Switch vs If ?

[makkawi]

Hi ,,When we use Switch Statement and when we use if...else if....else statement ?andWhat is the difference between them?thank you ..

[boen_robot]

You use if...else if...else when you need to test different variables, and you use switch() when you want to test a single variable against multiple values.In addition, switch() allows some very interesting constructs when omitting the break statement before a certain case. In particular:

switch(var) {  case 1:  case 2:	//This will execute if var is 1 or 2.	break;  case 3:	//This will execute if var is 3.  case 4:	//This will execute if var is 3 or 4 (if var is 3, execution will start at the previous code block and continue to this one)	break;  default:	//This will execute if var is nothing from the above (i.e. is not 1, 2, 3 or 4)  case 0:	//This will execute if var is nothing from the above (i.e. is not 1, 2, 3 or 4) or 0 (if var is 0, execution starts here, otherwise at the previous code block)	break;}

The equivalent in if...else if...else would be:

if(var == 1 || var == 2) {  //This will execute if var is 1 or 2.}else if(var === 3 || var === 4) {  if (var == 3) {	//This will execute if var is 3  }  //This will execute if var is 3 or 4 (if var is 3, execution will start at the previous code block and continue to this one)}else {  //This will execute if var is nothing from the above (i.e. is not 1, 2, 3 or 4)  if(var == 0) {	//This will execute if var is nothing from the above (i.e. is not 1, 2, 3 or 4) or 0 (if var is 0, execution starts here, otherwise at the previous code block)  }}

[makkawi]

thank you ..butIs it true that switch Statement faster than if Statement in the execute?

[jeffman]

According to http://www.phpbench.com/ they are quite comparable. Switch gets faster as the conditions become more complicated. As with many things, though, you need to execute a thousand before you can really tell the difference.

[boen_robot]

Remember the part I said about:

}else {  //This will execute if var is nothing from the above (i.e. is not 1, 2, 3 or 4)  if(var == 0) {	//This will execute if var is nothing from the above (i.e. is not 1, 2, 3 or 4) or 0 (if var is 0, execution starts here, otherwise at the previous code block)  }}

Actually, it's not equivalent. In swich(), if var is anything else, even the "0" code block will be executed. Here, it will not. To achieve a real if...else if... else equivalent, you need to use a function:

}else {  function ifVarIsNothingFromTheAbove() {	//This will execute if var is nothing from the above (i.e. is not 1, 2, 3 or 4) including 0  }  if(!var == 0) {	//This will execute if var is nothing from the above (i.e. is not 1, 2, 3 or 4)	ifVarIsNothingFromTheAbove();  }else {	ifVarIsNothingFromTheAbove();  }}

This essentially makes the code slower (or at least it should) as the act of creating and calling a function is more expensive than a single simple condition evaluation.The differences between switch() and if...else if...else are small really. The only thing that I sometimes don't like about switch() is that is forces loose typing (at least in JavaScript and PHP), i.e.

var == 0

is the same as

var == "0"

I'd have loved if I could adjust this behaviour to scrict type checking where

var === 0

is not the same as

var === "0"

Actually wait... does JavaScript support sctrict type checking? The spec implemented in browsers that is (ECMAScript 4 probably does, but that's like a brand new thing).

[Jesdisciple]

Actually wait... does JavaScript support sctrict type checking? The spec implemented in browsers that is (ECMAScript 4 probably does, but that's like a brand new thing).

If I found the correct spec, yes. Ctrl+F for "===" at http://www.ecma-international.org/publicat...ST/Ecma-262.pdf.But I thought even 0 would start at the default block?

[boen_robot]

But I thought even 0 would start at the default block?

You know, you may have a point there... I didn't really tested out my code. I just thought about all possible combos and this one springed into mind.In that case, I guess the one other combo that would work (properly) would be:

  case 0:	//This will execute if var is 0  default:	//This will execute if var is nothing from the above (in which case execution will begin here) or 0 (in which case execution starts at the previous code block)

Did I missed anything?

[Jesdisciple]

I tested it and 0 went to 0... Now I wonder about PHP, Java, C++, etc.

[jeffman]

My console let me test this in 30 seconds.

var x;x == 0;		 // returns falsex == false;	 // returns falsex == null;	  // returns truex == undefined; // returns true!x;			 // returns true

[Jesdisciple]

First, what console - Firefox's?And did you actually use a switch-statement with that? If not, you're apparently using an assumption I never have before, although I can't identify it immediately. I always thought of the interpreter scanning the cases until it found one that fit.