So I have a matrix X of size m,10 that is initialized to all zeros.
then I have a vector of size m,1 that contains digits from 1 to 10
what I want to do (hopefully in a single operation with no loops), is for each row of the matrix X and vector y, I want to put a '1' in the column indexed by the value written in the row of vector y.
Here's what I want with a small example: X = [0 0 0; 0 0 0; 0 0 0]; lets say y= [3; 2; 1];
then I would expect the operation to return X = [0 0 1; 0 1 0; 1 0 0]
Do you have a command that can do that easily ?
X(sub2ind(size(X),y',1:numel(y)))=1
or
X((0:numel(y)-1)*size(X,2) + y')=1
Related
I have two matrices. One is of size 1,000,000 x 9 and the other is 500,000 x 9.
The columns have the same meaning and the first 7 columns have the function of a key. Correspondingly, the last two columns have data character. There are many overlapping key values in both of the matrices and I would like to have a big matrix to compare the values. This big matrix should be of dimension 1,000,000 x 11.
For example:
A = [0 0 0 0 0 0 0 10 20; 0 0 0 0 0 0 1 30 40];
B = [0 0 0 0 0 0 0 50 60];
A merged matrix would look like this:
C = [0 0 0 0 0 0 0 10 20 50 60; 0 0 0 0 0 0 1 30 40 0 0];
As you can see, the first row of C has columns 8, 9 from matrix A and columns 10,11 from matrix B. The second row uses the columns 8, 9 from matrix A and 0,0 for the last to columns because there is no corresponding entry in matrix B.
I have accomplished this task theoretically, but it is very, very slow. I use loops a lot. In any other programming language, I would sort both tables, would iterate both of the tables in one big loop keeping two pointers.
Is there a more efficient algorithm available in Matlab using vectorization or at least a sufficiently efficient one that is idiomatic/short?
(Additional note: My largest issue seems to be the search function: Given my matrix, I would like to throw in one column vector 7x1, let's name it key to find the corresponding row. Right now, I use bsxfun for that:
targetRow = data( min(bsxfun(#eq, data(:, 1:7), key), [], 2) == 1, :);
I use min because the result of bsxfun is a vector with 7 match flags and I obviously want all of them to be true. It seems to me that this could be bottleneck of a Matlab algorithm)
Maybe with ismember and some indexing:
% locates in B the last ocurrence of each key in A. idxA has logicals of
% those keys found, and idxB tells us where in B.
[idxA, idxB] = ismember(A(:,1:7), B(:,1:7),'rows');
C = [ A zeros(size(A, 1), 2) ];
C(idxA, 10:11) = B(idxB(idxA), 8:9); % idxB(idxA) are the idxB != 0
I think this does what you want, only tested with your simple example.
% Initial matrices
A = [0 0 0 0 0 0 0 10 20;
0 0 0 0 0 0 1 30 40];
B = [0 0 0 0 0 0 0 50 60];
% Stack matrices with common key columns, 8&9 or 10&11 for data columns
C = [[A, zeros(size(A,1),2)]; [B(:,1:7), zeros(size(B,1),2), B(:,8:9)]];
% Sort C so that matching key rows will be consecutive
C = sortrows(C,1:7);
% Loop through rows
curRow = 1;
lastRow = size(C,1) - 1;
while curRow < lastRow
if all(C(curRow,1:7) == C(curRow+1,1:7))
% If first 7 cols of 2 rows match, take max values (override 0s)
% It may be safer to initialise the 0 columns to NaNs, as max will
% choose a numeric value over NaN, and it allows your data to be
% negative values.
C(curRow,8:11) = max(C(curRow:curRow+1, 8:11));
% Remove merged row
C(curRow+1,:) = [];
% Decrease size counter for matrix
lastRow = lastRow - 1;
else
% Increase row counter
curRow = curRow + 1;
end
end
Answer:
C = [0 0 0 0 0 0 0 10 20 50 60
0 0 0 0 0 0 1 30 40 0 0]
I have a matrix A with a row of 100 values. When I do
B=A(A>=0);
My new matrix only has 50 values and I can't plot it anymore, because I need to specifically plot 100 values. How would I keep the placement of the empty values at 0?
