Related
...or is it just a practice?
I'm asking this because of an argument with my professor: I lost credit for calling a function recursively on the basis that we did not cover recursion in class, and my argument is that we learned it implicitly by learning return and methods.
I'm asking here because I suspect someone has a definitive answer.
For example, what is the difference between the following two methods:
public static void a() {
return a();
}
public static void b() {
return a();
}
Other than "a continues forever" (in the actual program it is used correctly to prompt a user again when provided with invalid input), is there any fundamental difference between a and b? To an un-optimized compiler, how are they handled differently?
Ultimately it comes down to whether by learning to return a() from b that we therefor also learned to return a() from a. Did we?
To answer your specific question: No, from the standpoint of learning a language, recursion isn't a feature. If your professor really docked you marks for using a "feature" he hadn't taught yet, that was wrong.
Reading between the lines, one possibility is that by using recursion, you avoided ever using a feature that was supposed to be a learning outcome for his course. For example, maybe you didn't use iteration at all, or maybe you only used for loops instead of using both for and while. It's common that an assignment aims to test your ability to do certain things, and if you avoid doing them, your professor simply can't grant you the marks set aside for that feature. However, if that really was the cause of your lost marks, the professor should take this as a learning experience of his or her own- if demonstrating certain learning outcomes is one of the criteria for an assignment, that should be clearly explained to the students.
Having said that, I agree with most of the other comments and answers that iteration is a better choice than recursion here. There are a couple of reasons, and while other people have touched on them to some extent, I'm not sure they've fully explained the thought behind them.
Stack Overflows
The more obvious one is that you risk getting a stack overflow error. Realistically, the method you wrote is very unlikely to actually lead to one, since a user would have to give incorrect input many many times to actually trigger a stack overflow.
However, one thing to keep in mind is that not just the method itself, but other methods higher or lower in the call chain will be on the stack. Because of this, casually gobbling up available stack space is a pretty impolite thing for any method to do. Nobody wants to have to constantly worry about free stack space whenever they write code because of the risk that other code might have needlessly used a lot of it up.
This is part of a more general principle in software design called abstraction. Essentially, when you call DoThing(), all you should need to care about is that Thing is done. You shouldn't have to worry about the implementation details of how it's done. But greedy use of the stack breaks this principle, because every bit of code has to worry about how much stack it can safely assume it has left to it by code elsewhere in the call chain.
Readability
The other reason is readability. The ideal that code should aspire to is to be a human-readable document, where each line describes simply what it's doing. Take these two approaches:
private int getInput() {
int input;
do {
input = promptForInput();
} while (!inputIsValid(input))
return input;
}
versus
private int getInput() {
int input = promptForInput();
if(inputIsValid(input)) {
return input;
}
return getInput();
}
Yes, these both work, and yes they're both pretty easy to understand. But how might the two approaches be described in English? I think it'd be something like:
I will prompt for input until the input is valid, and then return it
versus
I will prompt for input, then if the input is valid I will return it, otherwise I get the input and return the result of that instead
Perhaps you can think of slightly less clunky wording for the latter, but I think you'll always find that the first one is going to be a more accurate description, conceptually, of what you are actually trying to do. This isn't to say recursion is always less readable. For situations where it shines, like tree traversal, you could do the same kind of side by side analysis between recursion and another approach and you'd almost certainly find recursion gives code which is more clearly self-describing, line by line.
In isolation, both of these are small points. It's very unlikely this would ever really lead to a stack overflow, and the gain in readability is minor. But any program is going to be a collection of many of these small decisions, so even if in isolation they don't matter much, it's important to learn the principles behind getting them right.
To answer the literal question, rather than the meta-question: recursion is a feature, in the sense that not all compilers and/or languages necessarily permit it. In practice, it is expected of all (ordinary) modern compilers - and certainly all Java compilers! - but it is not universally true.
As a contrived example of why recursion might not be supported, consider a compiler that stores the return address for a function in a static location; this might be the case, for example, for a compiler for a microprocessor that does not have a stack.
For such a compiler, when you call a function like this
a();
it is implemented as
move the address of label 1 to variable return_from_a
jump to label function_a
label 1
and the definition of a(),
function a()
{
var1 = 5;
return;
}
is implemented as
label function_a
move 5 to variable var1
jump to the address stored in variable return_from_a
Hopefully the problem when you try to call a() recursively in such a compiler is obvious; the compiler no longer knows how to return from the outer call, because the return address has been overwritten.
