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Replacing values in a Matrix

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gooniyath
gooniyath 2016-8-15
回答： Stephen23 2023-11-5
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
  2 个评论
Nilesh
Nilesh 2023-11-5
give a single matlab commandthat will overwritethe 3rd entryin vectorB with 12
DGM
DGM 2023-11-5
B(3) = 12;
If you need to resort to copy-pasting such a simple question, then good luck writing anything.

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回答（4 个）

dpb
dpb 2016-8-15
A(A==yourvalue)=NewValue;
  5 个评论
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Yogya Chawla
Yogya Chawla 2022-2-4
can you explain this syntax
dpb
dpb 2022-2-4
编辑：dpb 2022-2-4
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 2016-8-15
编辑：Thorsten 2016-8-15
[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));
BJ Anderson
BJ Anderson 2019-3-12
编辑：BJ Anderson 2019-3-12
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
  1 个评论
BJ Anderson
BJ Anderson 2019-3-12
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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Stephen23
Stephen23 2023-11-5
Ran in:
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

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