A somewhat painful triple-nested ternary operator

问题:

I went looking through Raphael.js's source code to find out how he converted RGB values to HSB. I found out the function he did it in and I was in the process of converting it to Python when I bumped into this nice triple-nested ternary operator:

H = (C == 0 ? null :
    V == r ? (g - b) / C :
    V == g ? (b - r) / C + 2 :
             (r - g) / C + 4
    );

It threw me for a loop because Python doesn't have the same kind of ternary operator that Javascript does. I spent a while looking over it and eventually hashed this somewhat saner code (using only if/else) out of it:

if (C == 0) {
    H = null;
} else {
    if(V == r) {
        H = (g - b) / C;
    } else {
        if(V == g) {
            H = (b - r) / C + 2;
        } else {
            H = (r - g) / C + 4;
        }
    }
}

Was my interpretation correct? I'm only asking this because if it isn't correct, I'm faced with a lot of debugging. So. Did I "get it"?

回答1:

I think you can have this to avoid the deep nesting:

var H

if(C == 0){
    H = null;
}
else if(V == r){
    H = (g - b) / C;
}
else if (V == g){
    H = (b - r) / C + 2;
}
else {
    H = (r - g) / C + 4;
}

回答2:

To my personal taste, a carefully aligned nested ternary beats the if-esle mess:

const H =
  C == 0 ? null            :
  V == r ? (g - b) / C     :
  V == g ? (b - r) / C + 2 :
           (r - g) / C + 4 ;

回答3:

H = C == 0 
    ? null 
    : V == r 
        ? (g - b) / C 
        : V == g 
            ? (b - r) / C + 2 
            : (r - g) / C + 4

I've seen Dan Abramov using this indentation placement pattern. While I don't like how the conditional operator ? no longer visually follows the condition, I prefer this to something like @lolmaus's example in that the indentation will always be consistent regardless the size of the conditional.

You actually start to look at it as ? true : false which is visually intuitive here. And this way, I find the ternary is much easier to spot and differentiate from the surrounding code.

回答4:

The same logic can be written in a simpler way:

var H

if (C == 0)
    H = null;
else if (V == r)
    H = (g - b) / C;
else if (V == g)
    H = (b - r) / C + 2;
else
    H = (r - g) / C + 4;

It's possible to omit the curly braces because there's a single statement in each condition. And given that the conditions are mutually exclusive, using else if is much clearer than nesting ifs.

回答5:

Yes, it's right (apart from capitalisation differences). Yet, it may be cleaner written without any parentheses, readable as elseif:

if (C == 0)
    h = null;
else if (V == r)
    h = (g - b) / C;
else if (V == g)
    h = (b - r) / C + 2;
else
    h = (r - g) / C + 4;

回答6:

If your JavaScript codebase contains nested ternary statements like the one in question, consider converting the formatting to daisy chained ternary statements instead.

H = (C == 0)           // Is C zero?
    ? null
    : (V == r)         // Is V equal to r?
    ? (g - b) / C
    : (V == g)         // Is V equal to g?
    ? (b - r) / C + 2
    : (r - g) / C + 4; // Fallback (default) value

They simply read top to bottom in a straight line, returning a value as soon as they hit a truthy condition or the fallback.

–Nested Ternaries are Great, Eric Elliot