Test loops at the top or bottom? (while vs. do while)

Question

When I was taking CS in college (mid 80's), one of the ideas that was constantly repeated was to always write loops which test at the top (while...) rather than at the bottom (do ... while) of the loop. These notions were often backed up with references to studies which showed that loops which tested at the top were statistically much more likely to be correct than their bottom-testing counterparts.

As a result, I almost always write loops which test at the top. I don't do it if it introduces extra complexity in the code, but that case seems rare. I notice that some programmers tend to almost exclusively write loops that test at the bottom. When I see constructs like:

if (condition)
{
    do
    {
       ...
    } while (same condition);
}

or the inverse (if inside the while), it makes me wonder if they actually wrote it that way or if they added the if statement when they realized the loop didn't handle the null case.

I've done some googling, but haven't been able to find any literature on this subject. How do you guys (and gals) write your loops?

Answer 1

I write mine pretty much exclusively testing at the top. It's less code, so for me at least, it's less potential to screw something up (e.g., copy-pasting the condition makes two places you always have to update it)

Answer 2

I always follow the rule that if it should run zero or more times, test at the beginning, if it must run once or more, test at the end. I do not see any logical reason to use the code you listed in your example. It only adds complexity.

Answer 3

For the sake of readability it seems sensible to test at the top. The fact it is a loop is important; the person reading the code should be aware of the loop conditions before trying to comprehend the body of the loop.

Answer 4

It's actually meant for a different things. In C, you can use do - while construct to achieve both scenario (runs at least once and runs while true). But PASCAL has repeat - until and while for each scenario, and if I remember correctly, ADA has another construct that lets you quit in the middle, but of course that's not what you're asking. My answer to your question : I like my loop with testing on top.

Answer 5

The use cases are different for the two. This isn't a "best practices" question.

If you want a loop to execute based on the condition exclusively than use for or while

If you want to do something once regardless of the the condition and then continue doing it based the condition evaluation. do..while

Answer 6

It really depends there are situations when you want to test at the top, others when you want to test at the bottom, and still others when you want to test in the middle.

However the example given seems absurd. If you are going to test at the top, don't use an if statement and test at the bottom, just use a while statement, that's what it is made for.

Answer 7

You should first think of the test as part of the loop code. If the test logically belongs at the start of the loop processing, then it's a top-of-the-loop test. If the test logically belongs at the end of the loop (i.e. it decides if the loop should continue to run), then it's probably a bottom-of-the-loop test.

You will have to do something fancy if the test logically belongs in them middle. :-)

Answer 8

For anyone who can't think of a reason to have a one-or-more times loop:

try {
    someOperation();
} catch (Exception e) {
    do {
        if (e instanceof ExceptionIHandleInAWierdWay) {
            HandleWierdException((ExceptionIHandleInAWierdWay)e);
        }
    } while ((e = e.getInnerException())!= null);
}

The same could be used for any sort of hierarchical structure.

in class Node:

public Node findSelfOrParentWithText(string text) {
    Node node = this;
    do {
        if(node.containsText(text)) {
            break;
        }
    } while((node = node.getParent()) != null);
    return node;
}

Answer 9

Difference is that the do loop executes "do something" once and then checks the condition to see if it should repeat the "do something" while the while loop checks the condition before doing anything

Answer 10

First one may not execute at all if condition is false. Other one will execute at least once, then check the conidition.

Answer 11

The first tests the condition before performing so it's possible your code won't ever enter the code underneath. The second will perform the code within before testing the condition.

Answer 12

The while loop will check "condition" first; if it's false, it will never "do something." But the do...while loop will "do something" first, then check "condition".

Answer 13

A while() checks the condition before each execution of the loop body and a do...while() checks the condition after each execution of the loop body.

Thus, do...while()s will always execute the loop body at least once.

Functionally, a while() is equivalent to

startOfLoop:
    if (!condition)
        goto endOfLoop;

    //loop body goes here

    goto startOfLoop;
endOfLoop:

and a do...while() is equivalent to

startOfLoop:

    //loop body

    //goes here
    if (condition)
        goto startOfLoop;

Note that the implementation is probably more efficient than this. However, a do...while() does involve one less comparison than a while() so it is slightly faster. Use a do...while() if:

• you know that the condition will always be true the first time around, or
• you want the loop to execute once even if the condition is false to begin with.

Answer 14

In a typical Discrete Structures class in computer science, it's an easy proof that there is an equivalence mapping between the two.

Stylistically, I prefer while (easy-expr) { } when easy-expr is known up front and ready to go, and the loop doesn't have a lot of repeated overhead/initialization. I prefer do { } while (somewhat-less-easy-expr); when there is more repeated overhead and the condition may not be quite so simple to set up ahead of time. If I write an infinite loop, I always use while (true) { }. I can't explain why, but I just don't like writing for (;;) { }.

Answer 15

Here is the translation:

do { y; } while(x);

Same as

{ y; } while(x) { y; }

Note the extra set of braces are for the case you have variable definitions in y. The scope of those must be kept local like in the do-loop case. So, a do-while loop just executes its body at least once. Apart from that, the two loops are identical. So if we apply this rule to your code

do {
    // do something
} while (condition is true);

The corresponding while loop for your do-loop looks like

{
    // do something
}
while (condition is true) {
    // do something
}

Yes, you see the corresponding while for your do loop differs from your while :)

Answer 16

Both conventions are correct if you know how to write the code correctly :)

Usually the use of second convention ( do {} while() ) is meant to avoid have a duplicated statement outside the loop. Consider the following (over simplified) example:

a++;
while (a < n) {
  a++;
}

can be written more concisely using

do {
  a++;
} while (a < n)

Of course, this particular example can be written in an even more concise way as (assuming C syntax)

while (++a < n) {}

But I think you can see the point here.

