# Remove value question: if previous row -row>+-3, then remove the row in matrix A

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tengteng QQ on 5 Dec 2021
Commented: Matt J on 6 Dec 2021
Hi, I am a beginner at coding. I would like to know why did my code is not working. I have a matrix A, I would like to subtract every two rows for all my rows in the same matrix, and if the subtract >+-3, then remove the row.
A = [1;9;5;4;3;6;764;465;6;8;7;5;3;6;8;8;56;6;4;3;6;8;2;4;6;8;9;22;4]
example:
if row(2,1)-row(1,1)>(+-3) then I want to remove row(1,1) in matrix A
i try to code this:
A = [1;9;5;4;3;6;764;465;6;8;7;5;3;6;8;8;56;6;4;3;6;8;2;4;6;8;9;22;4]
for j = 1:size(A)
if A(j+1,1)-A(j,1)>+-3
A(j,1)=[]
end
end

Matt J on 5 Dec 2021
Edited: Matt J on 5 Dec 2021
A(diff(A)>-3)=[]
or
A(abs(diff(A))<3)=[];
if that's what you really meant.
##### 2 CommentsShowHide 1 older comment
Matt J on 6 Dec 2021

Jan on 5 Dec 2021
• for j = 1:size(A)
Remember, that size() replies a vector containing the size of all dimensions. In the expression 1:size(A) only the first element of size(A) is used. Better:
for j = 1:numel(A) % or size(A, 1)
A 2nd problem: You check A(j+1). Then j is not allowed to be numel(A), but the loop must stop one element before.
• if A(j+1,1)-A(j,1)>+-3
The expression "+-" is not defined in Matlab. I assume you want:
if abs(A(j+1,1) - A(j,1)) > 3
• A(j,1)=[]
After you have deleted one element, the original array is shorter. Then the loop must fail, because elements after the last one are tested.
I assume you want:
A = [1;9;5;4;3;6;764;465;6;8;7;5;3;6;8;8;56;6;4;3;6;8;2;4;6;8;9;22;4]
delete = false(size(A));
for j = 1:numel(A) - 1
delete(j + 1) = abs(A(j+1,1) - A(j,1)) > 3;
end
A(delete) = [];
tengteng QQ on 6 Dec 2021
Your expanlation is so clear!!Thank you so much!!