I have a 30 x 30 matrix, called A, and I want to assign B as the leftmost 30 x 20 block of A how can I do that?
Is this the correct way to do it?
B = A[30 ; 20]
No the correct way is
B = A(:, 1:20);
where : is shorthand for all of the rows in A.
Matrix indexing in MATLAB uses round brackets, (). Square brackets, [], are used to declare matrices (or vectors) as in
>> v = [1 2 3; 4 5 6; 7 8 9]
v =
1 2 3
4 5 6
7 8 9
excaza provides a very good link on Matrix Indexing in MATLAB which should help you. There is also Matrix Indexing.
A_new = A(:,1:20)
takes all the rows from A with this part A(:,) and the first 20 columns with this part A(,1:20)
A_newis now 30x20
You can also iterate over elements in two loops, but the above answer is easiest
Lets say I have matrix such that A(:,:,1)=[1,2,3;2,3,4], A(:,:,2)=[3,4,5;4,5,6].
How is the easiest way of accessing and plotting the vectors (1,2,3),(2,3,4),(3,4,5),(4,5,6). I tried creating B=[A(:,:,1);A(:,:,2)], but i need a procedure to arbitrary number of A's.
Hope this isn't trivial and I've formulated myself satisfactory.
You should think 'vertically'. This will allow you to use colon indexing:
>> A(:,:,1) = [1,2,3;2,3,4].'; %'// NOTE: transpose of your original
>> A(:,:,2) = [3,4,5;4,5,6].'; %'// NOTE: transpose of your original
>> A(:,:)
ans =
1 2 3 4
2 3 4 5
3 4 5 6
The colon indexing with two colons works for any dimension A:
>> A(:,:,:,:,1,1) = [1 2 3; 2 3 4].'; %'
>> A(:,:,:,:,2,1) = [3 4 5; 4 5 6].'; %'
>> A(:,:,:,:,1,2) = [5 6 7; 6 7 8].'; %'
>> A(:,:,:,:,2,2) = [7 8 9; 8 9 0].'; %'
>> A(:,:)
ans =
1 2 3 4 5 6 7 8
2 3 4 5 6 7 8 9
3 4 5 6 7 8 9 0
Colon indexing in MATLAB is quite interesting and really powerful once you master it. For example, if you use fewer colons than there are dimensions in the array (like above), MATLAB will automatically concatenate the remainder of the data along the dimension equal to the colon count.
So, if A has 48 dimensions, but you index with just 2 colons: you'll get a 2D array, that is the concatenation of the remaining 46 dimensions along the 2nd dimension.
In general: if A has N dimensions, but you index with just M ≤ N colons: you'll get an M-D array, that is the concatenation of the remaining N-M dimensions along the Mth dimension.
So as long as you are free to define your A to contain vectors on the columns rather than the rows (you should advise everyone to do this, as virtually everything in MATLAB is a bit faster that way), I think this is the fastest and most elegant way to do what you want.
If not, well, then just reshape like Dan :)
Assuming the order does not matter, here is how you can do it for vectors of length 3:
B = reshape(shiftdim(A,2), [], 3)
plot(B')
For vectors of arbitrary dimensions, replace 3 by size(A,2)
I have a 3*3 matrix A
A = [1 2 3
4 5 6
7 8 9];
I want to duplicate only the first row and column of this matrix. It should look like
1 1 2 3
1 1 2 3
4 4 5 6
7 7 8 9
can someone tell how can i do this in matlab
I think this is a good way just using indexing
A([1, 1:end], [1, 1:end])
You can do that by concatenating different parts of the original matrix:
B=[A(1) A(1,:);A(:,1) A];
In this expression A(1) is the top left element of A, A(1,:) is the first row and A(:,1) is the first column.
See the documentation on the colon operator.
In the code below, A is your starting point and I believe E is what you want to achieve.
You can of course combine all the intermediate expressions to achieve the final result in one step.
A= [1 2 3; 4 5 6; 7 8 9]
B= A(1:3,1:1)
C= [B A]
D= C(1:1,1:4)
E= [D;C]
A bit late in the game, but worthwhile answering. You can use padarray for that :
B = padarray(A,[1 1],'replicate','pre')
It's a one liner and more generic if you want to add more than just a single first and column ...
I would like to ask a question about Matlab program.
I have vector a
a = [1 2 3 4 5 6 7 8 9 10 11 12];
I would like to convert vector a to 2D array. Normally, I use this code to convert it.
m =1;
for i=1:4
for j=1:3
b(i,j) = a(m);
m=m+1;
end
end
Then b is a 2D matrix.
b =
1 2 3
4 5 6
7 8 9
10 11 12
Anybody, have an idea to convert 1D to 2D without using loop.
Thanks,
Check out the reshape function and help page.
In particular,
B = reshape(A,m,n)
returns the m-by-n matrix B whose elements are taken column-wise from A. An error results if A does not have m*n elements.
Note that it is column-wise, so I suggest you make a matrix with 3 rows and 4 columns and then tip it on its side (A.' will take the transpose of a matrix).
Given a vector of the counter-diagonals of a matrix in matlab, is there an easy way to reconstruct the matrix?
For example, given
x = [1 2 3 4 5 6 7 8 9]
is there any easy way to reconstruct it to the following?
1 2 4
3 5 7
6 8 9
This is made slightly easier by the fact that the dimensions of the original block are known. Reconstructing a rotation or transposition of the original matrix is fine, since rotating and transposing are easy to undo. Faster is better, this calculation has to be done on many xs.
Thanks!
You can create the corresponding Hankel matrix and use it for sorting (works only if the output is a square matrix!):
x = [1 2 3 4 5 6 7 8 9];
%# find size of output (works only with square arrays)
n=sqrt(length(x));
%# create Hankel matrix
hh = hankel(1:n,n:(2*n-1));
%# sort to get order of elements (conveniently, sort doesn't disturb ties)
[~,sortIdx]=sort(hh(:));
%# reshape and transpose
out = reshape(x(sortIdx),n,n)'; %'# SO formatting
out =
1 2 4
3 5 7
6 8 9