finding an output from a matrix using a single syntax

Hi there guys I have a question. How do you get a single line output out of a matrix? for example here is the matrix:

Q =

    10    20    30    40    50    60    70
     8     9    10    11    12    13    14
    33    30    27    24    21    18    15
    28    35    42    49    56    63    70
    36    45    54    63    72    81    90
    -1    -2    -3    -4    -5    -6    -7
    64    69    74    79    84    89    94

and I am required to find:

R = [33 9 30 74 -4 72 63 15]

how do I get it using a single syntax? I've been struggling since I am just new into using Matlab. please help me...

[Fangjun Jiang]

What is the logic? How do you make the selection?

Answer 1

[Paulo Silva]

R=[Q(3:6:18) Q(21:6:45)]

Edit, there's one shorter version

R=Q([3:6:18 21:6:49])

[Andrei Bobrov]

+1

[Dwyane Wade]

how did you get this?

[Paulo Silva]

Please read what I said in your other similar question and also http://www.mathworks.in/help/techdoc/math/f1-85462.html

Answer 2

[Dwyane Wade]

Im not sure what the logic is, all I know is that we are required to find R = [33 9 30 74 -4 72 63 15] using a single syntax like: R = (solution) << something like this

[Paulo Silva]

The values are clearly in diagonals / , see my answer.

Answer 3

[Fangjun Jiang]

There is a way to select the element using logical index. It is like this:

Ind=false(size(Q));
Ind(3,1)=true;
Ind(2,2)=true;
Ind(1,3)=true;
Ind(7,3)=true;

R=Q(Ind)

You'll get the idea.

Or linear indexing

LinInd=[1 12 20 40];
S=R(LinInd)

Answer 4

[Andrei Bobrov]

a=spdiags(Q(end:-1:1,:))
a1=a(:,[3,9])
out=a1(a1~=0)

[Andrei Bobrov]

about logic:

a=spdiags(Q(end:-1:1,:))

a1=a(:,[0,size(Q,1)-1]+3)

out=a1(a1~=0)