# Precedence of %

9 messages
Open this post in threaded view
|

## Precedence of %

 Does anyone understand why the precedence of % is strange:                 test=> select -25 % -10;          ?column?         ----------               -35         (1 row)                 test=> select -25 % (-10);          ?column?         ----------                -5         (1 row) Is it treating the first as -25 - 10?  Why?  Why are parens necessary to get the right answer?  I see this in gram.y:         %left       '+' '-'         %left       '*' '/' '%' Look at this:         test=> select -25 + -10;          ?column?         ----------               -35         (1 row)                 test=> select -25 * -10;          ?column?         ----------               250         (1 row)                 test=> select -25 / -10;          ?column?         ----------                 2         (1 row)                 test=> select -25 % -10;          ?column?         ----------               -35         (1 row) Only the '%' case looks wrong. --   Bruce Momjian                        |  http://candle.pha.pa.us  [hidden email]               |  (610) 359-1001   +  If your life is a hard drive,     |  13 Roberts Road   +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [hidden email]
Open this post in threaded view
|

## Re: Precedence of %

 Bruce Momjian <[hidden email]> writes: > Does anyone understand why the precedence of % is strange: > test=> select -25 % -10; It's treating it as ((-25) %) - (10), which is probably not so surprising given the relative precedence of % and - ... though I have to admit I'm not totally clear why it's not (-(25 %)) - (10) instead. We could maybe hack the precedence of the productions for prefix/postfix %, but I wonder if it wouldn't be smarter to remove 'em altogether (along with the two existing unary % operators).                         regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster
Open this post in threaded view
|

## Re: Precedence of %

 I wrote: > Bruce Momjian <[hidden email]> writes: >> Does anyone understand why the precedence of % is strange: >> test=> select -25 % -10; > It's treating it as ((-25) %) - (10), which is probably not so > surprising given the relative precedence of % and - ... though > I have to admit I'm not totally clear why it's not (-(25 %)) - (10) > instead. Now that I'm fully awake, that last point is easily explained: the precedence of unary minus is higher than that of %, which in turn is higher than that of infix minus.  So the choice of (-25) % over -(25 %) is reasonable and correct.  Now when the parser is done with that, it is on the % with a lookahead of - and has to decide whether to reduce according to             | a_expr '%'                 { \$\$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "%", \$1, NULL); } or shift expecting to later reduce by             | a_expr '%' a_expr                 { \$\$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "%", \$1, \$3); } but the precedence of the '-' token is set up for infix minus so the choice is to reduce (see the Bison manual). We could possibly fix this by fooling with the precedence of the productions for postfix '%', but I'm worried that that would have unintended side-effects.  What I'd like to propose instead is that we remove prefix and postfix '%' entirely --- and also '^', which is the only other hard-wired operator that appears in all three forms in the grammar.  There are no actual uses of prefix or postfix '^' in pg_operator, so that loses us nothing.  Prefix and postfix '%' exist, but only for the float8 datatype, not anything else; and I can't imagine a good reason to write those rather than trunc() or round().  (Quick: which is which, and how would you remember?) round() and trunc() also have the virtue that they already have versions for type numeric.  If we keep the operators then we'll be right back with the complaint that was lodged the other day about exponentiation, namely unexpected precision loss for numeric inputs: regression=# select 12345678901234567890.55 %;        ?column? ----------------------  1.23456789012346e+19 (1 row) regression=# select round(12345678901234567890.55);         round ----------------------  12345678901234567891 (1 row) Comments?                         regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your       joining column's datatypes do not match
Open this post in threaded view
|

## Re: Precedence of %

 Tom Lane wrote: > We could possibly fix this by fooling with the precedence of the > productions for postfix '%', but I'm worried that that would have > unintended side-effects.  What I'd like to propose instead is that > we remove prefix and postfix '%' entirely --- and also '^', which > is the only other hard-wired operator that appears in all three > forms in the grammar.  There are no actual uses of prefix or postfix > '^' in pg_operator, so that loses us nothing.  Prefix and postfix '%' > exist, but only for the float8 datatype, not anything else; and I > can't imagine a good reason to write those rather than trunc() or > round().  (Quick: which is which, and how would you remember?) Agreed. I didn't know we even supported unary % and ~, and I doubt anyone else did either.  We just need to mark it as a non-backward compatible change in CVS commit so I mention it in the release notes. --   Bruce Momjian                        |  http://candle.pha.pa.us  [hidden email]               |  (610) 359-1001   +  If your life is a hard drive,     |  13 Roberts Road   +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your       joining column's datatypes do not match
Open this post in threaded view
|

## Re: Precedence of %

 In reply to this post by Tom Lane-2 > round() and trunc() also have the virtue that they already have versions > for type numeric.  If we keep the operators then we'll be right back > with the complaint that was lodged the other day about exponentiation, > namely unexpected precision loss for numeric inputs: > > regression=# select 12345678901234567890.55 %; >        ?column? > ---------------------- >  1.23456789012346e+19 > (1 row) I don't even grasp what unary modulo actually means??? Chris ---------------------------(end of broadcast)--------------------------- TIP 8: explain analyze is your friend
Open this post in threaded view
|

## Re: Precedence of %

 Christopher Kings-Lynne <[hidden email]> writes: > I don't even grasp what unary modulo actually means??? At some point in the dim mists of prehistory, somebody thought it would be cute to define prefix % as trunc() and postfix % as round().  I'm not aware of any precedent for that; it was probably mostly an exercise in testing out the grammar. Now that I look, it doesn't look like these operators are documented at all in the SGML docs, so it sure seems that removing them should be pretty painless.                         regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives?                http://archives.postgresql.org
Open this post in threaded view
|

## Re: Precedence of %

 On Jun 5, 2005, at 12:55 AM, Tom Lane wrote: > Now that I look, it doesn't look like these operators are documented > at all in the SGML docs, so it sure seems that removing them should be > pretty painless. I wonder what else is lurking around undocumented and unused? Might   be some other nuggets just waiting to be discovered! :) Michael Glaesemann grzm myrealbox com ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [hidden email]