Example:
A= [1 -1 2 -2 3 -3]
B would have to be
B = [1 0 2 0 3 0]
B = A;
B(A < 0) = 0;
A < 0 will return a a binary array [0 1 0 1 0 1] for your example. Calling B(A < 0) = 0 will set all positions of B with a 1 in A < 0 to 0.
Is there a vectorization way of returning the index of the last K nonzero elements of each row of a matrix?
For example, my matrix only contains 0 and 1 and the last column of each row is always 1. Then I want to find the index of the last K, where K>1, nonzero elements of each row. If a row only has M (less than K) nonzero elements, then the index for that row is just the index of the last M nonzero element. e.g.
A = [0 1 0 1;
1 1 0 1;
1 1 1 1;
0 0 0 1]
And my K = 2, then I expected to return a matrix such that
B = [0 1 0 1;
0 1 0 1;
0 0 1 1;
0 0 0 1]
Namely B is originally a zero matrix with same shape as A, then it copies each row of A where the corresponding column starts from the index of the last K non-zero element of the row of A (and if in one row of A there is only M < K non-zero element, then it starts from the index of the last M non-zero element of that row of A)
Knowing that elements are only 0 or 1, you can make a mask using cumsum on the flipped matrix A and throw away values with a cumulative sum greater than k:
A = [0 1 0 1;1 1 0 1;1 1 1 1;0 0 0 1]
k = 2;
C = fliplr(cumsum(fliplr(A), 2)); % take the cumulative sum backwards across rows
M = (C <= k); % cumsum <= k includes 0 elements too, so...
B = A .* M % multiply original matrix by mask
As mentioned in the comments (Thanks #KQS!), if you're using a recent version of MATLAB, there's a direction optional parameter to cumsum, so the line to generate C can be shortened to:
C = cumsum(A, 2, 'reverse');
Results:
A =
0 1 0 1
1 1 0 1
1 1 1 1
0 0 0 1
B =
0 1 0 1
0 1 0 1
0 0 1 1
0 0 0 1
knowing that find function can get indices of last k elements, we can use bsxfun to apply find to rows of a matrix to find which element in each row satisfy the condition. find again used to extract rows and columns of nonzero elements of the resultant matrix, so reducing size of data and complexity of operations. then save the result to a sparse matrix then convert to full matrix:
A = [0 1 0 1;
1 1 0 1;
1 1 1 1;
0 0 0 1]
k = 2;
[row , col]= size(A);
last_nz = bsxfun(#(a,b)find(a,b,'last'),A',(repmat(k, 1, row))); %get indices of last two nonzero elements for each row
[~,rr,cc]=find(last_nz); %get columns and rows of correspondong element for whole matrix
B = full(sparse(rr,cc,1));
I have a vector with alternating 0's and 1's and would like to convert each "1" to the length of the zeros that precede it. For example, I have x and would like to get to y:
x = [0 0 1 0 0 0 0 1 0 0 0 1 1 0 0 1]
y = [0 0 2 0 0 0 0 4 0 0 0 3 0 0 0 2]
I would really appreciate any suggestions on how to achieve this.
One approach with find & diff -
%// Initialize array, y with zeros and of length same as input, x
y = zeros(size(x))
%// Find places/indices where new values would be put
idx = find(x)
%// Calculate new values which would be the differentiated values of indices
%// and subtracted by 1 to account for the number of zeros in between two
%// non-zero values. We need to concatenate the indices array with one zero
%// at the start to account for the starting non-zero value in x
y(idx) = diff([0 idx])-1
I would like to display the coordinates of a matrix in terms of X and Y. For example
if matrix = [ 0 0 5 0; 0 0 1 0; 0 0 0 1; 0 0 0 0]
Say, i want the coordinate for 5...how can i write a code that says 5 = 1x and 3 y.
I don't want to display the element in the matrix, just the coordinates of that element.
use find
[y x] = find( matrix ~= 0 ); % gives you the x y coordinates of all non-zero elements
Note the order of y and x, since Matlab is indexing using row-column.