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Groovy collections performance considerations regarding space/time
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Closed 6 years ago.
Which of these two ways of transversing a list is recommended:
List list = [1, 2, 3]
list.each { num ->
// Do something
}
or
List list = [1, 2, 3]
for (num in list) {
// Do something
}
?
Does any of these make any difference in performance? I think, at least in terms of readability, they are both the same.
It depends on what you are doing inside of the block of code. You might want to be taking advantage of the closure's delegate or you might have a closure you want to re-use in different contexts, which would be arguments for using each. If none of that applies, the for loop probably makes more sense and will perform a small bit better.
From the usage POV, the main difference between them is the way how you abnormally terminate the loop.
With for-each it's straight-forward:
for (num in list) {
if( someCond ) break
}
whereas with each you would have to throw an exception:
list.each{
if( someCond ) throw new Exception( "" )
}
which is not so nice.
Related
What is closure in groovy?
Why we use this closure?
Are you asking about Closure annotation parameters?
[...
An interesting feature of annotations in Groovy is that you can use a closure as an annotation value. Therefore annotations may be used with a wide variety of expressions and still have IDE support. For example, imagine a framework where you want to execute some methods based on environmental constraints like the JDK version or the OS. One could write the following code:
class Tasks {
Set result = []
void alwaysExecuted() {
result << 1
}
#OnlyIf({ jdk>=6 })
void supportedOnlyInJDK6() {
result << 'JDK 6'
}
#OnlyIf({ jdk>=7 && windows })
void requiresJDK7AndWindows() {
result << 'JDK 7 Windows'
}
}
...]
Source:http://docs.groovy-lang.org/
Closures are a powerful concept with which you can implement a variety of things and which enable specifying DSLs. They are sort of like Java ( lambdas, but more powerful and versatile. You dont need to use closures, but they can make many things easier.
Since you didnt really specify a concrete question, I'll just point you to the startegy pattern example in the groovy docs:
http://docs.groovy-lang.org/latest/html/documentation/#_strategy_pattern
Think of the closure as an executable unit on its own, like a method or function, except that you can pass it around like a variable, but can do a lot of things that you would normally do with a class, for example.
An example: You have a list of numbers and you either want to add +1 to each number, or you want to double each number, so you say
def nums = [1,2,3,4,5]
def plusone = { item ->
item + 1
}
def doubler = { item ->
item * 2
}
println nums.collect(plusone)
println nums.collect(doubler)
This will print out
[2, 3, 4, 5, 6]
[2, 4, 6, 8, 10]
So what you achieved is that you separated the function, the 'what to do' from the object that you did it on. Your closures separate an action that can be passed around and used by other methods, that are compatible with the closure's input and output.
What we did in the example is that we had a list of numbers and we passed each of them to a closure that did something with it. Either added +1 or doubled the value, and collected them into another list.
And this logic opens up a whole lot of possibilities to solve problems smarter, cleaner, and write code that represents the problem better.
What I've seen in Java
Java 8 allows lazy evaluation of chained functions in order to avoid performance penalties.
For instance, I can have a list of values and process it like this:
someList.stream()
.filter( v -> v > 0)
.map( v -> v * 4)
.filter( v -> v < 100)
.findFirst();
I pass a number of closures to the methods called on a stream to process the values in a collection and then only grab the first one.
This looks as if the code had to iterate over the entire collection, filter it, then iterate over the entire result and apply some logic, then filter the whole result again and finally grab just a single element.
In reality, the compiler handles this in a smarter way and optimizes the number of iterations required.
This is possible because no actual processing is done until findFirst is called. This way the compiler knows what I want to achieve and it can figure out how to do it in an efficient manner.
Take a look at this video of a presentation by Venkat Subramaniam for a longer explanation.
What I'd like to do in Groovy
While answering a question about Groovy here on StackOverflow I figured out a way to perform the task the OP was trying to achieve in a more readable manner. I refrained from suggesting it because it meant a performance decrease.
Here's the example:
collectionOfSomeStrings.inject([]) { list, conf -> if (conf.contains('homepage')) { list } else { list << conf.trim() } }
Semantically, this could be rewritten as
collectionOfSomeStrings.grep{ !it.contains('homepage')}.collect{ it.trim() }
I find it easier to understand but the readability comes at a price. This code requires a pass of the original collection and another iteration over the result of grep. This is less than ideal.
