Two basic questions in matlab

Hello all, I used matlab for some time, but this did not made me thorough with basics.

1. If I have two matrices, I1 3x3 and I2 4x4. How I can concatenate them? Is it possible? If I do [I1,I2] it will not work as both have different dimensions.

2. If I have array of 100 random numbers and I want every 4th element starting from 20th location till end, can I do this? I want to do this without any loops.

Any ideas very welcome! Thanks in advance!!

[Adam]

What would you expect to be the result of concatenating a 3*3 matrix with a 4*4 matrix?

Answer 1

[Matt Tearle]

1. It won't let you. Two options: (a) pad with something, such as NaNs, (b) use cell arrays to store each matrix in a separate cell. For (a):

X = [[l1;NaN(1,3)] l2]

2. Use the range (:) operator with a step:

y = x(20:3:end)

(Assuming "every 4th element starting from 20th" you mean 20, 23, 26, etc)

[Image Analyst]

Wouldn't it be

y = x(20:4:end);

to give 20, 24, 28, 32, etc.?

[Matt Tearle]

I initially had that, but then somehow decided that "every 4th element" would mean like a musical interval 4th (thinking of blocks of 4 elements, including the last one). But now that I think about it again, I'm back with you -- x(20:4:end) would make more sense.

I guess Amit can figure out his interpretation and choose accordingly :)

[Amit Kumar]

Yup, I can figure out :) Thanks a lot Matt, Image Analyst!