javascript to objective-c conversion

Question

I know nothing about javascript and need help interpreting the following:

{
    var d = max - min;
    s = l > 0.5 ? d / (2 - max - min) : d / (max + min);
    switch(max){
      case r: h = (g - b) / d + (g < b ? 6 : 0); break;
      case g: h = (b - r) / d + 2; break;
      case b: h = (r - g) / d + 4; break;
    }
}

Could someone please explain what this means so I can code an equivalent in Objective-C ?

Thank you!

or to make my question even more to the point. could you please translate (to human language) the following line for me ?

s = l > 0.5 ? d / (2 - max - min) : d / (max + min);

Equivalent C (and thus Objective-C) code would look pretty much identical. What part do you not understand? — Ole Begemann, Dec 10 '10 at 20:35
the second line specifically .... s = l > 0.5 ? d / (2 - max - min) : d / (max + min); thank you. — EarlGrey, Dec 10 '10 at 20:36
`switch`/`case` with variable cases would be incompatible with C. — Greg Hewgill, Dec 10 '10 at 20:37
possible duplicate of [Question mark in JavaScript](http://stackoverflow.com/questions/1771786/question-mark-in-javascript) — Greg Hewgill, Dec 10 '10 at 20:43

filipe · Accepted Answer · 2010-12-10T21:56:48.650

s = l > 0.5 ? d / (2 - max - min) : d / (max + min);

the expression ? value1 : value2 operator evaluates the expression and, if it's true, returns value1, else returns value2. so in that particular case, it will evaluate l > 0.5, if that's true it'll set s to d / (2 - max - min), else it'll set s to d / (max + min)

EDIT in response to comment:

I don't really know a lot of Javascript at all, but as for the switch statement, it seems to be using variable cases. That's not allowed in Objective-C (or C, for that matter). But it seems to be trying to find which of the r, g or b variables is equal to the max variable...

In Objective-C (or C, or C++) a switch statement can only use constant case values, like this:

switch (myVariable)
{
    case 0:
        //do something if myVariable == 0
        break;
    case 1:
        //do something if myVariable == 1
        break;
    case 2:
        //do something if myVariable == 2
        break;
}

I'm gonna take a guess here and say that in javascript with variable cases, it would probably be equivalent to something like

if (max == r)
    h = (g - b) / d + (g < b ? 6 : 0);
else if (max == g)
    h = (b - r) / d + 2;
else if (max == b)
    h = (r - g) / d + 4;

if anyone who knows more about javascript could confirm that for us, it would be great.

yeah, that's what I ended up doing exactly. thanks for taking the time. — EarlGrey, Dec 12 '10 at 16:11

score 1 · Answer 2 · answered Dec 10 '10 at 20:34

1

This is actually very straightforward, all you have to do is change var to float.

answered Dec 10 '10 at 20:34

Jacob Relkin

161,348
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score 0 · Answer 3 · answered Dec 10 '10 at 20:37

Seems to be a fragment from an algorithm to assign new RGB colors to something. Can't tell without any info about the values max, min. d is probably "distance".

var s is assigned something with a ternary ?: operator, but s is not used again in the block-

score -1 · Answer 4 · answered Dec 10 '10 at 20:36

-1

I'd say it means that whomever you inherited that code from needs to learn how to name variables properly.

answered Dec 10 '10 at 20:36

Adam Milligan

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2

Just passing by, but can't help commenting: looks like this code converts RGB into HSL, which means that variables named h, s and l actually make perfect sense. And the purpose of the 'd' variable is perfectly clear from the first assignment (when you see d = max - min, you know that it stands for 'delta' or 'distance'). So I wouldn't say the names are bad. – Andrey Tarantsov Jul 09 '12 at 17:34

javascript to objective-c conversion

4 Answers4