point -> double,double ?

Greg Stark <gsstark@mit.edu> writes:

What are the operators or functions to extract the x and y portions of a
point? I can't find it in my book or in the online \do or \df output anywhere.

point[0] and point[1]. Not sure if this is adequately documented.

regards, tom lane

On Fri, Jan 17, 2003 at 14:04:21 -0500,
Greg Stark <gsstark@mit.edu> wrote:

What are the operators or functions to extract the x and y portions of a
point? I can't find it in my book or in the online \do or \df output anywhere.

One way to do it is the following:
bruno=> select height(box('(0,0)','(1,2)'));
height
--------
2
(1 row)

bruno=> select width(box('(0,0)','(1,2)'));
width
-------
1
(1 row)

Replace the the non '(0,0)' point with the point of interest.

This only works for nonnegative values.

Tom Lane <tgl@sss.pgh.pa.us> writes:

Greg Stark <gsstark@mit.edu> writes:

What are the operators or functions to extract the x and y portions of a
point? I can't find it in my book or in the online \do or \df output anywhere.

point[0] and point[1]. Not sure if this is adequately documented.

Thank you very much.

It may also be that I just don't have the right book. The book I have seems to
be more of a tutorial than a reference. Is there a recommended reference book
for postgres?

--
greg

Greg Stark <gsstark@mit.edu> writes:

Tom Lane <tgl@sss.pgh.pa.us> writes:

point[0] and point[1]. Not sure if this is adequately documented.

Thank you very much.

It may also be that I just don't have the right book. The book I have seems to
be more of a tutorial than a reference. Is there a recommended reference book
for postgres?

Book? Save a tree, use the online manuals ;-). I find this particular
item documented at the bottom of

http://www.ca.postgresql.org/users-lounge/docs/7.3/postgres/functions-geometry.html

regards, tom lane

Tom Lane <tgl@sss.pgh.pa.us> writes:

Greg Stark <gsstark@mit.edu> writes:

Tom Lane <tgl@sss.pgh.pa.us> writes:

point[0] and point[1]. Not sure if this is adequately documented.

Thank you very much.

Hm, this seems odd

slo=> select b,b[0],b[0][0] from t;
b | b | b
-----------------------------------------------+------------------------+---
(-75.493906,44.854114),(-75.493906,44.854114) | (-75.493906,44.854114) |
(1 row)

--
greg

Greg Stark <gsstark@mit.edu> writes:

Hm, this seems odd

slo=> select b,b[0],b[0][0] from t;
b | b | b
-----------------------------------------------+------------------------+---
(-75.493906,44.854114),(-75.493906,44.854114) | (-75.493906,44.854114) |
(1 row)

The problem here is that foo[m][n] is our notation for a
doubly-subscripted array --- and we don't have any concept in the type
system that a one-dimensional array is different from a two-dimensional
array. So the code goes down the primrose path of assuming that "b"
is a 2-D array of points, rather than expecting b[0] to yield a
separate datatype that should be separately subscripted.

We could perhaps hack something for this particular case, since box is
known not to be a general array but only a hardwired one-dimensional
array of points. I don't see a good general solution though.
Any thoughts?

regards, tom lane

Tom Lane <tgl@sss.pgh.pa.us> writes:

Greg Stark <gsstark@mit.edu> writes:

Hm, this seems odd

slo=> select b,b[0],b[0][0] from t;
b | b | b
-----------------------------------------------+------------------------+---
(-75.493906,44.854114),(-75.493906,44.854114) | (-75.493906,44.854114) |
(1 row)

The problem here is that foo[m][n] is our notation for a
doubly-subscripted array --- and we don't have any concept in the type
system that a one-dimensional array is different from a two-dimensional
array. So the code goes down the primrose path of assuming that "b"
is a 2-D array of points, rather than expecting b[0] to yield a
separate datatype that should be separately subscripted.

I guess arrays must be some deep voodoo. [] doesn't show up in the list of
operators. I would be happy if there was at least some way to trick the parser
into doing the right thing. I'm surprised something like this doesn't work:

slo=> select (b[0])[0] from t;
ERROR: parser: parse error at or near "[" at character 14

We could perhaps hack something for this particular case, since box is
known not to be a general array but only a hardwired one-dimensional
array of points. I don't see a good general solution though.
Any thoughts?

If [] were a normal operator then other datatypes could define operations
similar to point and box, but I imagine it's the way it is for some good
reason in the parser level.

--
greg

Greg Stark <gsstark@mit.edu> writes:

I guess arrays must be some deep voodoo. [] doesn't show up in the list of
operators. I would be happy if there was at least some way to trick the parser
into doing the right thing. I'm surprised something like this doesn't work:

slo=> select (b[0])[0] from t;
ERROR: parser: parse error at or near "[" at character 14

Yeah, that was my first thought too. It might not be difficult to make
it work --- there's no production in gram.y to accept this, but if there
were then I think that the rest of the code would do the right thing.

If [] were a normal operator then other datatypes could define operations
similar to point and box, but I imagine it's the way it is for some good
reason in the parser level.

You can make arrays of whatever datatype you want. The issue here
is just that there's a syntactic ambiguity between subscripting an
N-dimensional array and subscripting the element datatype of an
N-1-dimensional array ...

regards, tom lane