REAL(4) to REAL(8) conversion

[zzunic]

Dear All,

I have a problem with a real number conversion. I would like to convert a single precision real number into a double precision. I wrote a simple program which uses DBLE() function.

% cat conversion.f
program conversion

real(4) sx1
real(8) dx1

sx1=1.000010
dx1=dble(sx1)
write(*,*) sx1,dx1

end

When I run the program I get the following result:

% ./conversion
1.000010 1.00001001358032

I wish that the result of conversion is 1.00001000000000 (or perhaps 1.00001000000001 or. 1.00000999999999).

With Best Regards,

Zoran

[TimP]

If you are serious about this desire, you will have to WRITE the single precision value to a character string, and READ it back to the double precision value. Or, you could use
dx1 = 1.00001d0
The conversion you have written performs an exact binary extension. As 1.00001d0 is not exactly representable in single precision, it is different from 1.00001, as you showed.

[zzunic]

Dear Tim,

thanks for the answer. Yes, I was serious with this question, but I was thinking decimaly and not binary :-) .

Here is the listing of the expanded test:

% cat conversion.f
program conversion

real(4) sx1
real(8) dx1
character string*15

sx1=1.00001
dx1=dble(sx1)
write(*,*) sx1,dx1

write(string,*) sx1
read(string,*) dx1
write(*,*) sx1,dx1

end

% ./conversion
1.000010 1.00001001358032
1.000010 1.00001000000000

Regards, Zoran

[Steven_L_Intel1]

I would recommend you read the series of three articles The Perils of Real Numbers - it will help you understand what is going on here.