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Java Forum / General / December 2006Opinion poll: for loop vs while loop with Iterators.
Java 1.5 finaly gave us an elegant for each construct, but for-each lacks the ability to manipulate the underlying Iterable structure. Generally, the way to do this is (in pseudo-code:) Obtain the iterator. L: Check if it has a next element get the next element process the element. repeat from L This can be coded in Java a few ways. // For method: for (Iterator<E> iterator = iterable.iterator(); iterator.hasNext(); ) { E e = iterator.next(); if (shouldRemove(e)) { iterator.remove(e); } } // vs // While method: Iterator<E> iterator = iterable.iterator(); while (iterator.hasNext()) { E e = iterator.next(); if (shouldRemove(e)) { iterator.remove(e); } } Both approaches have their pros and cons, but I'm interested to see what people think. I'll post my opinion later. > Java 1.5 finaly gave us an elegant for each construct, but for-each > lacks the ability to manipulate the underlying Iterable structure. [quoted text clipped - 31 lines] > > I'll post my opinion later. I tend to use the while pattern for no particularly good reason except that the increment expression is not used in the for pattern. I guess it offends my sense of esthetics, which is reason enough to tip the balance, all else being equal. I might change my mind if someone can demonstrate a big advantage one way or the other. > Java 1.5 finaly gave us an elegant for each construct, but for-each > lacks the ability to manipulate the underlying Iterable structure. [quoted text clipped - 31 lines] > > I'll post my opinion later. The for loop is one line shorter but I've always preferred the while loop. It justs seems more natural and more readable to me. When I was at school, programming in BASIC and Pascal with less flexible for loops, for loops were for a fixed number of iterations known at the start, and while loops for an indeterminate number of iterations. With an iterator, you don't know how many iterations are required until you've finished (you can find out, but it's more work). So reading code that says "while there are more elements do x" just seems more correct. Dan.  SignatureDaniel Dyer http://www.uncommons.org > Java 1.5 finaly gave us an elegant for each construct, but for-each > lacks the ability to manipulate the underlying Iterable structure. [quoted text clipped - 31 lines] > > I'll post my opinion later. In "Effective Java" by Josh Bloch (p. 142 for all those who wish to check this), Josh argues that we should prefer using for loops as opposed to while loops. His reasoning is that the loop structure allows us an opportunity to limit the scope of variables. The for loop enables us to declare loop variables and thus limit their scope to the exact region needed. His caveat is if you happen to need the variable after the execution of the loop, you probably don't want to make it a loop variable. > > Java 1.5 finaly gave us an elegant for each construct, but for-each > > lacks the ability to manipulate the underlying Iterable structure. [quoted text clipped - 40 lines] > the execution of the loop, you probably don't want to make it a loop > variable. My personal preference is the while loop. I agree that limiting the scope of temporary variables is useful, however, there are clearer ways to accomplish this. My personal favorite is to introduce a method that does the work of the loop. That has the added benefit of explaining what the loop does, by giving it a name. Alternatively, to limit the scope other ways you can do stuff like: public static void main(String...args) { { final int myInt = 0; System.out.println(myInt); } { final int myInt = 2; System.out.println(myInt); } } > My personal preference is the while loop. I much prefer the for loop, except when there is no loop variable. I notice that the sun rises and sets irrespective of my preference, though. - Lew > > My personal preference is the while loop. > I much prefer the for loop, except when there is no loop variable. My opinion on the for statement: It is useful when there are 2 simple statements (initializer and post-loop), one conditional expression, and 1 more statement (compound or otherwise) The iterator concept does NOT fall into that category, unless you write it like this: E e; for (Iterator<E> i = getIterator(); i.hasNext(); doSomething(e)) { e = i.next(); } Which IMHO is ugly. Although, like you said > I notice that the sun rises and sets irrespective of my preference, though. Happens to be true of me too. > The iterator concept does NOT fall into that category, unless you write > it like this: [quoted text clipped - 4 lines] > > Which IMHO is ugly. How's this then? for (Iterator<E> i = getIterator(); i.hasNext(); ){ doSomething(i.next()); } > My opinion on the for statement: > It is useful when there are 2 simple statements (initializer and > post-loop), one conditional expression, and 1 more statement (compound > or otherwise) That use case lies far over toward the "for" end of the seesaw. > The iterator concept does NOT fall into that category, unless you write > it like this: [quoted text clipped - 4 lines] > > Which IMHO is ugly. That use case lives nearer the middle. I happen to like that idiom, except that I declare "E e" inside the loop and put the doSomething() there, too, but I understand perfectly well why it seems ugly. I rarely will put a workhorse expression inside the for () setup; they belong in the body. My version would be (assuming a collection as the progenitor of the iterator): for ( Iterator<E> iter = collection.getIterator(); iter.hasNext(); ) { E entry = iterator.next(); doSomething( entry ); // or just doSomething( iterator.next() ) } In most cases, that becomes for( E entry : collection ) { doSomething( entry ); } anyway, and the ugliness vanishes very far. >> I much prefer the for loop, except when there is no loop variable. I lied. I use for() {}, while () {} and do {} while ();. I prefer all three. I like "for ( ;; )" to set up unbounded loops. - Lew > I prefer all three. I like "for ( ;; )" to set up unbounded loops. Yuck! I have always used (and usually see everyone else use) "while (true)"... >> I prefer all three. I like "for ( ;; )" to set up unbounded loops. > Yuck! I have always used (and usually see everyone else use) "while > (true)"... Yuck? Here we have a perfectly useful idiom purpose-built to handle just this situation. I wonder why that seems so hideous to you. It's nice to have choices. - Lew >>> I prefer all three. I like "for ( ;; )" to set up unbounded loops. > [quoted text clipped - 7 lines] > > - Lew The difference is that the first time I saw a "for(;;)" I had to think to work out that it was going to loop forever. It depends on understanding the effect of omitting the continuation test. The first time I saw a "while(true)" it was instantly obvious that the intent was to loop as long as true is true. Patricia > The difference is that the first time I saw a "for(;;)" I had to think > to work out that it was going to loop forever. It depends on > understanding the effect of omitting the continuation test. > > The first time I saw a "while(true)" it was instantly obvious that the > intent was to loop as long as true is true. You seem to imply that a computer language should be explicable to those who do not know that language. There is a case for that. OTOH, the for ( ;; ) idiom has been around for decades and should be familiar to anyone versed in the C/Java family of languages. OTOOH, I might switch to using while ( true ) myself for the very reason you state. OTOOOH, the for ( ;; ) construction is so ugly-cute, and ugly-clever, and ugly-elitist, that I don't think I will ever give it up entirely. - Lew >> The difference is that the first time I saw a "for(;;)" I had to think >> to work out that it was going to loop forever. It depends on [quoted text clipped - 9 lines] > decades and should be familiar to anyone versed in the C/Java family of > languages. "know the language" is not a binary state. I prefer my code to be readable by as many people as possible. I have trouble imagining someone understanding "for(;;)" but not "while(true)". I'm not so sure the other way round. Patricia bjeremy skrev: > In "Effective Java" by Josh Bloch (p. 142 for all those who wish to > check this), Josh argues that we should prefer using for loops as [quoted text clipped - 4 lines] > the execution of the loop, you probably don't want to make it a loop > variable. Seconded, FWIW. ("Sun, arise!") .ed -- www.EdmundKirwan.com - Home of The Fractal Class Composition Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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