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

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

However, it is not working...please help me confirm the following information. I am appreciative of your help!!

Answer 1

[Matt J]

A(diff(A)>-3)=[]

or

A(abs(diff(A))<3)=[];

if that's what you really meant.

[tengteng QQ]

thank you for your help!! I really appreciative it!!

[Matt J]

You're welcome, but please Accept-click the answer if it solved your problem.

Answer 2

[Jan]

Your code contains some problems:

• 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]

Your expanlation is so clear!!Thank you so much!!