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Asked: 2026-05-15T16:01:19+00:00

I’m trying to work with large numbers (~10^14), and I need to be able

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I’m trying to work with large numbers (~10^14), and I need to be able to store them and iterate over loops of that length, i.e.

n=SOME_BIG_NUMBER
do i=n,1,-1

I’ve tried the usual star notation, kind=8 etc. but nothing seems to work.
Then I checked the huge intrinsic function, and the code:

program inttest

print *,huge(1)
print *,huge(2)
print *,huge(4)
print *,huge(8)
print *,huge(16)
print *,huge(32)

end program inttest

produces the number 2147483647 in all cases. Why is this? I’m using gfortran (f95) on a 64-bit machine.

If I’m going to need a bignum library, which one do people suggest?

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1 Answer

  1. Editorial Team

    The gfortran versions that I use, 4.3, 4.4 and 4.5 on a Mac, support 8-byte integers. The best way to select a variable type in Fortran >= 90 is to use an intrinsic function to specify the precision that you need. Try:

    integer, parameter :: LargeInt_K = selected_int_kind (18)
integer (kind=LargeInt_K) :: i, n

    to obtain at least 18 decimal digits, which will typically be a 8-byte integer.

    With gfortran 4.3, huge (1_LargeInt_K) outputs 9223372036854775807. When you wrote huge (1), etc., by default the constant was a default integer, here evidently 4-bytes since huge returned 2147483647. So sometimes you need to specify the precision of constants, not just variables — more commonly this trips people up when they lose significant figures on a real constant, which defaults to single precision.

    Also see Fortran: integer*4 vs integer(4) vs integer(kind=4)

    Usually gfortran has the command name gfortran. Could f95 be a different compiler? Try “gfortran -v” and “f95 -v”.

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