Re: Now what would you expect this to print out?
On Tuesday 20 May 2008 16:44, RW wrote: > On Tue, 20 May 2008 11:33:50 +0200 > > Jonathan McKeown <[EMAIL PROTECTED]> wrote: > > On Tuesday 20 May 2008 02:41, RW wrote: > > > On Mon, 19 May 2008 21:46:03 +1200 > > > > > > Jonathan Chen <[EMAIL PROTECTED]> wrote: > > > > find /usr/src \( -name Makefile -or -name '*.mk' \) -print > > > > > > Why does that make a difference, when print always evaluates to > > > true? > > > > > > x AND true = x > > > > > > so > > > > > > (a OR b) AND true = a OR b > > > a OR (b AND true) = a OR b > > > > It makes a difference (as in programming) because -print is used for > > its side-effect rather than its value, and the binding order > > influences when the side-effect happens. > > That's still a bit counter-intuitive because in normal programming > languages the binding order modifies side-effects via the evaluation > order. And in both cases the evaluation order would be expected to be > left-to-right, with -print running last. Yes. I'm actually talking rubbish. find evaluates its argument expression left-to-right, and the ``precedence'' actually applies to term grouping rather than evaluation order. (This does affect the outcome, but not in the way I glibly said it did). What I should have said is that like a lot of programming languages, find is lazy when it comes to Boolean expressions: when it gets a TRUE in an -or or a FALSE in an -and, the value of the whole expression must be TRUE or FALSE respectively, regardless of what the remaining terms are, so why bother evaluating them? (It's usually referred to as short-circuiting). > I guess what you are saying is that the side-effect of print is based-on > a Boolean "running-value". And without the brackets, the first test has > been evaluated, but not yet ORed into that "running-value", by the time > that print runs. That's not quite how it works. Rewriting find /usr/src -name Makefile -or -name '*.mk' -print using extra parens to emphasise the implicit grouping, and including the implicit -and, gives: find /usr/src -name Makefile -or \( -name '*.mk' -and -print \) in other words, an -or with two terms, one of which happens to be an expression. If -name Makefile is true, the -or is satisfied, so nothing else is evaluated, and find goes on to the next filename. Otherwise, the expression in the second term has to be evaluated. If -name '*.mk' is false, the -and is satisfied (which also satisfies the -or) and find moves to the next filename. If it's true, the -and can't be satisfied without evaluating the -print. The end result is that only files matching '*.mk' are printed. Rewriting the other case, find /usr/src \( -name Makefile -or -name '*.mk' \) -and -print If the first expression is false, the -and is satisfied and the -print is not evaluated. If the first expression is true (meaning either of the -name arguments is true), then the -and can't be satisfied without evaluating the -print. The last case is find /usr/src -name Makefile -or -name '*.mk' find quickly analyses this, finds no output action, and converts it to the second form above, internally placing parens around the whole expression and an -and -print after it. Jonathan ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
Re: Now what would you expect this to print out?
On Tue, 20 May 2008 11:33:50 +0200 Jonathan McKeown <[EMAIL PROTECTED]> wrote: > On Tuesday 20 May 2008 02:41, RW wrote: > > On Mon, 19 May 2008 21:46:03 +1200 > > > > Jonathan Chen <[EMAIL PROTECTED]> wrote: > > > find /usr/src \( -name Makefile -or -name '*.mk' \) -print > > > > Why does that make a difference, when print always evaluates to > > true? > > > > x AND true = x > > > > so > > > > (a OR b) AND true = a OR b > > a OR (b AND true) = a OR b > > It makes a difference (as in programming) because -print is used for > its side-effect rather than its value, and the binding order > influences when the side-effect happens. That's still a bit counter-intuitive because in normal programming languages the binding order modifies side-effects via the evaluation order. And in both cases the evaluation order would be expected to be left-to-right, with -print running last. I guess what you are saying is that the side-effect of print is based-on a Boolean "running-value". And without the brackets, the first test has been evaluated, but not yet ORed into that "running-value", by the time that print runs. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
Re: Now what would you expect this to print out?
On Tuesday 20 May 2008 02:41, RW wrote: > On Mon, 19 May 2008 21:46:03 +1200 > > Jonathan Chen <[EMAIL PROTECTED]> wrote: > > On Mon, May 19, 2008 at 01:49:35AM -0700, Garrett Cooper wrote: > > > Riddle for the day for folks that have source trees... what would > > > you expect this to print out (ask yourself the question and then > > > execute the command)? > > > > > > find /usr/src -name Makefile -or -name '*.mk' -print > > > > > > The expected output and what actual output differed in my mind, but > > > maybe somebody else can "shed some light" on the logic behind what > > > happened > > > > It's a problem that catches many young players with find(1). One has > > to remember from reading the man-page that all directives have an > > implicit AND operator on it; and that includes the "-print" directive. > > So to get what you want, you have to introduce brackets: > > > > find /usr/src \( -name Makefile -or -name '*.mk' \) -print > > Why does that make a difference, when print always evaluates to true? > > x AND true = x > > so > > (a OR b) AND true = a OR b > a OR (b AND true) = a OR b It makes a difference (as in programming) because -print is used for its side-effect rather than its value, and the binding order influences when the side-effect happens. Jonathan ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
Re: Now what would you expect this to print out?
