replacing different elements in a matrix according to different conditions

Hi

I have an array which contains integers from 0 to 5. Now I want to replace all 5s with a 0, all 0 with a 5, the 2s with a 4 and vice versa and the 3 would not be changed. I know that I could do that with several nested if loops - but is there a faster/smarter way?

[per isakson]

... and 1 would not change?

[Joseph Cheng]

What do you do with 1's

[MiauMiau]

you are right, so it would be

5 -> 0
4 -> 1
3 -> 2
2 -> 3
1 -> 4
5 -> 0

[Sebastian Castro]

Based on those rules, you could "cheat" and do this completely without logic:

out = 5 - in

[MiauMiau]

you are right, that is the fastest possibility indeed! thanks!

[Image Analyst]

There is a general function that lets you map any integer into any other number you want in a single line of code. It's general, not just "5-in" which coincidentally happens to work for this particular case. However based on your request - the only one I've ever gotten on over 15,000 questions answered - I won't answer.

Answer 1

[Sebastian Castro]

You can use logical indexing to find elements in a matrix that correspond to a certain value.

For instance, let's the take the logical test that a value is equal to 5. For some value x , you know you can enter the expression below and get a true-false response.

x == 5

In MATLAB, you can also do this for an entire matrix M . I've created a random matrix as follows:

M = randi(6,[10 10]) - 1

The following line will return a matrix the same size as M whose elements are 0 or 1 depending on the above condition (is equal to 5). Try it out for yourself on such a matrix.

M == 5

So, if you want to replace all those elements with a zero, use the logical indices and assign those to a value of 0:

M(M==5) = 0

- Sebastian

[Joseph Cheng]

If you go sebastian's route then you'd want to make sure you index each condition before you start swapping.

for ind = unique(M)
   swapele(ind,:) = M== ind; 
end

then go through a for loop to swap out (probably a switch statement would be best.)

[MiauMiau]

thanks!