multiple expressions in one switch statement

Question

It is my first time using the switch statement in Javascript. Is there a way to evaluate multiple conditions one switch statement, like so:

var i = 1;
switch(i && random(1)<0.3) {
    case (1):
        //code block
         break;
    case (2):
        //code block
}

So that the code blocks would execute if both of the conditions were true?

*So that the code blocks would execute if both of the conditions were true*: both of them? When should only the first be executed, when only the second, and when none of them? — trincot, Jul 24 '16 at 20:34

score 6 · Answer 1 · answered Jul 24 '16 at 21:04

It's possible to write switch statements this way:

switch (true) {
  case a && b:
    // do smth
    break;
  case a && !b:
    // do other thing
    break;
}

The only thing you need to keep in mind is that && can return not only boolean, but rather any other value if e.g. 'a' (in code snippet above) resolves to some false value. If 'b' is string - then a && b, where a is false, shall return a string.
So when you use this pattern always ensure that the right side of && expression resolves to a boolean.

eltonkamami · Answer 2 · 2016-07-24T20:42:39.443

0

You can do that but the switch statement will switch on the result of the expression you provide.

Given you have a logical and (&&) in your expression there are two possible outcomes defined on how && works.

if the left hand expression evaluates to true the expression will be equal to the evaluation of the second part.
if the left hand expression evaluates to false the whole expression will evaluate to false.

You can read more about the switch statement on the Ecmascript spec

edited Jul 24 '16 at 20:42

answered Jul 24 '16 at 20:33

eltonkamami

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Actually `&&` can evaluate to non-boolean results, but yes it's true that the current script will never match any of the cases – Bergi Jul 24 '16 at 20:38
I think I'd have to say `case(5 && true)` – Roman Jul 24 '16 at 20:43
@Roman again the same apply to the case statement block as described in the two points above. – eltonkamami Jul 24 '16 at 20:46
I see what you mean on multiple boolean expressions evaluating to either true or false. But what other boolean expressions can there be? – Roman Jul 24 '16 at 20:50
@Roman if you read number 1 and 2 of my answer you will get the idea. && will return false if the left hand of && is false. it will return the evaluation of the right hand otherwise. false && 4 -> false, true && 5 -> 5 – eltonkamami Jul 24 '16 at 20:52

trincot · Accepted Answer · 2016-07-24T20:48:27.550

You could do something like this:

var i = 1;
switch((i==1) + (Math.random(1)<0.3)*2) {
    case 0:
        //code block when neither is true
         break;
    case 1:
        //code block when only i == 1
         break;
    case 2:
        //code block when only random(1)<0.3
         break;
    case 3:
        //code block when both i==1 and random(1)<0.3
        break;
}

... but it is not really the nicest code, and can easily lead to mistakes when one of the tested expressions is anything else than 0 or 1 (false or true).

It is better to use if ... else constructs to deal with this:

var i = 1;
var small = Math.random(1)<0.3;
if (i==1) {
    if (small) {
        //code block when both i==1 and random(1)<0.3
    } else {
        //code block when only i == 1
    }
} else if (small) {
    //code block when only random(1)<0.3
} else {
    //code block when neither is true
}

multiple expressions in one switch statement

3 Answers3