## What does x(y) mean ?

### Jivitesh (view profile)

on 10 Jul 2013

If x and y are 3*3 matrices . then what'll be x(y) ?

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### Wayne King (view profile)

on 10 Jul 2013

I'm assuming that is just the composition of the two linear operators, in which case that is matrix multiplication. In other words, the definition of matrix multiplication comes from viewing the two matrices (x and y) as linear operators and composing them.

If it is something more than that, then I think we need more context.

### Jan Simon (view profile)

on 10 Jul 2013

Then y contains either linear or logical indices:

```x = reshape(1:9, 3, 3);
y = x;
x(y)   % same as x
```
```y = ones(3, 3);
x(y)   % Repeated first element of x
```
```y = true(3, 3);
y(2,2) = false;
x(y)   % All elements without the central value
```
```y = x + 5;
x(y)   % ERROR, the linear index must be 0 < index <= numel(x)
```

Jivitesh

### Jivitesh (view profile)

on 10 Jul 2013

if x and y are of same order then, for k=1:3 x(y~=k)=0; end what'll this code mean. found such code in color segmentation by k means example

Jan Simon

### Jan Simon (view profile)

on 10 Jul 2013

What does "of same order" mean here?

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