switch case efficiency question

I have a question about switch/case structure.

if true
  switch input[a,b] 
 case [1,1]
   do something 1
 case [1,2]
   do something 2
 case ...
 ... etc.

end

since a =1 to 40 and b = 1 to 11. So i will end up with a few hundred cases. How to make the code doing the selection more efficient? Thanks, Chong

Answer 1

[Image Analyst]

First of all, a few hundred tests is nothing - it will take just a very small fraction of a second to test 440 cases. Now if you had millions of tests, that might take some time. However looking at a switch statement that spans several thousand lines is difficult. So how you can compact that depends on what "something" is. If it's something that depends on a and b then you could just make the whole thing a single function. But if it really does unique operations in each case block, then you may be stuck. Is there anyway the "somethings" could be made into a single function taking a and b as an input? And I don't mean just transferring 440 tests inside the function. I mean like the something can be parameterized, like out=10*a+42*b or something like that.

[Image Analyst]

Responding to Chong's "Answer"... No, adding a break won't make it faster since it automatically breaks anyway once it does the case block.

Answer 2

[Chong Tao]

Thank you very much, Image Analyst.

"doing something " in each case is unique. it is not a function of a, b. Do you think add a break command to each case will make it faster since there will be only one match anyway?

switch input[a,b] 
case [1,1]
    M =18; break
case [1,2]
    M =20
case ...
... etc.
end

Answer 3

[Jan]

Please post the problem in valid Matlab syntax. "switch input[a,b]" could mean a variety of different things.

Your code example seems to be a look-up-table. Then this would be more efficient:

a   = 1;
b   = 2;

lut = [18, 20, 23;  19, 26, 65];
M   = lut(a, b);

[Chong Tao]

Thanks Jan. A look-up-table will definitely do the job. I never thought about this. :)

[Image Analyst]

I thought "Do something" meant executing a bunch of code that was all unique, not just assigning a single variable like M. If it's really as simple as assigning some variable, then a look up table would work. If they're integers you can use intlut() in the Image Processing Toolbox.