Subtracting values in a matrix

Asked by Jakub Nosek

Jakub Nosek (view profile)

on 6 Nov 2017
Latest activity Commented on by Jakub Nosek

Jakub Nosek (view profile)

on 6 Nov 2017
Hi, can I just ask you why the code below does not work? I am trying to subtract each value in a row from each other and if the condition is met then add 1 to matrix11(i,j). Apparently there's a problem with matrix10(i,:) expression.
matrix10=[1 7 20; 4 5 6; 7 8 9; 2 5 8];
[zk1, zk2] = size(matrix10);
matrix11=zeros(zk1,zk2);
for i=1:zk1
for j=1:zk2
if (abs(matrix10(i,j) - matrix10(i,:)) >= 2)
matrix11(i,j)=matrix11(i,j)+1;
end
end
end
matrix11 =
0 0 0
0 0 0
0 0 0
0 0 0

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Answer by Kaushik Lakshminarasimhan

Kaushik Lakshminarasimhan (view profile)

on 6 Nov 2017

The problem is with this expression in the if statement:
if abs(matrix10(i,j) - matrix10(i,:)) >= 2
The left hand side is a vector but right side is a scalar -- which means that the condition is only met if all 3 differences exceed 2. What is the actual condition you want to check? If it is whether the total sum of differences exceeds 2, then you need to change the statement to:
if sum(abs(matrix10(i,j) - matrix10(i,:))) >= 2
Or it could be something else depending on what your condition actually is.

Jakub Nosek

Jakub Nosek (view profile)

on 6 Nov 2017
Cool, thanks, this works fine. But what if I wanted to add another condition and to put the numbers into matrix11 only if both conditions are met. Let's say the second condition is for example abs(matrix12(i,j) - matrix12(i,:)) <=5. Would I need to use IF in this case or do I just define isTrue2?
Kaushik Lakshminarasimhan

Kaushik Lakshminarasimhan (view profile)

on 6 Nov 2017
If matrix10 and matrix12 have the same number of columns, then you won't need an if statement.
You can do something like:
isTrue1 = (abs(matrix10(i,j) - matrix10(i,:)) >= 2);
isTrue2 = (abs(matrix12(i,j) - matrix12(i,:)) <= 5);
isTrue = (isTrue1 & isTrue2); % if both must be true
Jakub Nosek

Jakub Nosek (view profile)

on 6 Nov 2017
Great! Thanks a lot!!