Answer 17

I guess some people test at the bottom because you could save one or a few machine cycles by doing that 30 years ago.

Answer 18

To write code that is correct, one basically needs to perform a mental, perhaps informal proof of correctness.

To prove a loop correct, the standard way is to choose a loop invariant, and an induction proof. But skip the complicated words: what you do, informally, is figure out something that is true of each iteration of the loop, and that when the loop is done, what you wanted accomplished is now true. The loop invariant is false at the end, for the loop to terminate.

If the loop conditions map fairly easily to the invariant, and the invariant is at the top of the loop, and one infers that the invariant is true at the next iteration of the loop by working through the code of the loop, then it is easy to figure out that the loop is correct.

However, if the invariant is at the bottom of the loop, then unless you have an assertion just prior to the loop (a good practice) then it becomes more difficult because you have to essentially infer what that invariant should be, and that any code that ran before the loop makes the loop invariant true (since there is no loop precondition, code will execute in the loop). It just becomes that more difficult to prove correct, even if it is an informal in-your-head proof.

Answer 19

This isn't really an answer but a reiteration of something one of my lecturers said and it interested me at the time.

The two types of loop while..do and do..while are actually instances of a third more generic loop, which has the test somewhere in the middle.

begin loop
  <Code block A>
  loop condition
  <Code block B>
end loop

Code block A is executed at least once and B is executed zero or more times, but isn't run on the very last (failing) iteration. a while loop is when code block a is empty and a do..while is when code block b is empty. But if you're writing a compiler, you might be interested in generalizing both cases to a loop like this.

Answer 20

I would say it is bad practice to write if..do..while loops, for the simple reason that this increases the size of the code and causes code duplications. Code duplications are error prone and should be avoided, as any change to one part must be performed on the duplicate as well, which isn't always the case. Also, bigger code means a harder time on the cpu cache. Finally, it handles null cases, and solves head aches.

Only when the first loop is fundamentally different should one use do..while, say, if the code that makes you pass the loop condition (like initialization) is performed in the loop. Otherwise, if it certain that loop will never fall on the first iteration, then yes, a do..while is appropriate.

Answer 21

From my limited knowledge of code generation I think it may be a good idea to write bottom test loops since they enable the compiler to perform loop optimizations better. For bottom test loops it is guaranteed that the loop executes at least once. This means loop invariant code "dominates" the exit node. And thus can be safely moved just before the loop starts.

Answer 22

Use while loops when you want to test a condition before the first iteration of the loop.

Use do-while loops when you want to test a condition after running the first iteration of the loop.

For example, if you find yourself doing something like either of these snippets:

func();
while (condition) {
   func();
}

//or:

while (true){
    func();
    if (!condition) break;
}

You should rewrite it as:

do{
    func();
} while(condition);

Answer 23

Here's a good real-world example I came across recently. Suppose you have a number of processing tasks (like processing elements in an array) and you wish to split the work between one thread per CPU core present. There must be at least one core to be running the current code! So you can use a do... while something like:

do {
    get_tasks_for_core();
    launch_thread();
} while (cores_remaining());

It's almost negligable, but it might be worth considering the performance benefit: it could equally be written as a standard while loop, but that would always make an unnecessary initial comparison that would always evaluate true - and on single-core, the do-while condition branches more predictably (always false, versus alternating true/false for a standard while).

Answer 24

As noted by Piemasons, the difference is whether the loop executes once before doing the test, or if the test is done first so that the body of the loop might never execute.

The key question is which makes sense for your application.

To take two simple examples:

1. Say you're looping through the elements of an array. If the array has no elements, you don't want to process number one of zero. So you should use WHILE.

2. You want to display a message, accept a response, and if the response is invalid, ask again until you get a valid response. So you always want to ask once. You can't test if the response is valid until you get a response, so you have to go through the body of the loop once before you can test the condition. You should use DO/WHILE.

Answer 25

Does avoiding do/while really help make my code more readable?

No.

If it makes more sense to use a do/while loop, then do so. If you need to execute the body of a loop once before testing the condition, then a do/while loop is probably the most straightforward implementation.

Answer 26

Yes, just like using for instead of while, or foreach instead of for improves readability. That said some circumstances need do while and I agree you would be silly to force those situations into a while loop.

Answer 27

It's more helpful to think in terms of common usage. The vast majority of while loops work quite naturally with while, even if they could be made to work with do...while, so basically you should use it when the difference doesn't matter. I would thus use do...while for the rare scenarios where it provides a noticeable improvement in readability.

Answer 28

I tend to prefer do-while loops, myself. If the condition will always be true at the start of the loop, I prefer to test it at the end. To my eye, the whole point of testing conditions (other than assertions) is that one doesn't know the result of the test. If I see a while loop with the condition test at the top, my inclination is to consider the case that the loop executes zero times. If that can never happen, why not code in a way that clearly shows that?

Answer 29

Generally, it depends on how you structure your code. And as someone already answered, some algorithms need at least one iteration performed. So, to escape additional iteration-count or at-least-one-interation-occured flag - you employ do/while.