For the compiler I actually used (late 70s or early 80s, I think) with no support for recursion the problem was slightly more subtle than that: the return address would be stored on the stack, just like in modern compilers, but local variables weren't. (Theoretically this should mean that recursion was possible for functions with no non-static local variables, but I don't remember whether the compiler explicitly supported that or not. It may have needed implicit local variables for some reason.)
Looking forwards, I can imagine specialized scenarios - heavily parallel systems, perhaps - where not having to provide a stack for every thread could be advantageous, and where therefore recursion is only permitted if the compiler can refactor it into a loop. (Of course the primitive compilers I discuss above were not capable of complicated tasks like refactoring code.)
The teacher wants to know whether you have studied or not. Apparently you didn't solve the problem the way he taught you (the good way; iteration), and thus, considers that you didn't. I'm all for creative solutions but in this case I have to agree with your teacher for a different reason: If the user provides invalid input too many times (i.e. by keeping enter pressed), you'll have a stack overflow exception and your solution will crash. In addition, the iterative solution is more efficient and easier to maintain. I think that's the reason your teacher should have given you.
Deducting points because "we didn't cover recursion in class" is awful. If you learnt how to call function A which calls function B which calls function C which returns back to B which returns back to A which returns back to the caller, and the teacher didn't tell you explicitly that these must be different functions (which would be the case in old FORTRAN versions, for example), there is no reason that A, B and C cannot all be the same function.
On the other hand, we'd have to see the actual code to decide whether in your particular case using recursion is really the right thing to do. There are not many details, but it does sound wrong.
There are many point of views to look at regarding the specific question you asked but what I can say is that from the standpoint of learning a language, recursion isn't a feature on its own. If your professor really docked you marks for using a "feature" he hadn't taught yet, that was wrong but like I said, there are other point of views to consider here which actually make the professor being right when deducting points.
From what I can deduce from your question, using a recursive function to ask for input in case of input failure is not a good practice since every recursive functions' call gets pushed on to the stack. Since this recursion is driven by user input it is possible to have an infinite recursive function and thus resulting in a StackOverflow.
There is no difference between these 2 examples you mentioned in your question in the sense of what they do (but do differ in other ways)- In both cases, a return address and all method info is being loaded to the stack. In a recursion case, the return address is simply the line right after the method calling (of course its not exactly what you see in the code itself, but rather in the code the compiler created). In Java, C, and Python, recursion is fairly expensive compared to iteration (in general) because it requires the allocation of a new stack frame. Not to mention you can get a stack overflow exception if the input is not valid too many times.
I believe the professor deducted points since recursion is considered a subject of its own and its unlikely that someone with no programming experience would think of recursion. (Of course it doesn't mean they won't, but it's unlikely).
IMHO, I think the professor is right by deducting you the points. You could have easily taken the validation part to a different method and use it like this:
public bool foo()
{
validInput = GetInput();
while(!validInput)
{
MessageBox.Show("Wrong Input, please try again!");
validInput = GetInput();
}
return hasWon(x, y, piece);
}
If what you did can indeed be solved in that manner then what you did was a bad practice and should be avoided.
Maybe your professor hasn't taught it yet, but it sounds like you're ready to learn the advantages and disadvantages of recursion.
The main advantage of recursion is that recursive algorithms are often much easier and quicker to write.
The main disadvantage of recursion is that recursive algorithms can cause stack overflows, since each level of recursion requires an additional stack frame to be added to the stack.
For production code, where scaling can result in many more levels of recursion in production than in the programmer's unit tests, the disadvantage usually outweighs the advantage, and recursive code is often avoided when practical.
Regarding the specific question, is recursion a feature, I'm inclined to say yes, but after re-interpreting the question. There are common design choices of languages and compilers that make recursion possible, and Turing-complete languages do exist that don't allow recursion at all. In other words, recursion is an ability that is enabled by certain choices in language/compiler design.
Supporting first-class functions makes recursion possible under very minimal assumptions; see writing loops in Unlambda for an example, or this obtuse Python expression containing no self-references, loops or assignments:
>>> map((lambda x: lambda f: x(lambda g: f(lambda v: g(g)(v))))(
... lambda c: c(c))(lambda R: lambda n: 1 if n < 2 else n * R(n - 1)),
... xrange(10))
[1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880]
Languages/compilers that use late binding, or that define forward declarations, make recursion possible. For example, while Python allows the below code, that's a design choice (late binding), not a requirement for a Turing-complete system. Mutually recursive functions often depend on support for forward declarations.
factorial = lambda n: 1 if n < 2 else n * factorial(n-1)
Statically typed languages that allow recursively defined types contribute to enabling recursion. See this implementation of the Y Combinator in Go. Without recursively-defined types, it would still be possible to use recursion in Go, but I believe the Y combinator specifically would be impossible.
