0 in the while statement

Question

i have one question and please help me.i have read on web page somthing about do while statement,different is that ,in while there is written 0,not boolean condition

do{
   // do some instruction
}while(condition );

is clearly understandable,but this one

 do
  {
    //again some instruction
  }while(0);

i can't guess what does it do?is it equivalence it to this: do something while(false)?or maybe is it infinity loop?please help me

What's the context of the loop? – Marlon Oct 18 '11 at 05:55 — Marlon, Oct 18 '11 at 05:55

score 6 · Accepted Answer · edited May 23 '17 at 12:27

6

It does something only once. It is widely used in macros to group statements and make the usage more natural (i.e. require ending semicolon).

Any compiler worth its salt should ignore the do .. while altogether and just run the enclosed statements.

edited May 23 '17 at 12:27

Community

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answered Oct 18 '11 at 05:54

cnicutar

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Doc Brown · Answer 2 · 2011-10-18T08:15:24.533

3

Though this "loop" is executed just once, one may use it this way:

do
{
  // declare some local variables only visible here
  // do something

  if(TestSomeConditions())
      break;

  // do something more
  // destructors of local variables run here automatically
}while(0);

IMHO, in most cases it is better to rewrite this in a more structured manner, either this way

  // do something
  if(!TestSomeConditions())
  {
      // do something more
  }

or by introducing a separate function, reflecting the original scope:

  void MyFunction()
  {
      // declare some local variables only visible here
      // do something

      if(TestSomeConditions())
          return;

      // do something more
  }

edited Oct 18 '11 at 08:15

answered Oct 18 '11 at 05:58

Doc Brown

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+1: that's one of the reason. But it looks like "I want GOTO, but I've been told I cannot so gimme a loop to escape from!" – Emilio Garavaglia Oct 18 '11 at 06:36
@EmilioGaravaglia: Yep, that was first coming into my mind when I saw this. – Doc Brown Oct 18 '11 at 07:21

Chethan Ravindranath · Answer 3 · 2011-10-18T06:07:44.390

1

The do while loop executes all the instructions in the body of the loop first and then checks for the condition. Hence, this loop just executes the instructions of the body of the loop once.

do
{
    //my instructions
}while(0)

is equivalent to writing

{
    //my instructions
}

The modern compilers are smart enough to optimize these things out!

edited Oct 18 '11 at 06:07

answered Oct 18 '11 at 05:57

Chethan Ravindranath

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Though this is true in many cases, technically this is not correct, since do{ - }while introduces a new scope. – Doc Brown Oct 18 '11 at 06:00
1

... but the question becomes another: WHY one should do that! – Emilio Garavaglia Oct 18 '11 at 06:01
I agree! Editing my answer to reflect the same – Chethan Ravindranath Oct 18 '11 at 06:07
@EmilioGaravaglia: see my answer for a possible explanation. – Doc Brown Oct 18 '11 at 06:32

score 1 · Answer 4 · answered Oct 18 '11 at 06:00

1

It will treat 0 as false and one time loop will execute and break as soon as reached to while (g++ compiler tested).

answered Oct 18 '11 at 06:00

UPT

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score 1 · Answer 5 · answered Oct 18 '11 at 06:07

Technically speaking, it makes it run once, as it checks (and fails the check) after it runs once.

Other than that, it's used in macros. For example:

#define blah(a) \
        something(1); \
        something(2)

if(a)
    blah(4);
else
    blah(19);

would lead to this:

...
if(a)
    something(1); something(2);
else
    something(1); something(2);

which is not valid syntax, as the else is not part of the if statement anymore. What you can do is:

 define blah(a) \
    do {
       something(1); something(2);
    } while(0)

    ...

which would translate into a single statement in the if block:

if(a)
    do { something(1); something(2); } while(0);
else
    ...

0 in the while statement

5 Answers5