CODEWITHSUNDEEP
javascriptbooleanlogic
LTS
LTS

Reputation: 23

Boolean equality

I got this out of an exam question, and couldn't understand how the solution works. This function is supposed to return "true" if the values "x" and "y" are equal, and return False otherwise.

The solution:

function equal_boolean (x , y) {
  return x ? y : y ? x : true;
}

Why does this work? As much as I can understand, it would turn out to be evaluating if X is true. If X is true, it would return Y. How is X supposed to be "true"?

If it isn't, it would evaluate whether Y is true, if it's true it would return X, and if it isn't - it would return True.

Is there something wrong with my understanding?

Upvotes: 2

Views: 770

Answers (4)

Tom Fenech
Tom Fenech

Reputation: 74705

The first thing you should do is group the ternary operators together:

(x ? y : (y ? x : true))
  • if x is true, then return y, whose value also tells you whether x and y are equal
  • if x is false, then start with the second ternary:
    • if y is true, then return x (false) as the two are not equal
    • if y is false, then x and y are equal, so return true

Upvotes: 3

georg
georg

Reputation: 215059

return x ? y : y ? x : true;

parses as

if x
   return y       // if y is true, then x == y. if y is false, then x != y
else (x is false)
   if y
      return x    // x is false and y is true, y != x, return false
   else
      return true // x is false and y is false, return true

This is of course a pretty convoluted way to express boolean equality (aka Logical biconditional aka iff). More natural would be an expression like this:

 (x && y) || (!x && !y)

Upvotes: 4

Volune
Volune

Reputation: 4339

  • First identify the expressions

    return x ? y : y ? x : true;
//turns into
return x ? y : (y ? x : true);

  • Replace the ?: ternary operators by if statements

    if (x) {
    return y;
} else {
    //here x is false, so we will be able to replace x by false in the next step
    if (y) {
        return x;
    } else {
        return true;
    }
}

  • Make if statements more verbose, replace return x by return false

    if (x == true) {
    return y;
} else {
    if (y == true) {
        return false;
    } else {
        return true;
    }
}

  • Finally replace return y; by if (y == true) { return true; } else { return false; }, and check all the possibilities

    if (x == true) {
    if (y == true) {
        return true; // x == true and y == true
    } else {
        return false; // x == true and y == false
    }
} else {
    if (y == true) {
        return false; // x == false and y == true
    } else {
        return true; // x == false and y == false
    }
}

It works. (As long as x and y are booleans)

Upvotes: 0

Design by Adrian
Design by Adrian

Reputation: 2235

Let's try expanding this a bit:

var myBool = x ? y : (y ? x : true)

return myBool;

First block

x ? y : ...

if X is true, then return value of Y. If Y happens to be true, both are equal. Otherwise return false (no match).

Second block if X was false:

y ? x : true

if Y is true, return X. X was false, so false is returned (no match) if Y is false, return true - both values are false.

Upvotes: 0

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