Is there ever a need for a "do {...} while ( )" loop?

Question

Bjarne Stroustrup (C++ creator) once said that he avoids "do/while" loops, and prefers to write the code in terms of a "while" loop instead. [See quote below.]

Since hearing this, I have found this to be true. What are your thoughts? Is there an example where a "do/while" is much cleaner and easier to understand than if you used a "while" instead?

In response to some of the answers: yes, I understand the technical difference between "do/while" and "while". This is a deeper question about readability and structuring code involving loops.

Let me ask another way: suppose you were forbidden from using "do/while" - is there a realistic example where this would give you no choice but to write unclean code using "while"?

From "The C++ Programming Language", 6.3.3:

In my experience, the do-statement is a source of errors and confusion. The reason is that its body is always executed once before the condition is evaluated. However, for the body to work correctly, something very much like the condition must hold even the first time through. More often than I would have guessed, I have found that condition not to hold as expected either when the program was first written and tested, or later after the code preceding it has been modified. I also prefer the condition "up front where I can see it." Consequently, I tend to avoid do-statements. -Bjarne

Answer 1

(Assuming you know the difference between the both)

Do/While is good for bootstrapping/pre-initializing code before your condition is checked and the while loop is run.

Answer 2

do-while is a loop with a post-condition. You need it in cases when the loop body is to be executed at least once. This is necessary for code which needs some action before the loop condition can be sensibly evaluated. With while loop you would have to call the initialization code from two sites, with do-while you can only call it from one site.

Another example is when you already have a valid object when the first iteration is to be started, so you don't want to execute anything (loop condition evaluation included) before the first iteration starts. An example is with FindFirstFile/FindNextFile Win32 functions: you call FindFirstFile which either returns an error or a search handle to the first file, then you call FindNextFile until it returns an error.

Pseudocode:

Handle handle;
Params params;
if( ( handle = FindFirstFile( params ) ) != Error ) {
   do {
      process( params ); //process found file
   } while( ( handle = FindNextFile( params ) ) != Error ) );
}

Answer 3

A do-while loop can always be rewritten as a while loop.

Whether to use only while loops, or while, do-while, and for-loops (or any combination thereof) depends largely on your taste for aesthetics and the conventions of the project you are working on.

Personally, I prefer while-loops because it simplifies reasoning about loop invariants IMHO.

As to whether there are situations where you do need do-while loops: Instead of

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

you can always

loopBody();
while(condition())
{
  loopBody();
}

so, no, you never need to use do-while if you cannot for some reason. (Of course this example violates DRY, but it's only a proof-of-concept. In my experience there is usually a way of transforming a do-while loop to a while loop and not to violate DRY in any concrete use case.)

"When in Rome, do as the Romans."

BTW: The quote you are looking for is maybe this one ([1], last paragraph of section 6.3.3):

From my experience, the do-statement is a source of error and confusion. The reason is that its body is always executed once before the condition is tested. For the correct functioning of the body, however, a similar condition to the final condition has to hold in the first run. More often than I expected I have found these conditions not to be true. This was the case both when I wrote the program in question from scratch and then tested it as well as after a change of the code. Additionally, I prefer the condition "up-front, where I can see it". I therefore tend to avoid do-statements.

(Note: This is my translation of the German edition. If you happen to own the English edition, feel free to edit the quote to match his original wording. Unfortunately, Addison-Wesley hates Google.)

[1] B. Stroustrup: The C++ programming language. 3rd Edition. Addison-Wessley, Reading, 1997.

Answer 4

Yes I agree that do while loops can be rewritten to a while loop, however I disagree that always using a while loop is better. do while always get run at least once and that is a very useful property (most typical example being input checking (from keyboard))

#include <stdio.h>

int main() {
    char c;

    do {
        printf("enter a number");
        scanf("%c", &c);

    } while (c < '0' ||  c > '9'); 
}

This can of course be rewritten to a while loop, but this is usually viewed as a much more elegant solution.

Answer 5

do { ... } while (0) is an important construct for making macros behave well.

Even if it's unimportant in real code (with which I don't necessarily agree), it's important for for fixing some of the deficiencies of the preprocessor.

Edit: I ran into a situation where do/while was much cleaner today in my own code. I was making a cross-platform abstraction of the paired LL/SC instructions. These need to be used in a loop, like so:

do
{
  oldvalue = LL (address);
  newvalue = oldvalue + 1;
} while (!SC (address, newvalue, oldvalue));

(Experts might realize that oldvalue is unused in an SC Implementation, but it's included so that this abstraction can be emulated with CAS.)