From what I can deduce from your question, using a recursive function to ask for input in case of input failure is not a good practice. Why?
Because every recursive functions call gets pushed on to the stack. Since this recursion is driven by user input it is possible to have an infinite recursive function and thus resulting in a StackOverflow :-p
Having a non recursive loop to do this is the way to go.
Recursion is a programming concept, a feature (like iteration), and a practice. As you can see from the link, there's a large domain of research dedicated to the subject. Perhaps we don't need to go that deep in the topic to understand these points.
Recursion as a feature
In plain terms, Java supports it implicitly, because it allows a method (which is basically a special function) to have "knowledge" of itself and of others methods composing the class it belongs to. Consider a language where this is not the case: you would be able to write the body of that method a, but you wouldn't be able to include a call to a within it. The only solution would be to use iteration to obtain the same result. In such a language, you would have to make a distinction between functions aware of their own existence (by using a specific syntax token), and those who don't! Actually, a whole group of languages do make that distinction (see the Lisp and ML families for instance). Interestingly, Perl does even allow anonymous functions (so called lambdas) to call themselves recursively (again, with a dedicated syntax).
no recursion?
For languages which don't even support the possibility of recursion, there is often another solution, in the form of the Fixed-point combinator, but it still requires the language to support functions as so called first class objects (i.e. objects which may be manipulated within the language itself).
Recursion as a practice
Having that feature available in a language doesn't necessary mean that it is idiomatic. In Java 8, lambda expressions have been included, so it might become easier to adopt a functional approach to programming. However, there are practical considerations:
the syntax is still not very recursion friendly
compilers may not be able to detect that practice and optimize it
The bottom line
Luckily (or more accurately, for ease of use), Java does let methods be aware of themselves by default, and thus support recursion, so this isn't really a practical problem, but it still remain a theoretical one, and I suppose that your teacher wanted to address it specifically. Besides, in the light of the recent evolution of the language, it might turn into something important in the future.
So, I saw someplace (I'm not sure where off the top of my head, but if I recall it I will post a link to it) that the following code was not example of recursion:
void f() {
f();
}
Now, this is a function that calls itself (albeit, infinitely). Why would that not be an example of recursion? It might not be the best example, but why would they go so far as stating that it's not recursion at all?
That's definitely an example of a recursive function, based solely on the definition of "recursive". Simply speaking, a recursive function is any one that calls itself.
It's hard to explain why someone said it wasn't without seeing their claim in context (and hopefully an attempt at a justification for that claim).
That said, it's not a very useful recursive function, and perhaps that was the point they were driving at. Any program that calls that function will eventually crash after it overflows the stack. Sometimes, this is called a "Stack Overflow" error. :-)
Useful recursive functions have to contain some kind of conditional code that causes the recursion to stop eventually.
Your code example does not have a way to terminate and would go on forever, likely causing a stack overflow exception. Recursive functions have a simple way to terminate through a base case condition. Read this article for a proper definition and explanation.
That is definitely recursion, and here are some notes form the University of Wisconsin-Madison. Actually using void f() as the example http://pages.cs.wisc.edu/~vernon/cs367/notes/6.RECURSION.html
The site could of been trying to drive home a point of how not to do recursion .. not sure without the source link.
It is example of recursion but there is no way to end it , you have to terminate it using condition.
My questions is basically all in the title. I have no real application in mind, I'm just trying to make sure I use generally accepted good coding practices, and I remember my CS professor saying that breaks are generally avoided when possible, but he never told us why.
I would ask him myself, but he's out of his office this summer.
This is a theological issue. People who are opposed to the use of break (or continue for that matter) say that it makes the control flow of the program harder to follow and thus makes the program less readable. But an easier way to make readable code is simply to make shorter functions, use meaningful variable names, comment appropriately, etc.
Sometimes professors are wrong.
He may object to skipping out of a loop right in the middle of it. This is akin to using a return statement in the middle of a function...some consider that blasphemy.
What ever is the simplest, and produces the most easily maintainable code, is what should be used.
