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sparge
Joined: 11 Apr 2005
Posts: 371
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 Posted: Tue Mar 23, 2010 1:19 pm Post subject: CHARACTER variable size limit query |
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Does anyone know of any "inherent" (i.e. not memory-related) limit on the length of a variable of type CHARACTER (len = ...)? Or any reason why code might behave unexpectedly if manipulating such variables with len = 2**16?
Last edited by sparge on Wed Mar 24, 2010 10:01 am; edited 1 time in total |
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JohnCampbell
Joined: 16 Feb 2006
Posts: 2615
Location: Sydney
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| Posted: Wed Mar 24, 2010 7:10 am Post subject: |
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It's not an array, it is a character variable.
I don't know for sure but I think the limit for length is 1024, as I get different errors in a module compilation if length > 1024.
You can certainly have an array of Character*1 much bigger. I have an array of 750 million and it works ok.
John |
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sparge
Joined: 11 Apr 2005
Posts: 371
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| Posted: Wed Mar 24, 2010 10:09 am Post subject: Re: |
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| JohnCampbell wrote: | | It's not an array, it is a character variable. |
Whoops, of course it is; thanks. Subject line duly edited. I was going to call it "String size limit query", then I remembered that string is not in the FORTRAN vocabulary (even though everyone would have known what I meant), then I realised I couldn't call it "Character size limit query" (Answer: yes, 1). I think at that point I got flustered and strove for brevity at the expense of accuracy 
| JohnCampbell wrote: | I don't know for sure but I think the limit for length is 1024, as I get different errors in a module compilation if length > 1024.
You can certainly have an array of Character*1 much bigger. I have an array of 750 million and it works ok.
John |
Yes, I noted your latest post with interest. Did we just happen to cross in the post, as it were?
30 years since I first dipped my toe in the waters of FORTRAN, and I still look forward to the day when someone can explain to me, even by analogy, why it is necessary or desirable to distinguish between CHARACTER (len = n) char and CHARACTER char (n) ... why will one or the other not do? |
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IanLambley
Joined: 17 Dec 2006
Posts: 506
Location: Sunderland
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| Posted: Wed Mar 24, 2010 1:31 pm Post subject: |
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CHARACTER (len = n) char
or
CHARACTER*(n) char
or
CHARACTER char*(n)
creates a character variable called "char" that can hold "n" characters
an individual character can be addressed using char(i:i)
groups of characters can be addressed using char(i:j)
CHARACTER char (n)
or
CHARACTER*1 char(n)
creates a character variable array containing "n" elements that can each hold 1 character
an individual character can be addressed using char(i)
groups of characters can't so easily be addressed for assignments & comparisons etc and is akin to the pre-F77 method of storing each character in an integer*1 array
It is probably better to use a different name for the variable than "char" to avoid confusion with the intrinsic "char" function.
Ian |
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JohnCampbell
Joined: 16 Feb 2006
Posts: 2615
Location: Sydney
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| Posted: Thu Mar 25, 2010 12:34 am Post subject: |
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I think I am wrong in my estimate of the limit of character*n, if my program is working as I expect. It shows a value of 200 million compiles and runs. The limit may be 2gb, meaning as much memory as available.
Ian, I have demonstrated the difference between the variable "character a*n" and array "character b(n)*1".
Both have disadvantages as :
1) setting a = ' ' can be inefficient, if you are only using a small part of a.
2) Using the array in a subroutine call will not work if you want to receive as character *(*)
3) using "a" in an internal write or read may have other limitations, based on the maximum record size for an internal read/write buffer. Again I don't know what the limit is or where in Help to find the list of compiler limits.
| Code: | integer*4, parameter :: thousand = 1000
integer*4, parameter :: million = thousand*thousand
integer*4, parameter :: char_len = 200*million
character (len=char_len) aa
character bb(char_len), c
integer*4 i
!
do i = 1,char_len
c = char ( ichar('0')+mod(i,53) )
aa(i:i) = c
bb(i) = c
end do
!
i = char_len-50
write (*,'(a,i0,a,i0,a)') 'Variable is *',char_len,' (',char_len/million,'m)'
write (*,*) 'Var: ',aa(i:) ! this is a single variable
write (*,*) 'Array: ',bb(i:) ! this is 50 variables
end
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It's interesting that "aa(i: )" and "bb(i: )" both code and print the same, but are different. They can require a different format statement if not using *. You can convert from one to the other via an internal write, but only aa(i: ) can be easily used as the internal r/w buffer.
I've always preferred the array format for large character data but the example above may show both are useable.
John
ps : I increased the size to 1.5gb, removed the array bb and it still ran. |
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