MATLAB Answers

Assign null to rows of a matrix

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Benedict Teng
Benedict Teng on 20 Sep 2019
Commented: Benedict Teng on 20 Sep 2019
Hi guys,
I wanted to assign increment rows of null to a variable "C". The variable C has a size of 138x6x69. My code is shown below.
a = 1;
for i = 1:69
C(1:a,:,i)=[];
a = a+2;
end
But the error was " A null assignment can have only one non-colon index.A null assignment can have only one non-colon index."
Can anyone teach me another way of modifying the code? I really need help.
Much Thanks
Ben

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Accepted Answer

Raj
Raj on 20 Sep 2019
You cannot assign 'null' like this to a 3D matrix while wanting to maintain the dimensions of the matrix. You can assign zero or NaN depending on what you want to achieve here.

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Raj
Raj on 20 Sep 2019
"matrix size change every time after I assign null the rows, and it makes the code more complicated" - Yes that is expected. Matrix dimension will change everytime you assign null to a row because you are effectively removing that row. You can do something like this to avoid this complication: Instead of assigning 'null', assign zero and then from the final matrix remove all rows which are all zeros.
Stephen Cobeldick
Stephen Cobeldick on 20 Sep 2019
"but it seems like the matrix size change every time after I assign null the rows"
Because what you are doing is removing rows:
MATLAB does not have a "null" concept, even though you keep using this term. You can't "assign null" as you write, because "null" does not exist.
Can you suggest any better way to do this?
Collect all of the required indices into a vector, use it to remove all rows at once after the loop.
Benedict Teng
Benedict Teng on 20 Sep 2019
I've got it, thanks for the advices guys. Really appreciate it.

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