getting a matrix out of a matrix

Hi there guys I have a question. How do you get an 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:

S=

   20 40 60

   9  11 13

   30 24 18

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

Answer 1

[Andrei Bobrov]

Q(1:3,[2 4 6])

[Sean de Wolski]

Q(1:3,2:2:6)

[Paulo Silva]

Almost identical question from the same person here http://www.mathworks.com/matlabcentral/answers/13650-finding-an-output-from-a-matrix-using-a-single-syntax

[Dwyane Wade]

may I know how did you get this?

[Walter Roberson]

Experience.

[Paulo Silva]

example Q=[1 3;2 4] , I choose the numbers so they are equal to the index positions:

Q(1) is 1

Q(2) is 2

Q(3) is 3

Q(4) is 4

MATLAB allows you to create vectors like this

1:3

ans=

1 2 3

You can use those vector to select certain index values like this:

Q(1:3) % the same as Q([1 2 3])

ans=

1 2 3

Another way to do it is with row and column numbers, notice the comma (,)

Q(1,2) is 3

Q(1,1) is 1

Other things to remember

The word end

Q(end) is the value of the last index, in this case is 4

The :

Q(:) means all values of Q, they appear in rows

The size function

size(Q) is 2 2 this means two rows and two columns

Q(1:2:end) in this case gives 1 3 the meaning is all values at index 1 until the end, the 2 is your step, try 1:2:4

Please read the documentation http://www.mathworks.in/help/techdoc/math/f1-85462.html