It doesn't look like the GDK's grep, collect and findAll methods are lazily evaluated like the methods in Java 8's streams API. Is there any way to have them behave like this? Is there any alternative library in Groovy that I could use?
I imagine it might be possible to use Java 8 somehow in Groovy and have this functionality. I'd welcome an explanation on the details but ideally, I'd like to be able to do that with older versions of Java.
I found a way to combine closures but it's not really what I want to do. I'd like to chain not only closures themselves but also the functions I pass them to.
Googling for Groovy and Streams mostly yields I/O related results. I haven't found anything of interest by searching for lazy evaluation, functional and Groovy as well.
Adding the suggestion as an answer taking cfrick's comment as an example:
#Grab( 'com.bloidonia:groovy-stream:0.8.1' )
import groovy.stream.Stream
List integers = [ -1, 1, 2, 3, 4 ]
//.first() or .last() whatever is needed
Stream.from integers filter{ it > 0 } map{ it * 4 } filter{ it < 15 }.collect()
Tim, I still know what you did few summers ago. ;-)
Groovy 2.3 supports jdk8 groovy.codehaus.org/Groovy+2.3+release+notes. your example works fine using groovy closures:
[-1,1,2,3,4].stream().filter{it>0}.map{it*4}.filter{it < 100}.findFirst().get()
If you can't use jdk8, you can follow the suggestion from the other answer or achieve "the same" using RxJava/RxGroovy:
#Grab('com.netflix.rxjava:rxjava-groovy:0.20.7')
import rx.Observable
Observable.from( [-1, 1, 2, 3, 4, 666] )
.filter { println "f1 $it"; it > 0 }
.map { println "m1 $it"; it * 4 }
.filter { println "f2 $it"; it < 100 }
.subscribe { println "result $it" }
I found myself confronted with an interview question where the goal was to write a sorting algorithm that sorts an array of unsorted int values:
int[] unsortedArray = { 9, 6, 3, 1, 5, 8, 4, 2, 7, 0 };
Now I googled and found out that there are so many sorting algorithms out there!
Finally I could motivate myself to dig into Bubble Sort because it seemed pretty simple to start with.
I read the sample code and came to a solution looking like this:
static int[] BubbleSort(ref int[] array)
{
long lastItemLocation = array.Length - 1;
int temp;
bool swapped;
do
{
swapped = false;
for (int itemLocationCounter = 0; itemLocationCounter < lastItemLocation; itemLocationCounter++)
{
if (array[itemLocationCounter] > array[itemLocationCounter + 1])
{
temp = array[itemLocationCounter];
array[itemLocationCounter] = array[itemLocationCounter + 1];
array[itemLocationCounter + 1] = temp;
swapped = true;
}
}
} while (swapped);
return array;
}
I clearly see that this is a situation where the do { //work } while(cond) statement is a great help to be and prevents the use of another helper variable.
But is this the only case that this is more useful or do you know any other application where this condition has been used?
In general:
use do...while when you want the body to be executed at least once.
use while... when you may not want the body to be executed at all.
EDIT: I'd say the first option comes up about 10% of the time and the second about 90%. You can always re-factor to use either, in either circumstance. Use the one that's closest to what you want to say.
do...while guarantees that the body of code inside the loop executes at least once. This can be handy under certain conditions; when coding a REPL loop, for example.
Anytime you need to loop through some code until a condition is met is a good example of when to use do...while or while...
A good example of when to use do...while or while... is if you have a game or simulation where the game engine is continuously running the various components until some condition occurs like you win or lose.
Of course this is only one example.
The above posts are correct regarding the two conditional looping forms. Some languages have a repeat until form instead of do while. Also there's a minimalist view where only the necessary control structures should exist in a language. The while do is necessary but the do while isn't. And as for bubble sort you'll want to avoid going there as it's the slowest of the commonly known sorting algorithms. Look at selection sort or insertion sort instead. Quicksort and merge sort are fast but are hard to write without using recursion and perform badly if you happen to chose a poor pivot value.
I have read quite a few articles on closures, and, embarassingly enough, I still don't understand this concept! Articles explain how to create a closure with a few examples, but I don't see any point in paying much attention to them, as they largely look contrived examples. I am not saying all of them are contrived, just that the ones I found looked contrived, and I dint see how even after understanding them, I will be able to use them. So in order to understand closures, I am looking at a few real problems, that can be solved very naturally using closures.
