Do-while statement

Question

Possible duplicate:
When is it suitable?

Could someone tell me what is the difference between these two statements and when to use each other?

var counterOne = -1; do { counterOne++; document.write(counterOne); } while(counterOne < 10); 

and

 var counterTwo = -1; while(counterTwo < 10) { counterTwo++; document.write(counterTwo); } 

http://fiddle.jshell.net/Shaz/g6JS4/

At this point in time, I do not see the point of the do statement if this can be done without specifying it inside the while statement.

Answer 1

Do / While VS. While the condition is checked.

A while loop checks the condition and then executes the loop. A Do / While runs a loop and then checks the conditions.

For example, if the counterTwo variable was 10 or more, then the / while loop ran once, while your regular while loop did not run the loop.

Answer 2

It is guaranteed that do-while works at least once. Although the while may not work at all.

Answer 3

The do statement usually guarantees that your code will be executed at least once (the expression is evaluated at the end), while it is evaluated at the beginning.

Answer 4

Suppose you want to process a block inside a loop at least once, regardless of the condition.

Answer 5

do while checks conditional state after block start. while checks the conditional expression before running it. This is usually used in situations where the code always runs at least once.

Answer 6

if you get the value of counterTwo as the return value of another function, in the first case you have to make sure that the if statement.

eg.

 var counterTwo = something.length; while(counterTwo > 0) { counterTwo--; document.write(something.get(counterTwo)); } 

or

 var counterTwo = something.length; if(counterTwo < 0) return; do { counterTwo--; document.write(something.get(counterTwo)); } while(counterTwo > 0); 

The first case is useful if you are processing data in an existing array. The second case is useful if you are "collecting" data:

 do { a = getdata(); list.push(a); } while(a != "i'm the last item");