LL and SC are an excellent example of a situation where do/while is significantly cleaner than the equivalent while form:

oldvalue = LL (address);
newvalue = oldvalue + 1;
while (!SC (address, newvalue, oldvalue))
{
  oldvalue = LL (address);
  newvalue = oldvalue + 1;
}

For this reason I'm extremely disappointed in the fact that Google Go has opted to remove the do-while construct.

Answer 6

It's useful for when you want to "do" something "until" a condition is satisfied.

It can be fudged in a while loop like this:

while(true)
{

    // .... code .....

    if(condition_satisfied) 
        break;
}

Answer 7

I hardly ever use them simply because of the following:

Even though the loop checks for a post-condition you still need to check for this post condition within your loop so that you don't process the post condition.

Take the sample pseudo code:

do {
   // get data 
   // process data
} while (data != null);

Sounds simple in theory but in real world situations it would probably turn out to look like so:

do {
   // get data 
   if (data != null) {
       // process data
   }
} while (data != null);

The extra "if" check just isn't worth it IMO. I have found very few instances where it's more terse to do a do-while instead of a while loop. YMMV.

Answer 8

It's all about readability.
More readable code leads to less headache in code maintenance, and better collaboration.
Other considerations (such as optimization) are, by far, less important in most cases.
I'll elaborate, since I got a comment here:
If you have a code snippet A that uses do { ... } while(), and it's more readable than its while() {...} equivalent B, then I'd vote for A. If you prefer B, since you see the loop condition "up front", and you think it's more readable (and thus maintainable, etc.) - then go right ahead, use B.
My point is: use the code that is more readable to your eyes (and to your colleagues'). The choice is subjective, of course.

Answer 9

In response to a question/comment from unknown (google) to the answer of Dan Olson:

"do { ... } while (0) is an important construct for making macros behave well."

#define M do { doOneThing(); doAnother(); } while (0)
...
if (query) M;
...

Do you see what happens without the do { ... } while(0)? It will always execute doAnother().

Answer 10

In our coding conventions

• if / while / ... conditions don't have side effects and
• varibles must be initialized.

So we have almost never a do {} while(xx) Because:

int main() {
    char c;

    do {
        printf("enter a number");
        scanf("%c", &c);

    } while (c < '0' ||  c > '9'); 
}

is rewritten in:

int main() {
    char c(0);
    while (c < '0' ||  c > '9');  {
        printf("enter a number");
        scanf("%c", &c);
    } 
}

and

Handle handle;
Params params;
if( ( handle = FindFirstFile( params ) ) != Error ) {
   do {
      process( params ); //process found file
   } while( ( handle = FindNextFile( params ) ) != Error ) );
}

is rewritten in:

Params params(xxx);
Handle handle = FindFirstFile( params );
while( handle!=Error ) {
    process( params ); //process found file
    handle = FindNextFile( params );
}

Answer 11

It is only personal choice in my opinion.

Most of the time, you can find a way to rewrite a do ... while loop to a while loop; but not necessarily always. Also it might make more logical sense to write a do while loop sometimes to fit the context you are in.

If you look above, the reply from TimW, it speaks for itself. The second one with Handle, especially is more messy in my opinion.

Answer 12

do {
    cout << "Is there ever a need for a do {…} while ( ) loop?";
    cin >> choice;
}
while (choice != 'Y'); // hee hee hee!

Answer 13

Read the Structured Program Theorem. A do{} while() can always be rewritten to while() do{}. Sequence, selection, and iteration are all that's ever needed.

Since whatever is contained in the loop body can always be encapsulated into a routine, the dirtiness of having to use while() do{} need never get worse than

LoopBody()
while(cond) {
    LoopBody()
}

Answer 14

consider something like this:

int SumOfString(char* s)
{
    int res = 0;
    do
    {
        res += *s;
        ++s;
    } while (*s != '\0');
}

It so happens that '\0' is 0, but I hope you get the point.

Answer 15

My problem with do/while is strictly with its implementation in C. Due to the reuse of the while keyword, it often trips people up because it looks like a mistake.

If while had been reserved for only while loops and do/while had been changed into do/until or repeat/until, I don't think the loop (which is certainly handy and the "right" way to code some loops) would cause so much trouble.

I've ranted before about this in regards to JavaScript, which also inherited this sorry choice from C.

Answer 16

The following common idiom seems very straightforward to me:

do {
    preliminary_work();
    value = get_value();
} while (not_valid(value));

The rewrite to avoid do seems to be:

value = make_invalid_value();
while (not_valid(value)) {
    preliminary_work();
    value = get_value();
}

That first line is used to make sure that the test always evaluates to true the first time. In other words, the test is always superfluous the first time. If this superfluous test wasn't there, one could also omit the initial assignment. This code gives the impression that it fights itself.

In cases such like these, the do construct is a very useful option.