i have a char matrix (in matlab) 18x6692 and i want this to be a matrix with 1 row and 6692x18=120450 column.
I'm not able to do this, can you help me?
I also tried with a smaller matrix: from 2x4 to 1x8 with no results.
thank you
Simply use the colon operator and transpose the vector:
A = A(:).'
You can use the reshape function:
B = reshape(A,1,[]);
where A is the input matrix, 1 is the number of rows and [] is to indicate that the number of columns is to be calculated from the number of elements in A.
Note that this stacks all columns of A. If you want to concatenate along the rows, you can do this by transposing A first
B = reshape(A.',1,[]);
Related
I need to know if there is any efficient way of doing the following in MATLAB.
I have several big sparse matrices, the size of each one is roughly 9000000x9000000.
I need to access multiple element of such matrix and assign to each selected element a different value stored in another array. I'll give an example:
What I have:
SPARSE MATRIX of size 9000000x9000000
Matrix with the list of indexes and values I want to access, this is a matrix like this:
[row1, col1, value1;
row2, col2, value2;
...
rowN, colN, valueN]
Where N is the length of such matrix.
What I need:
Assign to the SPARSE MATRIX the corresponding value to the corresponding index, this is:
SPARSE_MATRIX(row1, col1) = value1
SPARSE_MATRIX(row2, col2) = value2
...
SPARSE_MATRIX(rowN, colN) = valueN
Thanks in advance!
EDIT:
Thank you to both for answering, I think I did not explain myself well, I'll try again.
I already have a large SPARSE MATRIX of about 9000000 rows x 9000000 columns, it is a SPARSE MATRIX filled with zeros.
Then I have another array or matrix, let's call it M with N number of rows, where N could take values from 0 to 9000000; and 3 columns. The first two columns are used to index an element of my SPARSE MATRIX, and the third column stores the value I want to transfer to the SPARSE MATRIX, this is, given a random row of M, i:
SPARSE_MATRIX(M(i, 1), M(i, 2)) = M(i, 3)
The idea is to do that for all the rows, I have tried it with common indexing:
SPARSE_MATRIX(M(:, 1), M(:, 2)) = M(:, 3)
Now I would like to do this assignation for all the rows in M as fast as possible, because if I use a loop or common indexing it takes ages (I am using a 7th Gen i7 processor with 16 GB of RAM). And I also need to keep the zeros in the SPARSE_MATRIX.
EDIT 2: SOLVED! Thank you Metahominid, I was not thinking through, but yes the sparse function does solve my problem, I just think my brain circuits were shortcircuited yesterday and was unable to see through it hahaha. Thank you to both anyway!
Regards!
You can construct a sparse matrix like this.
A = sparse(i,j,v)
S = sparse(i,j,v) generates a sparse matrix S from the triplets i, j,
and v such that S(i(k),j(k)) = v(k). The max(i)-by-max(j) output
matrix has space allotted for length(v) nonzero elements. sparse adds
together elements in v that have duplicate subscripts in i and j.
So you can simply construct the row vector, column vector and value vector.
I am answering in part because I cannot comment. You question seems a little confusing to me. The sparse() function in MATLAB does just this.
You can enter your arrays of indices and values directly into the interface, or declare a sparse matrix of zeros and set each individually.
Given your data format make three vectors, ROWS = [row1; ...; rown], COLS = [col1; ...; coln], and DATA = [val1; ... valn]. I am assuming that your size is the overall size of the full matrix and not the sparse portion.
Then
A = sparse(ROWS, COLS, DATA) will do just what you want. You can even specify the original matrix size.
A = sparse(ROWS, COLS, DATA, 90...., 90....).
