Is while (true) with break bad programming practic

Question

I often use this code pattern:

while(true) {

    //do something

    if(<some condition>) {
        break;
    }

}   

Another programmer told me that this was bad practice and that I should replace it with the more standard:

while(!<some condition>) {

    //do something

}   

His reasoning was that you could "forget the break" too easily and have an endless loop. I told him that in the second example you could just as easily put in a condition which never returned true and so just as easily have an endless loop, so both are equally valid practices.

Further, I often prefer the former as it makes the code easier to read when you have multiple break points, i.e. multiple conditions which get out of the loop.

Can anyone enrichen this argument by adding evidence for one side or the other?

Answer 1

It depends on what you’re trying to do, but in general I prefer putting the conditional in the while.

• It’s simpler, since you don't need another test in the code.
• It’s easier to read, since you don’t have to go hunting for a break inside the loop.
• You’re reinventing the wheel. The whole point of while is to do something as long as a test is true. Why subvert that by putting the break condition somewhere else?

I’d use a while(true) loop if I was writing a daemon or other process that should run until it gets killed.

Answer 2

To quote that noted developer of days gone by, Wordsworth:

...
In truth the prison, unto which we doom
Ourselves, no prison is; and hence for me,
In sundry moods, 'twas pastime to be bound
Within the Sonnet's scanty plot of ground;
Pleased if some souls (for such their needs must be)
Who have felt the weight of too much liberty,
Should find brief solace there, as I have found.

Wordsworth accepted the strict requirements of the sonnet as a liberating frame, rather than as a straightjacket. I'd suggest that the heart of "structured programming" is about giving up the freedom to build arbitrarily-complex flow graphs in favor of a liberating ease of understanding.

I freely agree that sometimes an early exit is the simplest way to express an action. However, my experience has been that when I force myself to use the simplest possible control structures (and really think about designing within those constraints), I most often find that the result is simpler, clearer code. The drawback with

while (true) {
    action0;
    if (test0) break;
    action1;
}

is that it's easy to let action0 and action1 become larger and larger chunks of code, or to add "just one more" test-break-action sequence, until it becomes difficult to point to a specific line and answer the question, "What conditions do I know hold at this point?" So, without making rules for other programmers, I try to avoid the while (true) {...} idiom in my own code whenever possible.

Answer 3

No, that's not bad since you may not always know the exit condition when you setup the loop or may have multiple exit conditions. However it does require more care to prevent an infinite loop.

Answer 4

There is a discrepancy between the two examples. The first will execute the "do something" at least once every time even if the statement is never true. The second will only "do something" when the statement evaluates to true.

I think what you are looking for is a do-while loop. I 100% agree that while (true) is not a good idea because it makes it hard to maintain this code and the way you are escaping the loop is very goto esque which is considered bad practice.

Try:

do {
  //do something
} while (!something);

Check your individual language documentation for the exact syntax. But look at this code, it basically does what is in the do, then checks the while portion to see if it should do it again.

Answer 5

Javier made an interesting comment on my earlier answer (the one quoting Wordsworth):

I think while(true){} is a more 'pure' construct than while(condition){}.

and I couldn't respond adequately in 300 characters (sorry!)

In my teaching and mentoring, I've informally defined "complexity" as "How much of the rest of the code I need to have in my head to be able to understand this single line or expression?" The more stuff I have to bear in mind, the more complex the code is. The more the code tells me explicitly, the less complex.

So, with the goal of reducing complexity, let me reply to Javier in terms of completeness and strength rather than purity.

I think of this code fragment:

while (c1) {
    // p1
    a1;
    // p2
    ...
    // pz
    az;
}

as expressing two things simultaneously:

1. the (entire) body will be repeated as long as c1 remains true, and
2. at point 1, where a1 is performed, c1 is guaranteed to hold.

The difference is one of perspective; the first of these has to do with the outer, dynamic behavior of the entire loop in general, while the second is useful to understanding the inner, static guarantee which I can count on while thinking about a1 in particular. Of course the net effect of a1 may invalidate c1, requiring that I think harder about what I can count on at point 2, etc.

Let's put a specific (tiny) example in place to think about the condition and first action:

while (index < length(someString)) {
    // p1
    char c = someString.charAt(index++);
    // p2
    ...
}

The "outer" issue is that the loop is clearly doing something within someString that can only be done as long as index is positioned in the someString. This sets up an expectation that we'll be modifying either index or someString within the body (at a location and manner not known until I examine the body) so that termination eventually occurs. That gives me both context and expectation for thinking about the body.

The "inner" issue is that we're guaranteed that the action following point 1 will be legal, so while reading the code at point 2 I can think about what is being done with a char value I know has been legally obtained. (We can't even evaluate the condition if someString is a null ref, but I'm also assuming we've guarded against that in the context around this example!)

In contrast, a loop of the form:

while (true) {
    // p1
    a1;
    // p2
    ...
}

lets me down on both issues. At the outer level, I am left wondering whether this means that I really should expect this loop to cycle forever (e.g. the main event dispatch loop of an operating system), or whether there's something else going on. This gives me neither an explicit context for reading the body, nor an expectation of what constitutes progress toward (uncertain) termination.

At the inner level, I have absolutely no explicit guarantee about any circumstances that may hold at point 1. The condition true, which is of course true everywhere, is the weakest possible statement about what we can know at any point in the program. Understanding the preconditions of an action are very valuable information when trying to think about what the action accomplishes!

So, I suggest that the while (true) ... idiom is much more incomplete and weak, and therefore more complex, than while (c1) ... according to the logic I've described above.

Answer 6

The problem is that not every algorithm sticks to the "while(cond){action}" model.

The general loop model is like this :

loop_prepare
 loop:
  action_A
  if(cond) exit_loop
  action_B
  goto loop
after_loop_code

When there is no action_A you can replace it by :

loop_prepare
  while(cond)
  action_B
after_loop_code

When there is no action_B you can replace it by :

loop_prepare
  do action_A
  while(cond)
after_loop_code

In the general case, action_A will be executed n times and action_B will be executed (n-1) times.

A real life example is : print all the elements of a table separated by commas. We want all the n elements with (n-1) commas.

You always can do some tricks to stick to the while-loop model, but this will always repeat code or check twice the same condition (for every loops) or add a new variable. So you will always be less efficient and less readable than the while-true-break loop model.

Example of (bad) "trick" : add variable and condition

loop_prepare
b=true // one more local variable : more complex code
 while(b): // one more condition on every loop : less efficient
  action_A
  if(cond) b=false // the real condition is here
  else action_B
after_loop_code

Example of (bad) "trick" : repeat the code. The repeated code must not be forgotten while modifying one of the two sections.

loop_prepare
action_A
 while(cond):
  action_B
  action_A
after_loop_code

Note : in the last example, the programmer can obfuscate (willingly or not) the code by mixing the "loop_prepare" with the first "action_A", and action_B with the second action_A. So he can have the feeling he is not doing this.