Concept of the null operator question

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.

[the cyclist]

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]

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

Yes.

Answer 1

[John D'Errico]

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.

Answer 2

[Prudhvi Peddagoni]

Hi,

As seen in this answer , it does remove the 3rd element.

Hope this helps.