Generally speaking, you will use 'break' to get out of cycles when a certain condition is met (for instance, when you've found what you're looking for, etc.); I don't consider it 'evil' such as the infamous GOTO statement, but reasonable people will differ.
What Dijkstra said was harmful about goto statements - http://drdobbs.com/blogs/cpp/228700940
and https://softwareengineering.stackexchange.com/questions/58237/are-break-and-continue-bad-programming-practices
Your professor was saying "breaks should be avoided" because they are similar to the now-taboo goto statement. However to say "breaks should be avoided whenever possible" is just wrong.
Break exists for readability. You can do anything without break that you can with it, but that doesn't mean you should. Sometimes it just makes sense to make a statement like "In this condition, break out of this loop." or "In this condition, return from this function."
It's a bad idea to use break for program flow in a nested for loops in a way that might give a casual reader some confusion.
Simple rule of thumb: Does using break here (instead of additional if/thens, another boolean, etc.) make this easier to understand or harder to understand? It's that simple.
Ask your professor to try writing a switch statement in any C-style language without break. It can be done I'm sure, but the resulting code is not going to be any more readable.
switch (someExpression) {
case enumVar.CaseA:
// do things here, but don't you dare break!
case enumVar.CaseB:
// do other things - possibly unrelated or contradictory to CaseA
// ... and so on
}
I am currently working in PHP, so this example will be in PHP, but the question applies to multiple languages.
I am working on this project with a fiend of mine, and as always we were held up by a big problem. Now we both went home, couldn't solve the problem. That night we both found the solution, only I used a loop to tackle the problem, and he used recursion.
Now I wanted to tell him the difference between the loop and recursion, but I couldn't come up with a solution where you need recursion over a normal loop.
I am going to make a simplified version of both, I hope someone can explain how one is different from the other.
Please forgive me for any coding errors
The loop:
printnumbers(1,10);
public function printnumbers($start,$stop)
{
for($i=$start;$i<=$stop;$i++)
{
echo $i;
}
}
Now the code above just simply prints out the numbers.
Now let's do this with recursion:
printnumbers(1,10);
public function printnumbers($start,$stop)
{
$i = $start;
if($i <= $stop)
{
echo $i;
printnumbers($start+1,$stop);
}
}
This method above will do the exact same thing as the loop, but then only with recursion.
Can anyone explain to me what there is different about using one of these methods.
Loops and recursions are in many ways equivalent. There are no programs the need one or the other, in principle you can always translate from loops to recursion or vice versa.
Recursions is more powerful in the sense that to translating recursion to a loop might need a stack that you have to manipulate yourself. (Try traversing a binary tree using a loop and you will feel the pain.)
On the other hand, many languages (and implementations), e.g., Java, don't implement tail recursion properly. Tail recursion is when the last thing you do in a function is to call yourself (like in your example). This kind of recursion does not have to consume any stack, but in many languages they do, which means you can't always use recursion.
Often, a problem is easier expressed using recursion. This is especially true when you talk about tree-like data structures (e.g. directories, decision trees...).
These data structures are finite in nature, so most of the time processing them is clearer with recursion.
When stack-depth is often limited, and every function call requires a piece of stack, and when talking about a possibly infinite data structure you will have to abandon recursion and translate it into iteration.
Especially functional languages are good at handling 'infinite' recursion. Imperative languages are focused on iteration-like loops.
In general, a recursive function will consume more stack space (since it's really a large set of function calls), while an iterative solution won't. This also means that an iterative solution, in general, will be faster because.
I am not sure if this applies to an interpreted language like PHP though, it is possible that the interpreter can handle this better.
A loop will be faster because there's always overhead in executing an extra function call.
A problem with learning about recursion is a lot of the examples given (say, factorials) are bad examples of using recursion.
Where possible, stick with a loop unless you need to do something different. A good example of using recursion is looping over each node in a Tree with multiple levels of child nodes.
Recursion is a bit slower (because function calls are slower than setting a variable), and uses more space on most languages' call stacks. If you tried to printnumbers(1, 1000000000), the recursive version would likely throw a PHP fatal error or even a 500 error.
There are some cases where recursion makes sense, like doing something to every part of a tree (getting all files in a directory and its subdirectories, or maybe messing with an XML document), but it has its price -- in speed, stack footprint, and the time spent to make sure it doesn't get stuck calling itself over and over til it crashes. If a loop makes more sense, it's definitely the way to go.
Well, I don't know about PHP but most languages generate a function call (at the machine level) for every recursion. So they have the potential to use a lot of stack space, unless the compiler produces tail-call optimizations (if your code allows it).
