Re: arithmetic operator precedence

[Peter Stephenson <p.w.stephenson@ntlworld.com>]

On Mon, 16 Jun 2008 10:07:26 +0200
Vincent Lefevre <vincent@vinc17.org> wrote:
> Speaking of precedence, the following one is nasty:
> 
> vin% zsh -c 'echo $((-3**2))'
> 9 
> vin% bash -c 'echo $((-3**2))'
> 9
> vin% ksh93 -c 'echo $((-3**2))'
> 9
> 
> IMHO these shells should be fixed to give -9, i.e. ** should have
> the precedence over the unary -, like conventional math writing.

That's an interesting point for C_PRECEDENCES since I was trying to get
it behave as much as possible like Perl.  What does anyone else think?

Index: Doc/Zsh/arith.yo
===================================================================
RCS file: /cvsroot/zsh/zsh/Doc/Zsh/arith.yo,v
retrieving revision 1.13
diff -u -r1.13 arith.yo
--- Doc/Zsh/arith.yo	12 Jun 2008 13:45:05 -0000	1.13
+++ Doc/Zsh/arith.yo	16 Jun 2008 13:38:41 -0000
@@ -121,11 +121,13 @@
 
 With the option tt(C_PRECEDENCES) the precedences (but no other
 properties) of the operators are altered to be the same as those in
-most other languages that support the relevant operators:
+most other languages that support the relevant operators, in particular
+Perl (note that exponentiation, not present in C, has a higher
+precedence than unary operators):
 
 startsitem()
-sitem(tt(PLUS() - ! ~ PLUS()PLUS() --))(unary plus/minus, logical NOT, complement, {pre,post}{in,de}crement)
 sitem(tt(**))(exponentiation)
+sitem(tt(PLUS() - ! ~ PLUS()PLUS() --))(unary plus/minus, logical NOT, complement, {pre,post}{in,de}crement)
 sitem(tt(* / %))(multiplication, division, modulus (remainder))
 sitem(tt(PLUS() -))(addition, subtraction)
 sitem(tt(<< >>))(bitwise shift left, right)
Index: Src/math.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/math.c,v
retrieving revision 1.33
diff -u -r1.33 math.c
--- Src/math.c	12 Jun 2008 13:45:06 -0000	1.33
+++ Src/math.c	16 Jun 2008 13:38:42 -0000
@@ -193,9 +193,9 @@
 static int c_prec[TOKCOUNT] =
 {
 /*        M_INPAR   M_OUTPAR     NOT       COMP     POSTPLUS */
-/*  0 */     1,       137,        2,        2,         2,
+/*  0 */     1,       137,        3,        3,         3,
 /*        POSTMINUS   UPLUS     UMINUS     AND        XOR    */
-/*  5 */     2,         2,        2,        9,        10,
+/*  5 */     3,         3,        3,        9,        10,
 /*          OR         MUL       DIV       MOD       PLUS    */
 /* 10 */    11,         4,        4,        4,         5,
 /*         MINUS      SHLEFT   SHRIGHT     LES        LEQ    */
@@ -211,7 +211,7 @@
 /*        DANDEQ      DOREQ    DXOREQ    COMMA       EOI     */
 /* 40 */    17,        17,       17,       18,       200,
 /*       PREPLUS    PREMINUS     NUM        ID       POWER   */
-/* 45 */     2,         2,        0,        0,         3,
+/* 45 */     3,         3,        0,        0,         2,
 /*          CID      POWEREQ     FUNC  */
 /* 50 */     0,        17,        0
 };
@@ -514,13 +514,9 @@
 		ptr++;
 		return MINUSEQ;
 	    }
-	    if (unary) {
-		if (idigit(*ptr) || *ptr == '.') {
-		    ptr--;
-		    return lexconstant();
-		} else
-		    return UMINUS;
-	    } else
+	    if (unary)
+		return UMINUS;
+	    else
 		return MINUS;
 	case '(':
 	    return M_INPAR;


-- 
Peter Stephenson <p.w.stephenson@ntlworld.com>
Web page now at http://homepage.ntlworld.com/p.w.stephenson/