Unusual for loop

[roundcorners]

Can someone explain how this works please?for (var d=fieldChecks.length; d--; ) { initChecks(fieldChecks[d]); }Thank you in advance

[ApocalypeX]

Can someone explain how this works please?for (var d=fieldChecks.length; d--; ) { initChecks(fieldChecks[d]); }Thank you in advance

I think this is what you're trying to do:for(i=fieldChecks.length;i>0;i--){ initChecks(fieldChecks);}Its a loop that runs till it is true. i decrements till it is equal to or less that 0. After that it ends.Though I am not very clear on what you mean in your post.

[thescientist]

there is a missing conditional in that for loop, and its is needed because eventually it will throw an error once d reaches 0. (because array indexes are always >= 0)anyway, the for loop assigns a variable d to the length of array fieldChecks and after each iteration decrements it by -1. The for loop itself runs a function called initChecks that passes the value of whatever is in fieldCheck[d] at that time for relative to what d equals during that iteration. It basically starts at the end of an array and passes each of its values to a function starting from the end back to the beginning.

[thescientist]

I think this is what you're trying to do:for(i=0;i<fieldChecks.length;i++){ initChecks(fieldChecks);}Its a loop that runs till it is true. i increments till it is equal to or more that fieldChecks length. after that it ends.Though I am not very clear on what you mean in your post.

although it is missing the third conditional, there could be a very good reason why it is starting at the end and working backwards. LIFO comes to mind.

[ApocalypeX]

although it is missing the third conditional, there could be a very good reason why it is starting at the end and working backwards. LIFO comes to mind.

Didn't see the decrement. Let me edit that in.

[jeffman]

Since d is getting decremented with every iteration, eventually it becomes 0. At that point, the loop terminates because d evaluates to false. The third term of the for-loop initializer is optional as long as the ; delimiter is present. This is valid code and it should work just fine.I believe that was the intent of the original question. Not, "How do I fix it", but "This works and I'd like to know why."On the other hand, it is very old-fashioned spacing-saving code such as you might find in a C program circa 1978. There is no advantage to writing code this terse in a JavaScript environment. But there is a disadvantage: what it is supposed to do is not intuitively obvious to many developers. For this reason, I recommend writing it out the long way as in Post #2.

[thescientist]

hmmm...didn't know about the two part for loop conditional or that it in this case it would automatically terminate once it reaches 0. interesting stuff.

[jeffman]

It pays to know exactly what will evaluate to false and under which conditions. This problem is what led to the === operator, which older languages did not have. Sometimes, for clarity, it really pays to write out what you mean, as in:d === 0ord === falseA lot of expressions "work," but six months later you look at your code and say, "Huh?" Even worse when you put a coworker or client through a moment like that. And so unnecessary in environments where all the RAM you want is practically free.

[thescientist]

i think i see it....what confuses me is that when one increments often times it starts with 0, so why would 0 evaluate to false in this situation? for instance:

for(var i = 0, l = myArray.length; i < l; i++){  alert("i is: " + i);}

would print out i (obviously) but would start at 0...but yet wouldn't terminate so....what am I missing?

[jeffman]

For one thing, you're testing a comparison. Each of the following:0 < 1andfalse < 1evaluates as true.The original:(var d = fieldChecks.length; d--; ) // third expression legally omittedtests whether d evaluates to false or not. It's more like an existence test.The decrement operator, obviously, executes after the test, with the final value of d (after the loop) being -1. In fact, one simple (and more intuitive) way of rewriting this thing would be this:(var d = fieldChecks.length; d; d--)The difference being this: the decrement operator does not execute after the test, so the final value of d (after the loop) is 0.Terse code is efficient, but the results are not always obvious.

[thescientist]

ok, I see. Thanks!