I recently asked a question over on code review, where I asked for someone to help optimise my code. I was told over there that using a while (true) was not useful, and I shouldn't do it. I then came over to look it through and I founf this exact question, about when to use one, and then answer said, you usually see one in games.
My question is when should I use one? And if I shouldn't use it now, what should I use instead?
Sometimes you need to make a decision to exit the loop in the middle of loop's body. Java offers loop control in two places - at the beginning of each iteration (while and for) and at the end of each iteration (do-while). You use while (true) with a break in these situations:
while (true) {
... // Do something
if (exitCondition) {
break;
}
... // Do something else
}
Is it ok to do this to simplify a do-while loop? or should for only be used for incrementing/decrementing?
String res;
for(res=null;res==null;res=op.getOrderID());
instead of:
String res = null;
do {
res = op.getRetOrderID()
} while (res == null);
I think it's better to stick with the do-while loop. This is the standard idiom for the situation when you want the body of the loop to be executed at least once.
One good reason for sometimes using a for loop in preference to a while or do-while loop is that you can make the variable checked in the termination condition local to the loop. You definitely don't want that here as you need it afterwards.
Simple Advice :
use for loop when you know the number of repetitions .
use while loop when you do not know the number of repetitions .
use do-while loop when you want to run your code one time at least and do not know the number of repetitions .
Good Luck .
This question already has answers here:
Why use a "do while" loop? [closed]
(9 answers)
Closed 1 year ago.
I know what do does, and how it cooperates with the while loop, but won't a while loop code be the same, whether or not the do is there?
Consider the following:
while(condition){
myFunction();
}
and
do{
myFunction();
}while(condition);
The second form executes myFunction() at least once then checks the condition! To do so with a while loop you've to write:
myFunction();
while(condition){
myFunction();
}
The biggest difference between while and do-while statement is, whether it is executed at least one time or not.
while(false)
printf("print!");
with this statement, the program will never print the string. However,
do{
printf("print!");
}while(false);
with this statement, the program will print the string once.
But many people don't recommend to use do-while statement--because it can be substituted with while statement. It doesn't mean that do-while statement is critically harmful, but it is rarely used.
In fact, for loop is the most safe and recommended way because the programmer can handle the iteration number, loop conditions, and increasing variables easily. So if you don't have any specific reason that you have to use while loop, just use for loop.
Use do-while() construct when you have to get your task executed at least once even if condition fails.
Use while() when you your task to be executed only on certain condition success.
The difference is with "do-while" loop will be executed at least one time. With "while" loop with false condition the loop body will not be executed.
if you write the "loop" like so (without the do as in your question):
int i=0;
{
System.out.println(i++);
}while(i<10);
it will just print out 0 (nothing more), and not loop 10 times.. so no, the loop won't be the same if the do isnt there.
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I was wondering if it is a "bad practice" to use a break statement to exit a loop instead of fulfilling the loop condition?
I do not have enough insight in Java and the JVM to know how a loop is handled, so I was wondering if I was overlooking something critical by doing so.
The focus of this question: is there a specific performance overhead?
Good lord no. Sometimes there is a possibility that something can occur in the loop that satisfies the overall requirement, without satisfying the logical loop condition. In that case, break is used, to stop you cycling around a loop pointlessly.
Example
String item;
for(int x = 0; x < 10; x++)
{
// Linear search.
if(array[x].equals("Item I am looking for"))
{
//you've found the item. Let's stop.
item = array[x];
break;
}
}
What makes more sense in this example. Continue looping to 10 every time, even after you've found it, or loop until you find the item and stop? Or to put it into real world terms; when you find your keys, do you keep looking?
Edit in response to comment
Why not set x to 11 to break the loop? It's pointless. We've got break! Unless your code is making the assumption that x is definitely larger than 10 later on (and it probably shouldn't be) then you're fine just using break.
Edit for the sake of completeness
There are definitely other ways to simulate break. For example, adding extra logic to your termination condition in your loop. Saying that it is either loop pointlessly or use break isn't fair. As pointed out, a while loop can often achieve similar functionality. For example, following the above example..
while(x < 10 && item == null)
{
if(array[x].equals("Item I am looking for"))
{
item = array[x];
}
x++;
}
Using break simply means you can achieve this functionality with a for loop. It also means you don't have to keep adding in conditions into your termination logic, whenever you want the loop to behave differently. For example.
for(int x = 0; x < 10; x++)
{
if(array[x].equals("Something that will make me want to cancel"))
{
break;
}
else if(array[x].equals("Something else that will make me want to cancel"))
{
break;
}
else if(array[x].equals("This is what I want"))
{
item = array[x];
}
}
Rather than a while loop with a termination condition that looks like this:
while(x < 10 && !array[x].equals("Something that will make me want to cancel") &&
!array[x].equals("Something else that will make me want to cancel"))
Using break, just as practically any other language feature, can be a bad practice, within a specific context, where you are clearly misusing it. But some very important idioms cannot be coded without it, or at least would result in far less readable code. In those cases, break is the way to go.
