Thanks Alex, makes total sense with the examples you provided. The only remaining question in my mind is if the use of parens changes anything - a post yesterday suggested that putting a condition in parens causes it to be evaluated ahead of the other conditions but I can't make that happen in your example. I suspect confusion between precedence and evaluation again.
Pete Molly's Revenge <http://www.mollysrevenge.com> On Thu, Oct 6, 2011 at 3:55 AM, Alex Tweedly <a...@tweedly.net> wrote: > Don't confuse "operator precedence" and "order of evaluation". > > precedence tells you how the results of individual parts of the calculation > are combined. For instance, 1+2*3+4 will give 11 (i.e. multiplication is > higher precedence than addition, so the 2 is multiplied by the 3 and then > that is used in the lower precedence addition). If you wanted different, you > would need brackets ... (1+2)*(3+4) would give you 21 (3*7). > > order of evaluation (for LC) is left to right (regardless of precedence > !!). > > Try this cute test case > >> global C >> >> function getnext >> add 1 to C >> return C >> end getnext >> >> on mouseUp >> put 0 into C >> put getnext()+getnext()*getnext()+**getnext() >> end mouseUp >> > this results in 11 - i.e. 1 + 2*3 + 4 > > If evaluation order had followed precedence then it would have been 3 + > (1*2) + 4 ---> 9 > > The similar rules apply when evaluating logical expressions in a test > condition, with the additional proviso that LC will stop evaluating the > terms of the expression as soon as the eventual result is determined. So, > >> on mouseUp >> put 0 into C >> if getnext() > 0 and getnext() < 2 and getnext()< 3 and getnext()<3 then >> put "that's a surprise" >> else >> put getnext() >> end if >> end mouseUp >> > produces 3 (first term is true, second term is false and at that point we > know the whole expression must be false, so we stop evaluating - therefore > the next call to getnext() produces 3 > > -- Alex. > > On 06/10/2011 01:26, Pete wrote: > >> I just read the section in the manual about operator precedence and I >> think >> perhaps I'm even more confused than I was! It details a strict order of >> precedence which seems to contradict the knowledge that condition >> evaluation >> proceeds from left to right and stops as soon as a false one is found. >> >> Maybe I'll try a few test cases. >> >> >> Pete >> Molly's Revenge<http://www.**mollysrevenge.com<http://www.mollysrevenge.com> >> > >> >> > > ______________________________**_________________ > use-livecode mailing list > use-livecode@lists.runrev.com > Please visit this url to subscribe, unsubscribe and manage your > subscription preferences: > http://lists.runrev.com/**mailman/listinfo/use-livecode<http://lists.runrev.com/mailman/listinfo/use-livecode> > > _______________________________________________ use-livecode mailing list use-livecode@lists.runrev.com Please visit this url to subscribe, unsubscribe and manage your subscription preferences: http://lists.runrev.com/mailman/listinfo/use-livecode
