Replacing values in a Matrix

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gooniyath
gooniyath on 15 Aug 2016
Edited: dpb on 4 Feb 2022
Hi,
I have a matrix similar to this:
A =[ ]
25 40 40 40 25 25 25 25 25 25
25 40 40 40 40 25 25 25 25 25
25 25 40 40 40 40 25 25 25 25
25 25 40 40 40 40 40 25 25 25
25 25 25 40 40 40 40 40 25 25
25 25 25 40 40 40 40 40 40 25
25 25 12 12 40 40 40 40 40 40
25 12 12 12 12 40 40 40 40 40
12 12 12 12 12 12 40 40 40 40
12 12 12 12 12 12 25 40 40 40
12 12 12 12 12 25 25 40 40 40
How do I write a script to replace all the 25's, with a certain value, and the 40's with another value and 12's with another value?
Thanks

Answers (3)

dpb
dpb on 15 Aug 2016
A(A==yourvalue)=NewValue;
  5 Comments
dpb
dpb on 4 Feb 2022
Edited: dpb on 4 Feb 2022
A use of "logical indexing", one of the most powerful of MATLAB features, illustrated at <MatrixIndexingByLogicalExpression>.
A==yourvalue is a logical vector true where the values of A match yourvalue, false elsewhere. MATLAB then assigns the RHS to the true locations; ignoring the false positions.
It is the one way one can address an array with 0/1, but the values must be of class logical, not numeric.

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Thorsten
Thorsten on 15 Aug 2016
Edited: Thorsten on 15 Aug 2016
[Aval, ~, indAval] = unique(A);
Define the new values. Values are ordered from the smallest value to replace with to the largest, i.e., to replace 12 with 41, 25 with 26 and 40 with 13 defise Avalnew as
Avalnew = [41; 26; 13];
Anew = Avalnew(indAval);
Anew = reshape(Anew, size(A));
  1 Comment
Javier Cabello
Javier Cabello on 17 Apr 2020
Much appreciated. Very flexible and smart solution!

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BJ Anderson
BJ Anderson on 12 Mar 2019
Edited: BJ Anderson on 12 Mar 2019
The real answer you're looking for is changem:
The syntax looks like this:
B = changem(A,[0 0],[9 8])
where the latter two arguments are vectors, wherein the all elements in the last vector are replaced with their counterparts in the first vector, within data array A.
Syntax
mapout = changem(Z,newcode,oldcode)
Description
mapout = changem(Z,newcode,oldcode) returns a data grid mapout identical to the input data grid, except that each element of Z with a value contained in the vector oldcode is replaced by the corresponding element of the vector newcode.
oldcode is 0 (scalar) by default, in which case newcode must be scalar. Otherwise, newcode and oldcode must be the same size.
Examples
Invent a map:
A = magic(3)
A =
8 1 6
3 5 7
4 9 2
Replace instances of 8 or 9 with 0s:
B = changem(A,[0 0],[9 8])
B =
0 1 6
3 5 7
4 0 2
  1 Comment
BJ Anderson
BJ Anderson on 12 Mar 2019
A quick update on changem:
Sadly, if one inspects the actual code within changem, it functions as a loop. While it is a handy one-liner, it does not have the time-savings of moving from a looped function to an matrix-operation function.

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