randy@gtx.com (Randy D. Miller) (11/02/88)
I've seen conflicting information about the int result of a relational
operator in which the condition is satisfied. Does it evaluate to
exactly one, or just "non-zero" ? E.g., can we test whether two
comparisons have the same relational result with the following ?
(Obviously there are other ways of doing this, but that is not the
question here.)
int a,b,c,d;
if ((a > b) == (c > d)) {
/* both tests have the same relational result */
}
--
Randy D. Miller (602) 870-1696
GTX Corp., 8836 N. 23rd Ave., Phoenix, AZ 85021
{cbosgd,decvax,hplabs,amdahl,nsc}!sun!sunburn!gtx!randyguy@auspex.UUCP (Guy Harris) (11/02/88)
>I've seen conflicting information about the int result of a relational >operator in which the condition is satisfied. Does it evaluate to >exactly one, or just "non-zero" ? It evaluates to exactly one in all C implementations. (Implementations where it does not may purport to be C implementations, but they aren't.)
crossgl@ingr.UUCP (Gordon Cross) (11/02/88)
In article <787@gtx.com>, randy@gtx.com (Randy D. Miller) writes: > I've seen conflicting information about the int result of a relational > operator in which the condition is satisfied. Does it evaluate to > exactly one, or just "non-zero" ? According to the proposed ANSI standard, the result of the operations <, >, <=, >=, ==, !=, &&, and || has type "int" and a value of precisely zero or one. Hope that helps! Gordon Cross Intergraph Corp. Huntsville, AL
gwyn@smoke.BRL.MIL (Doug Gwyn ) (11/03/88)
In article <787@gtx.com> randy@gtx.UUCP (Randy D. Miller) writes: >I've seen conflicting information about the int result of a relational >operator in which the condition is satisfied. Does it evaluate to >exactly one, or just "non-zero"? Aaargh! Looking in K&R (1st Ed.) in the index under "relational expression, value of", I find that it's discussed on p.41. Turning to p.41 I find the answer. And so could you.
knudsen@ihlpl.ATT.COM (Knudsen) (11/03/88)
In article <787@gtx.com>, randy@gtx.com (Randy D. Miller) writes: > I've seen conflicting information about the int result of a relational > operator in which the condition is satisfied. Does it evaluate to > exactly one, or just "non-zero" ? E.g., can we test whether two > comparisons have the same relational result with the following ? > int a,b,c,d; > if ((a > b) == (c > d)) { > /* both tests have the same relational result */ > } Last year I had the same doubts you did, and asked the same question in a roundabout way. I was assured on the net (and by K&R) that Boolean TRUE is indeed exactly 1, not -1, 3, 7, 0xFF or whatever. Very nice, because you can pretty freely intermix bitwise and Boolean operations, at least below the top level of nested expressions. Your example could be done: if( !((a > b) ^ (c > d)) {...} Yes, you now have exclusive OR. Also, things like boolvar &= boolex; and boolvar |= boolex; Then there is the odd-number tester: #define odd(n) (n & 1) But don't use ~ for ! (twiddle for bang) or you'll get 0xFFFF for TRUE, but much worse 0xFFFE for FALSE -- not good. Also be very wary of functions whose result is often treated as Boolean, but is really integer or pointer so that "true" returns as anything nonzero. "Cast" these into Boolean if needed by !!fcn(...) BTW, I've always considered C to be schizoid wrt the Boolean type; there isn't any such type, but there are operators for it and you can assign them as integers! Well not exactly; && and || are really flow-control constructs. -- Mike Knudsen Bell Labs(AT&T) att!ihlpl!knudsen "Lawyers are like nuclear bombs and PClones. Nobody likes them, but the other guy's got one, so I better get one too."
blm@cxsea.UUCP (Brian Matthews) (11/03/88)
Randy D. Miller (randy@gtx.UUCP) writes: |I've seen conflicting information about the int result of a relational |operator in which the condition is satisfied. Does it evaluate to |exactly one, or just "non-zero" ? E.g., can we test whether two |comparisons have the same relational result with the following ? | int a,b,c,d; | if ((a > b) == (c > d)) { | /* both tests have the same relational result */ | } From Section 3.3.8 "Equality operators" of the ANSI draft standard, last paragraph under Semantics: "Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false. The result has type int." From Section 3.3.9 "Equality operators", first paragraph under Semantics: "The == (equal to) and the != (not equal to) operators are analogous to the relational operators except for their lower precedence." It is also guaranteed in K&R, but I don't have a K&R handy, so I can' give you a section or page number. -- Brian L. Matthews blm@cxsea.UUCP ...{mnetor,uw-beaver!ssc-vax}!cxsea!blm +1 206 251 6811 Computer X Inc. - a division of Motorola New Enterprises
scjones@sdrc.UUCP (Larry Jones) (11/03/88)
In article <787@gtx.com>, randy@gtx.com (Randy D. Miller) writes: > I've seen conflicting information about the int result of a relational > operator in which the condition is satisfied. Does it evaluate to > exactly one, or just "non-zero"? The draft standard says "Each of the operators ... shall yield 1 if the specified relation is true and 0 if it is false." I believe K&R made a similar statement. ---- Larry Jones UUCP: uunet!sdrc!scjones SDRC scjones@sdrc.uucp 2000 Eastman Dr. BIX: ltl Milford, OH 45150 AT&T: (513) 576-2070 "Save the Quayles" - Mark Russell
bill@twwells.uucp (T. William Wells) (11/06/88)
In article <7482@ihlpl.ATT.COM> knudsen@ihlpl.ATT.COM (Knudsen) writes:
: Also be very wary of functions whose result is often treated as
: Boolean, but is really integer or pointer so that "true" returns
: as anything nonzero. "Cast" these into Boolean if needed by
: !!fcn(...)
Clearer, and likely to generate better code, is:
fcn != 0
---
Bill
{uunet|novavax}!proxftl!twwells!bill