Concept of the null operator question

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Evan Bakalars 2021-3-4

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编辑： John D'Errico 2021-3-8

This is more of a conceputal question that I need help with.

If I have say sampleMatrix=[1,2,3,4,5] and then I type the code sampleMatrix(3)=[], does that make the matrix now look like [1,2,4,5]?

I ask because I want to know if you do the previously mentioned null operator code, does that now make the third element 4, or is 4 still the fourth element?

Thank you and I apologize if it is confusing.

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the cyclist 2021-3-4

编辑：the cyclist 2021-3-4

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That syntax fully removes the element from the vector. What used to be a length-5 vector is now a length-4 vector. The number 4 is now the 3rd element of that vector,

sampleMatrix = [1 2 3 4 5];
sampleMatrix(3)=[];
idx = find(sampleMatrix==4)
idx = 3

Stephen23 2021-3-5

"...does that make the matrix now look like [1,2,4,5]?"

Yes.

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Answer 1

John D'Errico 2021-3-8

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编辑：John D'Errico 2021-3-8

在 MATLAB Online 中打开

MATLAB does not allow arrays with empty elements. Double precision arrays are always nice rectangular things. So, for example, the array

[1 2 3 [] 5 6]
ans = 1×5
     1     2     3     5     6

is just a vector with 5 elements. The 4th element there is the number 5.

Similarly, you cannot insert an "empty" cell into a vector.

Therefore, the syntax in MATLAB to delete an element of a vector is to set it equal to [], empty brackets.

V = 1:5;
V(4) = []
V = 1×4
     1     2     3     5

Again, the result is a vector of length 4. Everything to the right of that element gets shuffled over.

You can use [] to delete multiple elements at once, thus

V = 1:10;
V(2:2:end) = []
V = 1×5
     1     3     5     7     9

And, for example, the third element of the new vector V is just 5.

V(3)
ans = 5

All of that said, you need to be careful when working with cell arrays, because cell arrays CAN contain empty cells. But for regular flat arrays, [] is just the tool to delete elements.