Loops are more 'efficient' in that sense because they don't grow the stack. Recursion has the advantage of being able to express some tasks more naturally though.
In this specific case, from a conceptual (rather than implementative) point of view, the two solutions are totally equivalent.
Compared to loops, a function call has its own overhead like allocating stack etc. And in most cases, loops are more understandable than their recursive counterparts.
Also, you will end up using more memory and can even run out of stack space if the difference between start and stop is high and there are too many instances of this code running simultaneously (which can happen as you get more traffic).
You don't really need recursion for a flat structure like that. The first code I ever used recursion in involved managing physical containers. Each container might contain stuff (a list of them, each with weights) and/or more containers, which have a weight. I needed the total weight of a container and all it held. (I was using it to predict the weight of large backpacks full of camping equipment without packing and weighing them.) This was easy to do with recursion and would have been a lot harder with loops. But many kinds of problems that naturally suit themselves to one approach can also be tackled with the other.
Stack overflow.
And no, I don't mean a website or something. I MEAN a "stack overflow".
I'm facing a problem where both recursion and using a loop seem like natural solutions. Is there a convention or "preferred method" for cases like this? (Obviously it is not quite as simple as below)
Recursion
Item Search(string desired, Scope scope) {
foreach(Item item in scope.items)
if(item.name == desired)
return item;
return scope.Parent ? Search(desired, scope.Parent) : null;
}
Loop
Item Search(string desired, Scope scope) {
for(Scope cur = scope; cur != null; cur = cur.Parent)
foreach(Item item in cur.items)
if(item.name == desired)
return item;
return null;
}
I favor recursive solutions when:
The implementation of the recursion is much simpler than the iterative solution, usually because it exploits a structural aspect of the problem in a way that the iterative approach cannot
I can be reasonably assured that the depth of the recursion will not cause a stack overflow, assuming we're talking about a language that implements recursion this way
Condition 1 doesn't seem to be the case here. The iterative solution is about the same level of complexity, so I'd stick with the iterative route.
If performance matters, then benchmark both and choose on a rational basis. If not, then choose based on complexity, with concern for possible stack overflow.
There is a guideline from the classic book The Elements of Programming Style (by Kernighan and Plauger) that algorithm should follow data structure. That is, recursive structures are often processed more clearly with recursive algorithms.
Recursion is used to express an algorithm that is naturally recursive in a form that is more easily understandable. A "naturally recursive" algorithm is one where the answer is built from the answers to smaller sub-problems which are in turn built from the answers to yet smaller sub-problems, etc. For example, computing a factorial.
In a programming language that is not functional, an iterative approach is nearly always faster and more efficient than a recursive approach, so the reason to use recursion is clarity, not speed. If a recursive implementation ends up being less clear than an iterative implementation, then by all means avoid it.
In this particular case, I would judge the iterative implementation to be clearer.
If you are using a functional language (doesn't appear to be so), go with recursion. If not, the loop will probably be better understood by anyone else working on the project. Of course, some tasks (like recursively searching a directory) are better suited to recursion than others.
Also, if the code cannot be optimized for tail end recursion, the loop is safer.
Well I saw tons of answers and even accepted answer but never saw the correct one and was thinking why...
Long story short :
Always avoid recursions if you can make same unit to be produced by loops!
How does recursion work?
• The Frame in Stack Memory is being allocated for a single function call
• The Frame contains reference to the actual method
• If method has objects, the objects are being put into Heap memory and Frame will contain reference to that objects in Heap memory.
•These steps are being done for each single method call!
Risks :
• StackOverFlow when the stack has no memory to put new recursive methods.
• OutOfMemory when the Heap has no memory to put recursive stored objects.
How Does loop work?
• All the steps before, except that the execution of repeatedly code inside the loop will not consume any further data if already consumed.
Risks :
• Single risk is inside while loop when your condition will just never exist...
Well that won't cause any crashes or anything else, it just won't quit the loop if you naively do while(true) :)
Test:
Do next in your software:
private Integer someFunction(){
return someFunction();
}
You will get StackOverFlow exception in a second and maybe OutOfMemory too
Do second:
while(true){
}
The software will just freeze and no crash will happen:
Last but not least - for loops :
Always go with for loops because this or that way this loop somewhat forces you to give the breaking point beyond which the loop won't go, surely you can be super angry and just find a way to make for loop never stop but I advice you to always use loops instead of recursion in sake of memory management and better productivity for your software which is a huge issue now days.
