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My code on n consecutive elements in a matrix

Asked by Qingchuan Lyu on 11 Jul 2018
Latest activity Edited by Matt J
on 11 Jul 2018
Hi, I got a question to figure out the largest sum of n consecutive elements in a matrix. The order can be in the same row, column, diagonal, or reverse diagonal. Here is my code. However, the matlab keeps telling me variables "i","j", "u", "w","z" may be unused.Could you give me a hint how to make these variables valid? Also, I guess there is a way to simply my code a little bit? Thanks!
A numerical example would be: Suppose A=[1 2; 3 4] and n=2. We need to find the largest sum of 2 consecutive elements in all directions. In this case, we have
rows: 1+2, 3+4
columns: 1+3, 2+4
diagonal: 1+4
reverse diagonal: 2+3
Obviously, the output of this function should be 3+4. How could I write a function without loops? Any hint will be appreciated.
% function x=maxsum(A, n)
[p,q]=size(A);
k=zeros(1,q*floor(p/n));
%rows
for i =1:p
j=1:q-n+1;
u=1:q*floor(p/n);
k(u)=sum(A(i,j:j+n-1));
i=i+1;
j=j+1;
u=u+1;
end
K=max(k);
%columns
y=zeros(1,p*floor(q/n));
for w=1:q-n+1
v=1:q;
z=1:p*floor(q/n);
y(z)=sum(A(w:w+n-1, v);
w=w+1;
v=v+1;
z=z+1;
end
Y=max(y);
%diagonal
t=zeros(1,p-n+1);
for h=1:p-n+1
B=A(h:h+n-1,h:h+n-1);
t(h)=diag(B);
h=h+1;
end
T=max(t);
%reverse diagonal
m=zeros(1,p-n+1);
for o=1:p-n+1
c=flip(A(o:o+n-1,o:o+n-1));
m(o)=diag(c);
o=o+1;
end
M=max(m);
%compare all
x=max([M T Y K]);
% end

  3 Comments

You are changing the looping variable inside the loop itself. In a MATLAB for, you need not increment the looping variable inside the loop, it is done automatically.
In addition, you have written j=1:q-n+1; and also, k(u)=sum(A(i,j:j+n-1));. This is inconsistent as j is a vector.
KSSV
on 11 Jul 2018
Your loop index is replaced with same variable inside loop. This is not good code. YOu need to rethink. Give us a numerical example with data, you will get help and this task can be achieved without loop.
@KSSV A numerical example would be: Suppose A=[1 2; 3 4] and n=2. We need to find the largest sum of 2 consecutive elements in all directions. In this case, we have
rows: 1+2, 3+4
columns: 1+3, 2+4
diagonal: 1+4
reverse diagonal: 2+3
Obviously, the output of this function should be 3+4. How could I write a function without loops? Any hint will be appreciated.

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1 Answer

Answer by Matt J
on 11 Jul 2018
Edited by Matt J
on 11 Jul 2018
 Accepted Answer

To avoid loops, use convolution. For example
forwardDiags = conv2(yourMatrix,eye(3),'valid')
will compute all forward diagonal sums of length 3. Similarly,
reverseDiags = conv2(yourMatrix,fliplr(eye(3)),'valid')
will compute reverse diagonals, and so on.

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