For instance, a natural way to explain recursion to a person could be to explain the computation of n!. It is very natural to understand a problem like computing the factorial of a number using recursion. Similarly, it is almost a no-brainer to find an element in an unsorted array by reading each element, and comparing with the number in question. Also, at a different level, doing Object-oriented programming also makes sense.
So I am trying to find a number of problems that could be solved with or without closures, but using closures makes thinking about them and solving them easier. Also, there are two types to closures, where each call to a closure can create a copy of the environment variables, or reference the same variables. So what sort of problems can be solved more naturally in which of the closure implementations?
Callbacks are a great example. Let's take JavaScript.
Imagine you have a news site, with headlines and short blurbs and "Read More..." buttons next to each one. When the user clicks the button, you want to asynchronously load the content corresponding to the clicked button, and present the user with an indicator next to the requested headline so that the user can "see the page working at it".
function handleClickOnReadMore(element) {
// the user clicked on one of our 17 "request more info" buttons.
// we'll put a whirly gif next to the clicked one so the user knows
// what he's waiting for...
spinner = makeSpinnerNextTo(element);
// now get the data from the server
performAJAXRequest('http://example.com/',
function(response) {
handleResponse(response);
// this is where the closure comes in
// the identity of the spinner did not
// go through the response, but the handler
// still knows it, even seconds later,
// because the closure remembers it.
stopSpinner(spinner);
}
);
}
Alright, say you're generating a menu in, say, javascript.
var menuItems = [
{ id: 1, text: 'Home' },
{ id: 2, text: 'About' },
{ id: 3, text: 'Contact' }
];
And your creating them in a loop, like this:
for(var i = 0; i < menuItems.length; i++) {
var menuItem = menuItems[i];
var a = document.createElement('a');
a.href = '#';
a.innerHTML = menuItem.text;
// assign onclick listener
myMenu.appendChild(a);
}
Now, for the onclick listener, you might attempt something like this:
a.addEventListener('click', function() {
alert( menuItem.id );
}, false);
But you'll find that this will have every link alert '3'. Because at the time of the click, your onclick code is executed, and the value of menuItem is evaluated to the last item, since that was the value it was last assigned, upon the last iteration of the for loop.
Instead, what you can do is to create a new closure for each link, using the value of menuItem as it is at that point in execution
a.addEventListener('click', (function(item) {
return function() {
alert( item.id );
}
})( menuItem ), false);
So what's going on here? We're actually creating a function that accepts item and then immediately call that function, passing menuItem. So that 'wrapper' function is not what will be executed when you click the link. We're executing it immediately, and assigning the returned function as the click handler.
What happens here is that, for each iteration, the function is called with a new value of menuItem, and a function is returned which has access to that value, which is effectively creating a new closure.
Hope that cleared things up =)
Personally I hated to write sort routines when I had a list of objects.
Usually you had the sort function and a separate function to compare the two objects.
Now you can do it in one statement
List<Person> fList = new List<Person>();
fList.Sort((a, b) => a.Age.CompareTo(b.Age));
Well, after some time, spent with Scala, I can not imagine a code, operating on some list without closures. Example:
val multiplier = (i:Int) => i * 2
val list1 = List(1, 2, 3, 4, 5) map multiplier
val divider = (i:Int) => i / 2
val list2 = List(1, 2, 3, 4, 5) map divider
val map = list1 zip list2
println(map)
The output would be
List((2,0), (4,1), (6,1), (8,2), (10,2))
I'm not sure, if it is the example, you are looking for, but I personally think, that best examples of closures's real power can be seen on lists-examples: all kinds of sortings, searchings, iterating, etc.
I personally find massively helpful presentation by Stuart Langridge on Closures in JavaScript (slides in pdf). It's full of good examples and a bit of sense of humour as well.
In C#, a function can implement the concept of filtering by proximity to a point:
IEnumerable<Point> WithinRadius(this IEnumerable<Point> points, Point c, double r)
{
return points.Where(p => (p - c).Length <= r);
}
The lambda (closure) captures the supplied parameters and binds them up into a computation that will performed at some later time.