References:
Stack-Based memory allocation
Use the loop. It's easier to read and understand (reading code is always a lot harder than writing it), and is generally a lot faster.
It is provable that all tail-recursive algorithms can be unrolled into a loop, and vice versa. Generally speaking, a recursive implementation of a recursive algorithm is clearer to follow for the programmer than the loop implementation, and is also easier to debug. Also generally speaking, the real-world performance of the loop implementation will be faster, as a branch/jump in a loop is typically faster to execute than pushing and popping a stack frame.
Personally speaking, for tail-recursive algorithms I prefer sticking with the recursive implementation in all but the most-performance-intensive situations.
I prefer loops as
Recursion is error-prone
All the code remains into one function / method
Memory and speed savings
I use stacks (LIFO schema) to make the loops work
In java, stacks is covered with Deque interface
// Get all the writable folders under one folder
// java-like pseudocode
void searchWritableDirs(Folder rootFolder){
List<Folder> response = new List<Folder>(); // Results
Deque<Folder> folderDeque = new Deque<Folder>(); // Stack with elements to inspect
folderDeque.add(rootFolder);
while( ! folderDeque.isEmpty()){
Folder actual = folder.pop(); // Get last element
if (actual.isWritable()) response.add(actual); // Add to response
for(Folder actualSubfolder: actual.getSubFolder()) {
// Here we iterate subfolders, with this recursion is not needed
folderDeque.push(actualSubfolder);
}
}
log("Folders " + response.size());
}
Less complicated, more compact than
// Get all the writable folders under one folder
// java-like pseudocode
void searchWritableDirs(Folder rootFolder){
List<Folder> response = new List<Folder>(); // Results
rec_searchWritableDirs(actualSubFolder,response);
log("Folders " + response.size());
}
private void rec_searchWritableDirs(Folder actual,List<Folder> response) {
if (actual.isWritable()) response.add(actual); // Add to response
for(Folder actualSubfolder: actual.getSubFolder()) {
// Here we iterate subfolders, recursion is needed
rec_searchWritableDirs(actualSubFolder,response);
}
}
The latter has less code, but two functions and it's harder to understand code, IMHO.
I would say the recursion version is better understandable, but only with comments:
Item Search(string desired, Scope scope) {
// search local items
foreach(Item item in scope.items)
if(item.name == desired)
return item;
// also search parent
return scope.Parent ? Search(desired, scope.Parent) : null;
}
It is far easier to explain this version. Try to write a nice comment on the loop version and you will see.
I find the recursion more natural, but you may be forced to use the loop if your compiler doesn't do tail call optimization and your tree/list is too deep for the stack size.
If the system you're working on has a small stack (embedded systems), the recursion depth would be limited, so choosing the loop-based algorithm would be desired.
I usually prefer the use of loops. Most good OOP designs will allow you to use loops without having to use recursion (and thus stopping the program from pushing all those nasty parameters and addresses to the stack).
It has more of a use in procedural code where it seems more logical to think in a recursive manner (due to the fact that you can't easily store state or meta-data (information?) and thus you create more situations that would merit it's use).
Recursion is good for proto-typing a function and/or writing a base, but after you know the code works and you go back to it during the optimization phase, try to replace it with a loop.
Again, this is all opinionated. Go with what works best for you.
You can also write the loop in a more readable format. C's for(init;while;increment) have some readability drawbacks since the increment command is mentioned at the start but executed at the end of the loop.
Also YOUR TWO SAMPLES ARE NOT EQUIVALENT. The recursive sample will fail and the loop will not, if you call it as: Search(null,null). This makes the loop version better for me.
Here are the samples modified (and assuming null is false)
Recursion (fixed and tail-call optimizable)
Item Search(string desired, Scope scope) {
if (!scope) return null
foreach(Item item in scope.items)
if(item.name == desired)
return item;
//search parent (recursive)
return Search(desired, scope.Parent);
}
Loop
Item Search(string desired, Scope scope) {
// start
Scope cur = scope;
while(cur) {
foreach(Item item in cur.items)
if(item.name == desired)
return item;
//search parent
cur = cur.Parent;
} //loop
return null;
}
If your code is compiled it will likely make little difference. Do some testing and see how much memory is used and how fast it runs.
Avoid recursion. Chances are that piece of code will have to be maintained eventually at some point and it'll be easier if it is not done with recursion. Second, it'll most likely have a slower execution time.