This related question How can I apply a function to every row/column of a matrix in MATLAB? seems to indicate one way to do this is using num2cell, which I kind of want to stay away from.
Here's what I want to do. I've got an index list for a triangle mesh, the indices index the vertex list.
I want to run func(a,b,c) on the first 3 indices, then the next three indices, and so on.
So I could reshape(idxs,3,[]) so now i've got my data into triplets as column vectors. But arrayfun does not do what I want it to do.
Looking for something like a column-map operator.
First, get your func properly vectorized, if necessary, such that the arguments can be lists of equal length:
vec_func = #(a,b,c)(arrayfun(#func,a,b,c))
Then, you can directly access every third element of idxs:
vec_func( idxs(1:3:end), idxs(2:3:end), idxs(3:3:end) )
Related
I have a row cell vector M, containing matrices in each cell. Every matrix m (matrix inside the big matrix M) is made of 2 channels (columns), of which I only want to use the first.
The approach I thought about was going through each m, check if it has 2 channels, and if that is the case delete the second channel.
Is there a way to just slice it in matlab? or loop it and obtain the matrix M as the matrix m would disappear.
First code is:
load('ECGdata.mat')
I have the below.
when I double-click in one of the variable , here is what I can see:
As you can see the length of each matrix in each cell is different. Now let's see one cell:
The loop I'm trying to get must check the shape of the matrix (I'm talking python here/ I mean if the matrix has 2 columns then delete the second) because some of the variables of the dataframe have matrix containing one column (or just a normal column).
In here I'm only showing the SR variable that has 2 columns for each matrix. Its not the case for the rest of the variables
You do not need to delete the extra "channel", what you can do is quite simple:
newVar = cellfun(#(x)x(:,1), varName, 'UniformOutput', false);
where varName is SR, VF etc. (just run this command once for each of the variables you load).
What the code above does is go over each element of the input cell (an Nx2 matrix in your example), and select the first column only. Then it stores all outputs in a new cell array. In case of matrices with a single column, there is no effect - we just get the input back.
(I apologize in advance if there is some typo / error in the code, as I am writing this answer from my phone and cannot test it. Please leave a comment if something is wrong, and I'll do my best to fix it tomorrow.)
Might sound too simple to you but I need some help in regrad to do all folowings in one shot instead of defining redundant variables i.e. tmp_x, tmp_y:
X= sparse(numel(find(G==0)),2);
[tmp_x, temp_y] = ind2sub(size(G), find(G == 0));
X(:)=[tmp_x, tmp_y];
(More info: G is a sparse matrix)
I tried:
X(:)=ind2sub(size(G), find(G == 0));
but that threw an error.
How can I achieve this without defining tmp_x, tmp_y?
A couple of comments with your code:
numel(find(G == 0)) is probably one of the worst ways to determine how many entries that are zero in your matrix. I would personally do numel(G) - nnz(G). numel(G) determines how many elements are in G and nnz(G) determines how many non-zero values are in G. Subtracting these both would give you the total number of elements that are zero.
What you are doing is first declaring X to be sparse... then when you're doing the final assignment in the last line to X, it reconverts the matrix to double. As such, the first statement is totally redundant.
If I understand what you are doing, you want to find the row and column locations of what is zero in G and place these into a N x 2 matrix. Currently with what MATLAB has available, this cannot be done without intermediate variables. The functions that you'd typically use (find, ind2sub, etc.) require intermediate variables if you want to capture the row and column locations. Using one output variable will give you the column locations only.
You don't have a choice but to use intermediate variables. However, if you want to make this more efficient, you don't even need to use ind2sub. Just use find directly:
[I,J] = find(~G);
X = [I,J];
I have looked around for an answer to this but could not see anything relevant.
I am timing some (Matlab) code and there is one part which takes up a large amount of time. I have a vector, V, of integers (that may contain repeats) and I have a much smaller vector, v, contain indices of elements that I wish to delete. I am currently implementing this as:
V(v) = [];
'V' is something that I am selecting indices from at random and without replacement. 'v' must not contain duplicates.
Any suggestions?
I have a (5160 X 4) matrix.
I want to extract just (1,1),(41,1),(81,1),(121,1)........ in a uniform interval, only from the first column of the matrix.
Assuming that data is your matrix, you can do this:
A = data(1:40:5160,1);
The 1:40:5160 will create an array such that it starts at 1, and goes up in increments of 40 as much as possible up until 5160. Once you create 1:40:5160, you can use this array and access the corresponding rows, and you are accessing the first column using the index of 1 for the second parameter. Actually, the last row that gets extracted is 5121. We aren't able to go up to 5161 due to the fact that your matrix has 5160 rows, and we have also specified 5160 as the ending of the indexing.
NB: This is very basic MATLAB syntax. Any standard MATLAB tutorial should teach you this.
I am getting stock prices from yahoo, and want to have each stock have its own time series data structure, but also don't want to have hundreds of variables, so naturally I would want to have an array, but when I do something like array = [stock1 stock2]; it actually merges the series together. How can I make a real array?
Thanks,
CP
[x x] notation in matlab is not an array, it is a vector. It is assumed that what you're putting together belongs together. What you probably want is a cell array which is indexed with a curly brace, ie myArray{1} = stock1; myArray{2} = stock2;. Reference here.
Ah, since you have row vectors, [stock1 stock2] is a concatenation. If you want to create a 2-by-x array instead, do something like this [stock1; stock2], which will place one array above the other.
Joining vectors using [x y] has different results depending on whether your vectors are rows or columns. If rows, then joining them with [x y] makes a longer row vector, but if columns, you'll get a Nx2 matrix. You should probably convert them to column vectors using the TRANSPOSE operator thus: [x' y']. Although you should check if transpose means the same thing with Time Series objects as at does with regular vectors.