matlab replace number

in a column, value 2 replace with 1 and value 1 & 3 replace with 2. The code i wrote below got problem:

S=[1 1 1 2 2 3 3 3 3];
S(S==2)=1; S(S==1)=2; S(S==3)=2;

result:

S=[2 2 2 2 2开发者_开发知识库 2 2 2 2]

However, the result i wan to get is S=[2 2 2 1 1 2 2 2 2]. does anyone can help?

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That is happening because when in the S(S==1)=2; step, you are affected by the modifications from the S(S==2)=1; step. Try this

S = [1 1 1 2 2 3 3 3 3];
S_copy = S;
S(S_copy == 2) = 1; S(S_copy == 1) = 2; S(S_copy == 3) = 2;

or you could also save the results of the tests into separate variables:

S = [1 1 1 2 2 3 3 3 3];
f1 = (S == 2);
f2 = (S == 1);
f3 = (S == 3);
S(f1) = 1; S(f2) = 2; S(f3) = 2;

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Instead of manually replacing each value, you can use an extra matrix to define a "map" from input values in S to output values.

>> S = [1 1 1 2 2 3 3 3 3];  % input
>> M = [2 1 2];              % M[i] = j -> map value i to j
>> S = M(S)                  % compute output
S =
     2     2     2     1     1     2     2     2     2

This operation should be really fast in Matlab.

Note that this methods works as long as the values in S can be interpreted as index values (that is, they are integers and not too large).

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your are getting closer but the problem arises once you change all the 2's to one. after this statement S(S==2)=1; the array looks like this S=[1 1 1 1 1 3 3 3 3]; and after the other two statements S(S==1)=2; S(S==3)=2; your array will obviously have all 2's.

Instead of S(S==2)=1; S(S==1)=2; S(S==3)=2;

you can do like this:

S(S==2)=-1; S(S==1)=2; S(S==3)=2;S(S==-1)=1;

i.e. in the first step change all the 2's to some other value(e.g. -1 here) and then do the required conversion i.e. S(S==-1)=1;