In other words, don't listen to any blanket, unqualified advice—about break or anything else. It is not once that I've seen code totally emaciated just to literally enforce a "good practice".
Regarding your concern about performance overhead, there is absolutely none. At the bytecode level there are no explicit loop constructs anyway: all flow control is implemented in terms of conditional jumps.
The JLS specifies a break is an abnormal termination of a loop. However, just because it is considered abnormal does not mean that it is not used in many different code examples, projects, products, space shuttles, etc. The JVM specification does not state either an existence or absence of a performance loss, though it is clear code execution will continue after the loop.
However, code readability can suffer with odd breaks. If you're sticking a break in a complex if statement surrounded by side effects and odd cleanup code, with possibly a multilevel break with a label(or worse, with a strange set of exit conditions one after the other), it's not going to be easy to read for anyone.
If you want to break your loop by forcing the iteration variable to be outside the iteration range, or by otherwise introducing a not-necessarily-direct way of exiting, it's less readable than break.
However, looping extra times in an empty manner is almost always bad practice as it takes extra iterations and may be unclear.
In my opinion a For loop should be used when a fixed amount of iterations will be done and they won't be stopped before every iteration has been completed. In the other case where you want to quit earlier I prefer to use a While loop. Even if you read those two little words it seems more logical. Some examples:
for (int i=0;i<10;i++) {
System.out.println(i);
}
When I read this code quickly I will know for sure it will print out 10 lines and then go on.
for (int i=0;i<10;i++) {
if (someCondition) break;
System.out.println(i);
}
This one is already less clear to me. Why would you first state you will take 10 iterations, but then inside the loop add some extra conditions to stop sooner?
I prefer the previous example written in this way (even when it's a little more verbose, but that's only with 1 line more):
int i=0;
while (i<10 && !someCondition) {
System.out.println(i);
i++;
}
Everyone who will read this code will see immediatly that there is an extra condition that might terminate the loop earlier.
Ofcourse in very small loops you can always discuss that every programmer will notice the break statement. But I can tell from my own experience that in larger loops those breaks can be overseen. (And that brings us to another topic to start splitting up code in smaller chunks)
Using break in loops can be perfectly legitimate and it can even be the only way to solve some problems.
However, it's bad reputation comes from the fact that new programmers usually abuse it, leading to confusing code, especially by using break to stop the loop in conditions that could have been written in the loop condition statement in the first place.
No, it is not a bad practice to break out of a loop when if certain desired condition is reached(like a match is found). Many times, you may want to stop iterations because you have already achieved what you want, and there is no point iterating further. But, be careful to make sure you are not accidentally missing something or breaking out when not required.
This can also add to performance improvement if you break the loop, instead of iterating over thousands of records even if the purpose of the loop is complete(i.e. may be to match required record is already done).
Example :
for (int j = 0; j < type.size(); j++) {
if (condition) {
// do stuff after which you want
break; // stop further iteration
}
}
It isn't bad practice, but it can make code less readable. One useful refactoring to work around this is to move the loop to a separate method, and then use a return statement instead of a break, for example this (example lifted from #Chris's answer):
String item;
for(int x = 0; x < 10; x++)
{
// Linear search.
if(array[x].equals("Item I am looking for"))
{
//you've found the item. Let's stop.
item = array[x];
break;
}
}
can be refactored (using extract method) to this:
public String searchForItem(String itemIamLookingFor)
{
for(int x = 0; x < 10; x++)
{
if(array[x].equals(itemIamLookingFor))
{
return array[x];
}
}
}
Which when called from the surrounding code can prove to be more readable.
There are a number of common situations for which break is the most natural way to express the algorithm. They are called "loop-and-a-half" constructs; the paradigm example is
while (true) {
item = stream.next();
if (item == EOF)
break;
process(item);
}
If you can't use break for this you have to repeat yourself instead:
item = stream.next();
while (item != EOF) {
process(item);
item = stream.next();
}
It is generally agreed that this is worse.
Similarly, for continue, there is a common pattern that looks like this:
for (item in list) {
if (ignore_p(item))
continue;
if (trivial_p(item)) {
process_trivial(item);
continue;
}
process_complicated(item);
}
This is often more readable than the alternative with chained else if, particularly when process_complicated is more than just one function call.
