# I want to create a n by 1 matrix. The FDF2=2 is on the top row, while BDF2=3 is on the bottom. And the values of the middle row should all equal to 3. How do I input the values in such matrix?

3 views (last 30 days)
kingsley on 17 Apr 2017
Commented: John D'Errico on 19 Apr 2017
Should I create a zeros(n+1,1) matrix first?
##### 0 CommentsShow -2 older commentsHide -2 older comments

Sign in to comment.

### Accepted Answer

John D'Errico on 17 Apr 2017
Edited: John D'Errico on 17 Apr 2017
I'm not sure why you would initially create a VECTOR of length n+1, if your final goal is an nx1 vector. :)
No. There is no need to preallocate the vector with zeros, although your mentioning that idea is the only reason I was willing to answer your question. There are lots of ways to create this vector though. A simple one is:
V = [FDF2;repmat(3,n-2,1);BDF2];
An alternative is to preallocate a vector, then stuff the first and last elements.
V = ones(n,1)*3;
V([1 end]) = [FDF2,BDF2];
Lots of other ways to do this, but that covers the first ones I might think of. You can see that the first solution involves concatenation of the indicated elements, whereas the second had me create a vector of the final size, then stuffing the first and last elements as desired. Either is fine.
##### 2 CommentsShow NoneHide None
kingsley on 17 Apr 2017
Edited: kingsley on 17 Apr 2017
got it! thank you. I'm new to mathlab. So the question I ask might sound stupid. But what if the value on the on the middle rows. And it has to be 2*fi, where i is from 1 to n-1, and the function of f=sin(i)? Do I need to write a loop in that case ?
John D'Errico on 19 Apr 2017
Learn to think in terms of vectors. What is
1:(n-1)
This is a row vector. But if you want a column vector, then what does this do?
(1:(n-1))'
Can you compute the sine of ALL of those values in one call to sin? Of course you can.
Think at a high level, in terms of vectors & arrays, not in terms of loops.

Sign in to comment.

### Categories

Find more on Matrix Indexing in Help Center and File Exchange

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!