Well, I use Closures on a daily basis in the form of a function composition operator and a curry operator, both are implemented in Scheme by closures:
Quicksort in Scheme for instance:
(define (qsort lst cmp)
(if (null? lst) lst
(let* ((pivot (car lst))
(stacks (split-by (cdr lst) (curry cmp pivot))))
(append (qsort (cadr stacks) cmp)
(cons pivot
(qsort (car stacks) cmp))))))
Where I cmp is a binary function usually applied as (cmp one two), I curried it in this case to split my stack in two by creating a unitary predicate if you like, my curry operators:
(define (curry f . args)
(lambda lst (apply f (append args lst))))
(define (curryl f . args)
(lambda lst (apply f (append lst args))))
Which respectively curry righthanded and lefthanded.
So currying is a good example of closures, another example is functional composition. Or for instance a function which takes list and makes from that a predicate that tests if its argument is a member of that list or not, that's a closure too.
Closure is one of the power strengths of JavaScript, because JavaScript is a lambda language. For more on this subject go here:
Trying to simplify some Javascript with closures
is there a way to 'break' out of a groovy closure.
maybe something like this:
[1, 2, 3].each {
println(it)
if (it == 2)
break
}
I often forget that Groovy implements an "any" method.
[1, 2, 3].any
{
println it
return (it == 2)
}
12/05/2013 Heavily Edited.
Answering the question that was asked.
Is it possible to break out of a Closure?
You would "break" out of a closure by issuing the return keyword. However that isn't helpful in the example that is given. The reason for this is that the closure (think of it as a method) is called by the each method for every item in the collection.
If you run this example you will see it will print 1 then 3.
[1, 2, 3].each {
if (it == 2) return
println(it)
}
Why break in the context of each doesn't make sense.
To understand why you cannot break out of the each method like you could break out of a for loop you need to understand a bit of what is actually happening. Here is a gross simplification what the each method on a collection does.
myEach([0,1,3])
void myEach(List things) {
for (i in things) {
myEachMethod(i)
}
}
void myEachMethod(Object it) { // this is your Closure
if(it == 2) return
println it
}
As you can see the closure is basically a method that can be passed around. Just as in java you cannot break from within method call or closure.
What to do instead of breaking from each.
In Groovy you are supposed to express your code using high level abstractions as such primitive looping is not idiomatic. For the example that you gave I would consider making use of findAll. For example:
[1,2,3].findAll { it < 2 }.each { println it }
I hope this helps you understand what is going on.
Answering the implied question.
Can you break out of the Collection.each iterations against your supplied closure?
You cannot break out of the each method without throwing and catching an exception as John Wagenleitner has said. Although I would argue that throwing and catching an exception in the name of flow control is a code smell and a fellow programmer might slap your hands.
You can throw an exception:
try {
[1, 2, 3].each {
println(it)
if (it == 2)
throw new Exception("return from closure")
}
} catch (Exception e) { }
Use could also use "findAll" or "grep" to filter out your list and then use "each".
[1, 2, 3].findAll{ it < 3 }.each{ println it }
Take a look at Best pattern for simulating continue in groovy closure for an extensive discussion.
Try to use any instead of each
def list = [1, 2, 3, 4, 5, -1, -2]
list.any { element ->
if (element > 3)
return true // break
println element
}
The result : 1, 2, 3
Just using special Closure
// declare and implement:
def eachWithBreak = { list, Closure c ->
boolean bBreak = false
list.each() { it ->
if (bBreak) return
bBreak = c(it)
}
}
def list = [1,2,3,4,5,6]
eachWithBreak list, { it ->
if (it > 3) return true // break 'eachWithBreak'
println it
return false // next it
}
There is an other solution. Although, that groovy stuff like each/find/any is quite cool: if it doesn't fit, don't use it. You can still use the plain old
for (def element : list)
Especially, if you want to leave the method, too. Now you are free to use continue/break/return as you like. The resulting code might not be cool, but it is easy and understandable.
This is in support of John Wagenleiter's answer. Tigerizzy's answer is plain wrong. It can easily be disproved practically by executing his first code sample, or theoretically by reading Groovy documentation. A return returns a value (or null without an argument) from the current iteration, but does not stop the iteration. In a closure it behaves rather like continue.
You won't be able to use inject without understanding this.
There is no way to 'break the loop' except by throwing an exception. Using exceptions for this purpose is considered smelly. So, just as Wagenleiter suggests, the best practice is to filter out the elements you want to iterate over before launching each or one of its cousins.
With rx-java you can transform an iterable in to an observable.
Then you can replace continue with a filter and break with takeWhile
Here is an example:
import rx.Observable
Observable.from(1..100000000000000000)
.filter { it % 2 != 1}
.takeWhile { it<10 }
.forEach {println it}