Further reading: Loop Exits and Structured Programming:
Reopening the Debate
If you start to do something like this, then I would say it starts to get a bit strange and you're better off moving it to a seperate method that returns a result upon the matchedCondition.
boolean matched = false;
for(int i = 0; i < 10; i++) {
for(int j = 0; j < 10; j++) {
if(matchedCondition) {
matched = true;
break;
}
}
if(matched) {
break;
}
}
To elaborate on how to clean up the above code, you can refactor, moving the code to a function that returns instead of using breaks. This is in general, better dealing with complex/messy breaks.
public boolean matches()
for(int i = 0; i < 10; i++) {
for(int j = 0; j < 10; j++) {
if(matchedCondition) {
return true;
}
}
}
return false;
}
However for something simple like my below example. By all means use break!
for(int i = 0; i < 10; i++) {
if(wereDoneHere()) { // we're done, break.
break;
}
}
And changing the conditions, in the above case i, and j's value, you would just make the code really hard to read. Also there could be a case where the upper limits (10 in the example) are variables so then it would be even harder to guess what value to set it to in order to exit the loop. You could of course just set i and j to Integer.MAX_VALUE, but I think you can see this starts to get messy very quickly. :)
No, it is not a bad practice. It is the most easiest and efficient way.
While its not bad practice to use break and there are many excellent uses for it, it should not be all you rely upon. Almost any use of a break can be written into the loop condition. Code is far more readable when real conditions are used, but in the case of a long-running or infinite loop, breaks make perfect sense. They also make sense when searching for data, as shown above.
If you know in advance where the loop will have to stop, it will probably improve code readability to state the condition in the for, while, or `do-while loop.
Otherwise, that's the exact use case for break.
break and continue breaks the readability for the reader, although it's often useful.
Not as much as "goto" concept, but almost.
Besides, if you take some new languages like Scala (inspired by Java and functional programming languages like Ocaml), you will notice that break and continue simply disappeared.
Especially in functional programming, this style of code is avoided:
Why scala doesn't support break and continue?
To sum up: break and continueare widely used in Java for an imperative style, but for any coders that used to practice functional programming, it might be.. weird.
The following is part of the code for my college assignment.
else if (!codeList.contains(userCode)) {
i--; // i is the counter for the for-loop
}
else if (userQuantity[i]==0) {
i--;
}
The first part makes sure that if the user enters the wrong code, the counter i does not increment 1, or rather, it subtracts 1 from the recently incremented counter. This part works fine.
The second part however is what I seem to be having problems with. userQuantity[] is an int array and it has to be an array. This does not seem to do anything to the code. Even if 0 is entered for the quantity, it still incrememnts the counter which is not desireable.
I should explain, to avoid confusion, that this is an infinite for-loop (with a break statement). The reason I am doing a for-loop is because I am required to. Is it because of my for-loop that the condition isn't working or am I doing something completely wrong with it?
This is for my college assignment so I would appreciate answers with explanation and not just quick-fixes. If you need me to explain, let me know please.
Although it's not strictly illegal in Java, it's not a good idea to change the value of the for loop control variable from within the loop. (Such modification is illegal in some other languages.)
By changing the loop iteration variable within the loop, you're messing with the implicit assumptions offered by your use of a for loop. For example, if a reader sees:
for (int i = 0; i < 10; i++) {
// ...
}
the reader will rightfully assume that the loop is intended to execute exactly 10 times (or, no more than 10 if there is a break in there). However, if you go changing the value of i within the loop, this assumption is no longer valid.
If you must change the value of the counter, I would suggest writing this as a while loop instead:
int i = 0;
while (i < 10) {
// ...
i++;
}
along with a comment that explains why you are changing i within the loop and what it means to do so.
This is very bad practice. Change the for loop to a while loop and only increment if
codeList.contains(userCode)==true or userQuantity[i]!=0.
I should explain, to avoid confusion, that this is an infinite for-loop (with a break statement). The reason I am doing a for-loop is because I am required to. Is it because of my for-loop that the condition isn't working or am I doing something completely wrong with it?
I have a feeling that you are misunderstanding the requirements (e.g. you are not required to use a for loop), or that there is a mistake in your thinking; i.e. there is a simpler solution that doesn't involve the counter going backwards.
(It is surprising that a programming exercise would require you to write code that most experienced Java programmers would agree is bad code. The simple explanation is that it is not.)
Either way:
Changing the loop variable in a for loop is bad practice, for the reasons described by Greg.
The idea of an "infinite for loop" is really strange. The following is legal Java ...
for (int i = 0; true; i++) {
...
}
but the idiomatic way to write it is:
int i = 0;
while (true) {
...
i++; // ... at the appropriate point / points
}
... which in most cases means that you don't need to make the variable go backwards at all.