chris@mimsy.UUCP (Chris Torek) (02/18/89)
[sigh... all in the name of correctness] Names no longer matter; someone wrote: >>>... TRUE is not 0 in our implimentation. so in article <15908@mimsy.UUCP> I added: >>... the true-value of any comparison operator in *any* correct C >>compiler is 1 .... In article <EJP.89Feb16173101@faron.icd.abnet.COM> ejp@faron.icd.abnet.COM (Ed Prochak) writes: >Chris, > He said nothing about the value that resulted from the comparison. Sure enough---but consider the original phrasing: `TRUE is not 0 *in our implementation*'. This implies that true (to be distinguished from his TRUE, which was probably a macro defined as 1) has different values in different implementations. It does not. C comparison operators (which are the ones that produce true/false values) produce 0 and 1. Always. >He said, "TRUE is not 0". and that is correct. Indeed. But `3 < 5' is 1, not some implementation-defined value. >A TRUE value in C does not have to be 1 (one) for it to be a >"correct C compiler". A truth value produced by an ostensibly boolean operator DOES have to be 1 (one) for it to be correct. Read on: >In C > FALSE = 0 > TRUE = NOT FALSE No, for C defines neither `TRUE' nor `FALSE'. What it *does* define is both the domain and the range for ostensibly-boolean tests. Anything that *accepts* an ostensibly-boolean value accepts *all* nonzero integer values as `true' and only the integer value 0 as `false', but anything that *produces* such a value produces either 0 or 1. The *type* of a `boolean' operator is `int'; the *range* of such an operator is the set {0, 1}. Specifically: if (expr) ... accepts any non-zero value for `expr'. !(expr) converts any non-zero expr to 0, and a zero-valued expr to 1. expr1 comparison-op expr2 performs the comparison, and if true, produces 1, otherwise produces 0. Since if, while, do/while, and for conditions allow pointer-valued expressions, the simplest way to think about them is to imagine that all such statements are converted from if (expr) to if ((expr) != 0) during compilation. The result is then tested against 0 and 1 and no other values (none other being possible). Thus only the logical negation operator `!' need consider the possibility of values not in the set {0, 1}. (Obviously some computation can be elided by the compiler wherever the value itself is not used---typically all `boolean' tests discard the precise value, caring only about zero/nonzero---which is probably what leads to the question over the `correct' value for a macro for TRUE. FALSE is easy; only 0 qualifies. Any nonzero integer tests true, but only one such integer---namely 1---is ever produced, so while there is no `correct' answer, 1 is probably best.) There, sufficient now? :-) -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris
ejp@faron.icd.abnet.COM (Ed Prochak) (02/21/89)
Chris,
Let me publicly admit that I was wrong. Comparison operators
are required to generate TRUE = 1 in C. The fact that logical
operators such as ! (not) accept nonzero values as TRUE was
irrelevant. (The freedom to use TRUE = nonzero is a nice feature,
but is not allowed for the comparison operators. so compiler
writers should be aware of this.)
TTFN
Ed
--
Edward J. Prochak (216)646-4663 I think.
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