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Real representation

 
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christyleomin



Joined: 08 Apr 2011
Posts: 155
PostPosted: Sat Nov 12, 2011 10:10 pm    Post subject: Real representation Reply with quote
I am using a routine to convert a character to real.

I have:

'7.500000000000000E-2' (character)

The routine which converts character to real can read only the first 20 characters.

Please advise how to represent only 20 characters in '7.500000000000000E-2' such that the correct real number is obtaned.

I do not have access to the routine which carries out the conversion - so please help howw to send the 20 characters such that the correct real number 7.5E-2 is obtain ed.
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2555
Location: Sydney
PostPosted: Mon Nov 14, 2011 6:19 am    Post subject: Reply with quote
You could provide your own routine to read, such as adapting from the following example:
Code:
 character real_string*25, aa(3)*25
 real*10    x
 real*8     y
 integer*4 iostat,i
!
 aa(1) = '7.500000000000000E-2'
 aa(2) = '3.500000000000003400E-2'
 aa(3) = '3.500003400000000000E-20'
!
 do i = 1,3
   real_string = aa(i)
   read (real_string, fmt='(bn,f25.0)', iostat=iostat) x
   write (*,*) 'String = ',real_string
   write (*,*) 'X = ',x
   write (*,*) 'error = ',iostat
   y = x
   write (*,*) 'Y = ',y
   call trim_real_string (real_string)
   write (*,*) 'String = ',real_string
 end do
!
end
 
subroutine trim_real_string (real_string)
!
!  trims training 0 from real number string
!
 character real_string*(*)
!
 character real_part*25, exponent_part*10
 integer*4 ie,i
!
 ie = index(real_string, 'E')
!
 if (ie > 0) then
   real_part     = real_string(1:ie-1)
   exponent_part = real_string(ie:)
 else
   real_part     = real_string
   exponent_part = ' '
 end if
!
 do i = len_trim(real_part),1,-1
   if (real_part(i:i) /= '0') exit
 end do
!
 if (index(real_part, '.') > 0) real_string = real_part(:i) // exponent_part
!
end subroutine trim_real_string


John
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mecej4



Joined: 31 Oct 2006
Posts: 1887
PostPosted: Mon Nov 14, 2011 4:24 pm    Post subject: Re: Real representation Reply with quote
christyleomin wrote:

Please advise how to represent only 20 characters in '7.500000000000000E-2' such that the correct real number is obtained.


Neither 4-byte nor 8-byte IEEE floating point formats can represent 0.075 exactly.

In hex, the number is either Z'3BF5C28F' (32-bit) or Z'3FB3333333333333' (64-bit).

In general, when such numbers are read and printed from source code, compiled and run, there is no guarantee that the output number will match the input number in decimal representation.
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