I want to create a 4 by 4 sparse matrix A. I want assign values (e.g. 1) to following entries:
A(2,1), A(3,1), A(4,1)
A(2,2), A(3,2), A(4,2)
A(2,3), A(3,3), A(4,3)
A(2,4), A(3,4), A(4,4)
According to the manual page, I know that I should store the indices by row and column respectively. That is, for row indices,
r=[2,2,2,2,3,3,3,3,4,4,4,4]
Also, for column indices
c=[1,2,3,4,1,2,3,4,1,2,3,4]
Since I want to assign 1 to each of the entries, so I use
value = ones(1,length(r))
Then, my sparse matrix will be
Matrix = sparse(r,c,value,4,4)
My problem is this:
Indeed, I want to construct a square matrix of arbitrary dimension. Says, if it is a 10 by 10 matrix, then my column vector will be
[1,2,..., 10, 1,2, ..., 10, 1,...,10, 1,...10]
For row vector, it will be
[2,2,...,2,3,3,...,3,...,10, 10, ...,10]
I would like to ask if there is a quick way to build these column and row vector in an efficient manner? Thanks in advance.
I think the question aims to create vectors c,r in an easy way.
n = 4;
c = repmat(1:n,1,n-1);
r = reshape(repmat(2:n,n,1),1,[]);
Matrix = sparse(r,c,value,n,n);
This will create your specified vectors in general.
However as pointed out by others full sparse matrixes are not very efficient due to overhead. If I recall correctly a sparse matrix offers advantages if the density is lower than 25%. Having everything except the first row will result in slower performance.
You can sparse a matrix after creating its full version.
A = (10,10);
A(1,:) = 0;
B = sparse(A);
I have two column matrices:
size(X) = 50 x 1
size(Y) = 50 x 1
which I got from ind2sub
I want to create a structure str such that
str(i).XYZ returns [X(i), Y(i)] for i in [1,50]
I am trying to use
str = struct('XYZ', num2cell([X,Y]));
However, I believe for this to work properly, I need to modify [X,Y] to a matrix of row vectors, each row vector being [X(i), Y(i)]. Not sure if there is a better approach
You are basically on the right way, but num2cell([X,Y]) creates a 2x50 cell, which results in a 2x50 struct. You want to split your input matrix [X,Y] only among the second dimensions so each cell is a 2x1, use num2cell([X,Y],2)
str = struct('XYZ', num2cell([X,Y],2));
Concatenate the two vectors, convert from matrix to cell, and then from cell to struct array:
str = cell2struct(mat2cell([X Y], ones(1,size(X,1)), size(X,2)+size(Y,2) ).', 'XYZ');
A----is 3*1 matrix
B----is 4*4 matrix
C----is 3*1 matrix
the formuale is [A;1]=B*[C;1] from this A needs to be extracted.That is add a row of 1 to matrix C. Then extract the first 3 elements of B*C and store it to matrix A.
what i tried was
A = B*[C;1]([0 1 2])------did not work.
cannot use any other matrices other than A,B and C.
Indexing starts with 1 in matlab, the first three elements are [1,2,3] or [1:3]. You also need to split this into two lines:
t = B*[C;1]
A=t(1:3)
Sorry for the title. I could not think of something better.
I have the following problem.
I have two four-column matrices build up like this:
Property | X | Y | Z
The two matrices have different sizes, since matrix 1 has a large amount of additional rows compared to matrix 2.
What I want to do is the following:
I need to create a third matrix that only features those rows (of the large matrix) that are identical in columns X, Y and Z to rows in matrix2(the property column is always different).
I tried an if-statement but it did not really work out due to my programming syntax. Has somebody a tip?
Thank you!
I tried something like this: (in this case A is the larger matrix and I want its property column for X,Y,Z-positions that are identical to another matrix B.. I am terrible with the MatLab-syntax..
if (A(:,2) == B(:,2) and (A(:,3) == B(:,3) and (A(:,4) == B(:,4))
newArray(:,1) = A(:,1);
end
Use ismember with the 'rows' option to find the desired rows, and then use that as an index to build the result:
ind = ismember(A(:,2:4), B(:,2:4), 'rows');
C = A(ind,:);
I have assumed that a row of A is selected if its last three columns match those of any row of B.