if vs switch

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PChoppala
PChoppala 2011-9-20
Hi just a simple question, Which is a better use, 'if' or 'switch'? Thanx P
  1 个评论
Jan
Jan 2011-9-20
Please add the relevant part of the code to your question. without seeing any details a decision is not possible.

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Fangjun Jiang
Fangjun Jiang 2011-9-20
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
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Jan
Jan 2011-9-20
@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
Fangjun Jiang 2011-9-20
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.

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更多回答（1 个）

Walter Roberson
Walter Roberson 2011-9-20
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.
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PChoppala
PChoppala 2011-9-20
Yeah, that's right. Thanks. Can you look into my comment to Fangjun?

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