if vs switch

Hi just a simple question, Which is a better use, 'if' or 'switch'? Thanx P

[Jan]

Please add the relevant part of the code to your question. without seeing any details a decision is not possible.

Answer 1

[Fangjun Jiang]

I guess you are asking to compare switch-case with nested if-elseif. I prefer switch-case. I think the efficiency is the same but switch-case provides better readability and the flexibility to provide multiple value for each choice.

For example

switch Str
case {'a','b'}
 disp('It is a or b');
case 'c'
 disp('it is c');
otherwise
 disp('not valid');
end

Compare to

if strcmp(Str,'a') || strcmp(Str,'b')
     disp('It is a or b');
elseif strcmp(Str,'c')
     disp('it is c');
else
    disp('not valid');
end

[PChoppala]

Hi

Is there any computational complexity advantage in using 'switch'?

[Fangjun Jiang]

I don't think so. In terms of how many conditions to check before reaching a particular condition, I think the two approaches are the same.

Let's see if anyone else want to chime in.

[PChoppala]

Hmm... Okay then.

I wrote a function that gives the output y=x^2 / 25*x + sqrt(Q).

I want to use it twice in the main program as

y1=x^2 / 25*x + sqrt(Q)

and

y2=x^2 / 25*x

Although I can use switch and if, can I use nargin?

[Fangjun Jiang]

What do you mean? It sounds like a totally different question.

[PChoppala]

Is it?

I used switch in the function to call

either

y1=x^2 / 25*x + sqrt(Q)

or

y2=x^2 / 25*x

I am just wondering if I can do it using nargin or nargout instead of switch?

[Oleg Komarov]

nargin tells you how many inputs were supplied, nargout how many outputs were asked for, so how does it relate to switch an if?

[Jan]

@praveen: It is not clear how you use IF, SWITCH or NARGIN to perform the calculations. Therefore it is impossible give a meaningful advice.

Please insert the relevant part of the code in your original question by editing - and do *not* insert it as comment to an answer.

[Fangjun Jiang]

If you have a function like y=f(x,varargin) where Q be the optional input argument, then yes, you need to use nargin. Whether use if-elseif or switch-case probably doesn't make a difference. My suggestion, if you are literally design function y1=x^2 / 25*x + sqrt(Q), I would suggest making y1=x^2 / 25*x a function and add sqrt(Q) as needed. But of course, if you want to practice using nargin,if-elseif or switch-case, that is a different story.

Answer 2

[Walter Roberson]

Switch evaluates the value of the switch portion, and then it evaluates each case expression in the list until finally one of the cases matches the switch value. This is pretty much the same complexity as the corresponding if/elseif cascade.

Most people write code as if the switch cases must be constants, but that is not the case in MATLAB: they can be full expressions with function calls and all.

This needs to be compared to other computer languages such as C, in which the case labels do need to be constants. In those other languages, it is possible to do preprocessing such that the time required to decide which branch to use is decided in constant time (or near constant time.) In those languages, switch is more efficient than if/elseif chains.

In MATLAB, if/elseif chains are often able to optimize groups of cases by using extended logical conditions with && and || operators; it is difficult to do that kind of optimization using switch.

[PChoppala]

Yeah, that's right. Thanks. Can you look into my comment to Fangjun?