OpenEuphoria

Re: Why equal(x[n], x[n..n])=0 ?

• Posted by Jason Gade <jaygade at yaho?.?om> Sep 24, 2007
• 711 views

Fernando Bauer wrote:
> 
> Hi Derek! Thanks for your reply.
> 
> Derek Parnell wrote:
> > 
> > Fernando Bauer wrote:
> > > 
> > > Hi All,
> > > 
> > > I was debugging a function when I noticed that a slice with equal indexes
> > > is
> > > different from an access with one index. x[n] is different from x[n..n]
> > > where
> > > n is a valid index.
> > > According to the manual, a slice always result in a sequence (also when
> > > x[n]
> > > is an atom). But, in particular, when we have something like x[n..n], the
> > > result
> > > is x[n] with the its depth incremented by 1. In other words, the structure
> > > of
> > > the element depends on the access form (subscription or slicing). For me,
> > > this
> > > is a surprising fact. 
> > > This kind of implementation affects some algorithms, because we have to
> > > test
> > > when the indexes are equal in order to not use slice.
> > > So, why equal(x[n], x[n..n])=0 ?
> > 
> > Because the syntax [n..m] is a slice which is a sequence - always. And the
> > syntax
> > [n] is a element reference whose value depends in what is at location 'n'.
> > Specifically,
> > the syntax [n..n] is a sequence which has a length of one. 
> > 
> > Given the sequence (string) s = "qwerty", then s[3..3] is the string "e" and
> > s[3] is the character 'e'. And characters are not strings. 
> > 
> > Another way to look at is this ...
> > 
> >   s[3 .. 4] is "er"
> >   s[3 .. 3] is "e"
> Ok. But why s[3..3] is not 'e'? Why the original 'e' is transformed in "e" ?
> 
> >   s[3 .. 2] is ""
> > 
> > so making s[3..3] ("e") mean the same as s[3] ('e') is inconsistent.
> Why do you think this is inconsistent? I could think the opposite. Why does
> the slice s[3..3] change the type of s[3]?

I thought that it was already adequately explained, but let me try again.

s[3..3] is *not* the same as s[3]. s[3] is an element of a sequence, which may
be a sequence or an atom. s[3..3] is a sequence with the elements from index 3 to
index 3 (which happen to be the same index).

A slice is *always* a sequence. See http://www.rapideuphoria.com/refman_2.htm#1
section 2.2.6. With your idea, how would s[3..2] work? What is it supposed to
return? Currently it returns the empty sequence.

--
A complex system that works is invariably found to have evolved from a simple
system that works.
--John Gall's 15th law of Systemantics.

"Premature optimization is the root of all evil in programming."
--C.A.R. Hoare

j.