxlen = [4.0, 5.0, 7.0] real, parameter :: mindist = 0.1 integer ::i    write(*,"(a)") "operation on array"   print*, floor(boxlen/mindist)   write(*,"(/a)") "operation on individual elements"   i=1,3      print*, i, floor(boxlen(i)/mindist)   enddo

this when run code.

operation on array       40          50          70  operation on individual elements        1          39        2          49        3          69

can explain why 2 calculations (one using operation on array , using operation on individual elements) giving different results? think should same.

for me gfortran 4.8.2 on x86_64-linux-gnu get

 operation on array           40          50          70  operation on individual elements            1          40            2          50            3          70

peering crystall ball, if you're using 32-bit x86, there difference in order of how computed values stored memory , printed.

also, looking @ generated intermediate code "-fdump-tree-original", seems array operation, division done @ compile time, (so should @ least correctly rounded mpfr).


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