JS do ... while

Question

It seems to me that i misunderstand the behavior of the do ... while loop in JS. Let's say we have a code like:

var a = [1,2,3,4,5];
var b = [];
var c;
do {c = a[Math.floor(Math.random()*a.length)];
 b.push(c);}
while(c===4);
console.log(b);

Which is intended to roll out random item from array a if that item is not 4. But if we roll several times we'll see that it doesn't actually prevent 4 from getting to array b. Why? I thought that it would work like this:

1. Roll random item from array a, store it to c and push c to b;
2. Check if (c===4) is true;
3. If it is — go to paragraph 1;
4. If it's not — log b to console.

Where am I mistaking and why does this code work in such a way? What are others way to 'ban' some item from array from being rolled randomly (except filtering the array) if this approach can't help me?

• Artyom Neustroev: You actually said it yourself: you push the item into the array and only then check if it is 4 to exit the loop.
• tristantzara: @ArtyomNeustroev So, do.. while actually does work as described in ordered list, that I've posted?
• Xion Dark: What type of values for b are you expecting?

Answer 1

Do while runs and THEN checks. So it will get a random number from A, store that in C and push that to B, and THEN if C is 4, it will do another loop.

So if C is 4, it will still push it to B, it just won't continue after that.

You could do it like this:

var a = [1,2,3,4,5];
var b = [];
var c = a[Math.floor(Math.random()*a.length)];
while (c !== 4) {
 b.push(c);
 c = a[Math.floor(Math.random()*a.length)];
}
console.log(b);

I think this is what you're trying to do? Continuously push a random item from A into B unless you get the result 4, in which case, quit and go to console.log?

Answer 2

As explained by the commenters, you're still pushing 4. You can avoid it by make it very explicit what happens when.

var a = [1,2,3,4,5];
var b = [];
var c;
var keep_going = true;
while (keep_going) {
 c = a[Math.floor(Math.random()*a.length)];
 if (c === 4) {
 keep_going = false;
 } else {
 b.push(c);
 }
}
console.log(b);

Answer 3

So the way your code is written, you are not "bannning" 4 from being added to b. The code you have written will add a value from a to b and, if the value added equals 4, continue to add values from a to b until the last value added does not equal 4. So you will get results like:

b == [1];
b == [5];
b == [4,1];
b == [4,4,4,4,4,4,3];

Since do-while is a looping mechanism, I'm assuming you want to keep trying to add a value from a to b until you find one that is not 4. That would be

var a = [1,2,3,4,5],
 b = [],
 c;
do {
 c = a[Math.floor(Math.random()*a.length)];
 if(c!==4) { b.push(c); }
} while(c===4);
console.log(b);

This will produce the following values for b

b == [1];
b == [2];
b == [3];
b == [5];