.substring bug - indicies don't work as documented(?)

[Jim Avera]

Hi,

There may be a bug in the .substring function in 
GNU troff version 1.16.1.

The man page says:
       .substring xx n1 [n2]
              Replace the string in register  xx  with  the  sub­
              string defined by the indices n1 and n2.  The first
              character in the string has index one.   If  n2  is
              omitted,  it  is  taken to be equal to the string's
              length.  If the index value n1 or n2 is negative or
              zero,  it  will  be  counted  from  the  end of the
              string, going backwards:  The  last  character  has
              index  0,  the  character before the last character
              has index -1, etc.

Are n1 and n2 supposed to be the starting and ending indicies
of the substring to be retained?

Here are some results:

for s=abcdefg .substring s 0 1 yields "abcdefg"
for s=abcdefg .substring s 0 2 yields "bcdefg"
for s=abcdefg .substring s 0 3 yields "cdefg"
for s=abcdefg .substring s 1 1 yields ""
for s=abcdefg .substring s 1 2 yields "a"
for s=abcdefg .substring s 1 3 yields "ab"
for s=abcdefg .substring s 2 1 yields "a"

If this is not a bug, can someone explain the output to me?

Thanks...
-Jim Avera

P.S. Attached is a test program.




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substring_test.tr
Description: substring_test.tr