generic swap method in javascript
in [JavaScript]
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From: proxygeek on 16 Jun 2010 03:40 I could not find any worthwhile discussion on implementing a generic swap(a,b) function in Javascript. Please point me to a discussion/link if it's already covered there. Problem: I need a method swap(a,b) which I can call with two variables as params and it would swap the values of the two vars. <snip> .... .... function swap(a,b){ // This would NOT work as intended t = a; a = b; b = t; } .... .... var x = 10, y = 20; swap(x,y); alert("x = " + x + ", y = " + y); // should give x = 20, y = 10 .... </snip> Above implementation would not work as here swap() does not know a thing about x or y. What would be the best solution for this? Regards, proxygeek
From: Thomas 'PointedEars' Lahn on 16 Jun 2010 04:13 proxygeek wrote: > I could not find any worthwhile discussion on implementing a generic > swap(a,b) function in Javascript. Because there isn't one. > Problem: > I need a method swap(a,b) which I can call with two variables as > params and it would swap the values of the two vars. No, you don't. > <snip> > .... > .... > function swap(a,b){ // This would NOT work as > intended Original code should be posted so that it is readable and executable when wrapped to the customary 72 to 80 characters per line. Always use multi- line pre-comments, not single-line post-comments, for documentation comments. > t = a; You "forgot" to declare `t', so it leaks to the outer scope. > a = b; > b = t; > } > .... > .... > var x = 10, y = 20; > swap(x,y); > alert("x = " + x + ", y = " + y); // should give x = window.alert(…); > 20, y = 10 See above > .... > </snip> > > Above implementation would not work as here swap() does not know a > thing about x or y. Congratulations, you have discovered call-by-value. > What would be the best solution for this? Do not use a function/method. JavaScript 1.7+: /* Destructuring assignment */ [y, x] = [x, y]; Other versions/implementations: as in swap() See also <http://PointedEars.de/es-matrix> PointedEars -- Danny Goodman's books are out of date and teach practices that are positively harmful for cross-browser scripting. -- Richard Cornford, cljs, <cife6q$253$1$8300dec7(a)news.demon.co.uk> (2004)
From: VK on 16 Jun 2010 04:39 On Jun 16, 12:13Â pm, Thomas 'PointedEars' Lahn <PointedE...(a)web.de> wrote: > JavaScript 1.7+: > > Â /* Destructuring assignment */ > Â [y, x] = [x, y]; Not "JavaScript 1.7+" but Mozilla JavaScript 1.7+ thus Gecko platforms thus 10%-20% of average visitors. IMO way below the level of an acceptably universal solution. > Other versions/implementations: as in swap() ? Do you imply that other platforms do implement call-by-reference for primitives? It is not true. To OP: to my surprise this question seems never was answered properly in this newsgroup. The only relevant discussion I found was from 2000: http://groups.google.com/group/comp.lang.javascript/browse_frm/thread/b72a4c3346057c66/e80dfd68d712c73c and it doesn't contain any valuablÑ answer. JavaScript as currently defined by ECMA-262 3rd ed. by design doesn't allow primitives' swap in a separate function, because the function gets such arguments by values, not by references. If your code requires frequent primitive swaps, you may define an intermediary var at the beginning so do not create it over and over again and then: var swap = null; // ... var x = 10, y = 20; // ... swap = x, x = y, y = swap; window.alert("x = " + x + ", y = " + y);
From: Thomas 'PointedEars' Lahn on 16 Jun 2010 04:50 VK wrote: > Thomas 'PointedEars' Lahn wrote: >> JavaScript 1.7+: >> >> /* Destructuring assignment */ >> [y, x] = [x, y]; > > Not "JavaScript 1.7+" but Mozilla JavaScript 1.7+ "Mozilla" is superfluous when talking about "JavaScript". I have explicitly pointed out the other implementations, so there can not be ambiguity. Unless, of course, one *wants* to misunderstand in order to have a case for bickering and fairytales. > thus Gecko platforms JavaScript is not limited to "Gecko platforms". > thus 10%-20% of average visitors. That depends on the country to begin with. > IMO way below the level of an acceptably universal solution. Your opinion is worthless as you don't know what you are talking about. >> Other versions/implementations: as in swap() > > ? Do you imply that other platforms do implement call-by-reference for > primitives? No. > It is not true. You are jumping to conclusions. > To OP: to my surprise this question seems never was answered properly > in this newsgroup. The only relevant discussion I found was from > 2000: OP: Do not listen to, and do not feed the troll. PointedEars -- Anyone who slaps a 'this page is best viewed with Browser X' label on a Web page appears to be yearning for the bad old days, before the Web, when you had very little chance of reading a document written on another computer, another word processor, or another network. -- Tim Berners-Lee
From: VK on 16 Jun 2010 05:27
On Jun 16, 12:50 pm, Thomas 'PointedEars' Lahn <PointedE...(a)web.de> wrote: > "Mozilla" is superfluous when talking about "JavaScript". Do you have links to documents proving Mozilla's exclusive rights to JavaScript standardization or/and Mozilla's extensions "standardization priority" ? > > thus 10%-20% of average visitors. > > IMO way below the level of an acceptably universal solution. > > Your opinion is worthless as you don't know what you are talking about. I am talking about the destructuring assignment JavaScript extension and its support across used browsers. What are you talking about remains a mystery to the side readers. > > ? Do you imply that other platforms do implement call-by-reference for > > primitives? > > No. That's good to know. > OP: Do not listen to, and do not feed the troll. You worthless trolling is annoying. Make your *working* version of swap() for an ECMA-262 3rd ed. compliant platforms other than newest Fx or just stop the tread pollution. |