On Mon, 19 May 2008 12:40:46 +0200, Jonathan McKeown <[EMAIL PROTECTED]> wrote: > I forget where I saw this quote first, but the last five words always > make me think of the find command: > > Real Programmers consider "what you see is what you get" to be just as > bad a concept in Text Editors as it is in women. No, the Real Programmer > wants a "you asked for it, you got it" text editor - complicated, cryptic, > powerful, unforgiving, dangerous. The page at http://www.pbm.com/~lindahl/real.programmers.html suggests that this quote is from ``A letter to the editor of Datamation, volume 29 number 7, July 1983.'' The author of that page writes ``I've long ago lost my dog-eared photocopy, but I believe this was written (and is copyright) by Ed Post, Tektronix, Wilsonville OR USA,'' so it may be a bit tricky to verify the claim. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
Re: Now what would you expect this to print out?
On Mon, 19 May 2008 21:46:03 +1200 Jonathan Chen <[EMAIL PROTECTED]> wrote: > On Mon, May 19, 2008 at 01:49:35AM -0700, Garrett Cooper wrote: > > Riddle for the day for folks that have source trees... what would > > you expect this to print out (ask yourself the question and then > > execute the command)? > > > > find /usr/src -name Makefile -or -name '*.mk' -print > > > > The expected output and what actual output differed in my mind, but > > maybe somebody else can "shed some light" on the logic behind what > > happened > > It's a problem that catches many young players with find(1). One has > to remember from reading the man-page that all directives have an > implicit AND operator on it; and that includes the "-print" directive. > So to get what you want, you have to introduce brackets: > > find /usr/src \( -name Makefile -or -name '*.mk' \) -print > Why does that make a difference, when print always evaluates to true? x AND true = x so (a OR b) AND true = a OR b a OR (b AND true) = a OR b ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
Re: Now what would you expect this to print out?
On Monday 19 May 2008 11:46, Jonathan Chen wrote: > On Mon, May 19, 2008 at 01:49:35AM -0700, Garrett Cooper wrote: > > Riddle for the day for folks that have source trees... what would you > > expect this to print out (ask yourself the question and then execute the > > command)? > > > > find /usr/src -name Makefile -or -name '*.mk' -print > > > > The expected output and what actual output differed in my mind, but maybe > > somebody else can "shed some light" on the logic behind what happened > > It's a problem that catches many young players with find(1). One has > to remember from reading the man-page that all directives have an > implicit AND operator on it; and that includes the "-print" directive. > So to get what you want, you have to introduce brackets: > > find /usr/src \( -name Makefile -or -name '*.mk' \) -print Or, slightly bizarrely, just leave the -print off altogether - as the manpage says, If none of -exec, -ls, -print0, or -ok is specified, the given expression shall be effectively replaced by ( given expression ) -print. [Note the parens around given expression] I forget where I saw this quote first, but the last five words always make me think of the find command: Real Programmers consider "what you see is what you get" to be just as bad a concept in Text Editors as it is in women. No, the Real Programmer wants a "you asked for it, you got it" text editor - complicated, cryptic, powerful, unforgiving, dangerous. Jonathan ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
Re: Now what would you expect this to print out?
> Riddle for the day for folks that have source trees... what would you expect > this to print out (ask yourself the question and then execute the command)? > > find /usr/src -name Makefile -or -name '*.mk' -print > > The expected output and what actual output differed in my mind, but maybe > somebody else can "shed some light" on the logic behind what happened [I > read through the find(1) code and can see why it does what it does, but I > still don't find the result useful]. Looks like you wanted to do this: find /usr/src \( -name Makefile -o -name '*.mk' \) -print Implicit operator is and (-a) and is arithmetic, and is equivalent to multiplication, or is equivalent to addition ... so and as the priority. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
Re: Now what would you expect this to print out?
On Mon, May 19, 2008 at 01:49:35AM -0700, Garrett Cooper wrote: > Riddle for the day for folks that have source trees... what would you expect > this to print out (ask yourself the question and then execute the command)? > > find /usr/src -name Makefile -or -name '*.mk' -print > > The expected output and what actual output differed in my mind, but maybe > somebody else can "shed some light" on the logic behind what happened It's a problem that catches many young players with find(1). One has to remember from reading the man-page that all directives have an implicit AND operator on it; and that includes the "-print" directive. So to get what you want, you have to introduce brackets: find /usr/src \( -name Makefile -or -name '*.mk' \) -print Cheers. -- Jonathan Chen <[EMAIL PROTECTED]> -- If you're right 90% of the time, why quibble about the remaining 3%? ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
Now what would you expect this to print out?
Riddle for the day for folks that have source trees... what would you expect this to print out (ask yourself the question and then execute the command)? find /usr/src -name Makefile -or -name '*.mk' -print The expected output and what actual output differed in my mind, but maybe somebody else can "shed some light" on the logic behind what happened [I read through the find(1) code and can see why it does what it does, but I still don't find the result useful]. Cheers, -Garrett ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"
