Replacing values in a Matrix

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

[Nilesh]

give a single matlab commandthat will overwritethe 3rd entryin vectorB with 12

[DGM]

B(3) = 12;

If you need to resort to copy-pasting such a simple question, then good luck writing anything.

Answer 1

[dpb]

A(A==yourvalue)=NewValue;

[Thorsten]

This does work for a single value. But it fails if you want to swap values, like replacing all 25's with 12's and vice versa; or if some of the NewValues is the same as one of the old values and the order of replacements is not carefully chosen to be correct.

[jony nasir]

Thanks for the solution !!

[Cory Cress]

This is great, thanks!

[Yogya Chawla]

can you explain this syntax

[dpb]

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.

Answer 2

[Thorsten]

 [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));

[Javier Cabello]

Much appreciated. Very flexible and smart solution!

Answer 3

[BJ Anderson]

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.

(From https://www.mathworks.com/help/map/ref/changem.html:)

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

[BJ Anderson]

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.

Answer 4

[Stephen23]

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]

A = 11×10

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

old = [12,25,40];
new = [99,23,42];
B = interp1(old,new,A)

B = 11×10

23 42 42 42 23 23 23 23 23 23 23 42 42 42 42 23 23 23 23 23 23 23 42 42 42 42 23 23 23 23 23 23 42 42 42 42 42 23 23 23 23 23 23 42 42 42 42 42 23 23 23 23 23 42 42 42 42 42 42 23 23 23 99 99 42 42 42 42 42 42 23 99 99 99 99 42 42 42 42 42 99 99 99 99 99 99 42 42 42 42 99 99 99 99 99 99 23 42 42 42