I have a structure called "xyz" with the following contents. How can I make a new variable (a 141x136 double) having an element wise average of the fields.
You can follow these steps:
Convert the struct to a cell array.
Generate a comma-separated list from the cell, and concatenate the cells' contents along the third dimension, producing a 3D numeric array.
Compute the mean along the 3rd dimension.
Example:
xyz.v1 = ones(5,6);
xyz.v2 = 2*ones(5,6); % example data
c = struct2cell(xyz); % step 1
c = cat(3, c{:}); % step 2
result = mean(c, 3); % step 3
Consider if you really need a struct. Maybe you can have the 3D array instead (depending on the rest of your code). That's usually more time- and memory-efficient. The fields would correspond to the 3rd-dim slices, and you would only need step 3 above, that is, mean(..., 3).
Related
Suppose I have a 3x1 cell array called subj that has the following elements:
cell 1: 300x20 double
cell 2: 300x15 double
cell 3: 300X18 double
I want to remove the last quarter rows from every element in each cell as follows:
subj{1}(length(subj{1})*0.25+1:end,:) = []
subj{2}(length(subj{2})*0.25+1:end,:) = []
subj{3}(length(subj{3})*0.25+1:end,:) = []
However I want to be do this in one line and can't figure out a way to do this in Matlab. I messed around with converting the cell array to a matrix, but since there different numbers of columns, it makes it slightly more complicated. Is there a vectorized way to do this in one line? I will be applying machine learning algorithms to each element of subj and it would be great to have this be vectorized for later parts of my code.
Better not assigning empty arrays to a matrix (actually that doesn't work) but reassign the matrix itself:
% loop over the cell array
for i = 1:length(subj)
% determine the index/number of rows. Don't forget to round as MATLAB requires integers for slicing!
idx = round( length(subj{i})*0.25 );
% get the new matrix
NewMat = subj{i}(1:idx,:);
% assign new matrix to the old address, i.e. the content of the cell
subj{i} = NewMat;
end
MATLAB allocates contiguous memory for matrices, so cropping it shouldn't cause much overhead. In particular not as it uses "lazy copying" and; therefore, only copies the matrix when you change any of its values.
I want to generate a 3D cell array called timeData so that timeData(:,:,a) for some integer a is an nx1 matrix of data, and the number of rows n varies with the value of a in a 1:1 correspondence. To do this, I am generating a 2D array of data called data that is nx1. This assignment statement takes place within a for loop as follows:
% Before iterating, I define an array of indices where I want to store the
% data sets in timeData. This choice of storage location is for
% organizational purposes.
A = [2, 5, 9, 21, 34, 100]; % Notice they are in ascending order, but have
% gaps that have no predictability.
sizeA = size(A);
numIter = A(1);
for m = 1:numIter % numIter is the number of data sets that I need to store
% in timeData
% At this point, some code that is entirely irrelevant to my question
% generates a nx1 array of data. One example of this data array is below.
data = [1.1;2.3;5.5;4.4]; % This is one example of what data could be. Its
% number of rows, n, changes each iteration, as
% do its contents.
B = size(data);
timeData(1:B(1),1,A(m)) = num2cell(data);
end
This code does put all contents of data in the appropriate locations within timeData as I want. However, it also adds {0x0 double} rows to all 2D arrays of timeData(:,:,a) for any a whose corresponding number of rows n was not the largest number of rows. Thus, there are many of these 2D arrays that have 10 to a couple hundred 0-valued rows that I don't want. For values of a that did not have a corresponding data set, the content of timeData(:,:,a) is an nx1 array of {0x0 double}.
I need to iterate over the contents of timeData in subsequent code, and I need to be able to find the size of the data set that is in timeData(:,:,a) without somehow discounting all the {0x0 double}.
How can I modify my assignment statement to fix this?
Edit: Desired output of the above example is the following with n = 5. Let this data set be represented by a = 9.
timeData(:,:,9) = {[1.1]}
{[2.3]}
{[5.5]}
{[8.6]}
{[4.4]}
Now, consider the possibility that a previous or subsequent value of the A matrix had a data set with n = 7, and n = 7 is the largest data set (largest n value). timeData(:,:,9) outputs like so in my code:
timeData(:,:,9) = {[1.1]}
{[2.3]}
{[5.5]}
{[8.6]}
{[4.4]}
{[0x0 double]}
{[0x0 double]}
#Dev-iL, as I understand it, your answer gives me the ability to delete the cells that have {[0x0 double]} in them (this is what I mean by "discounting"). This is a good plan B, but is there a way to prevent the {[0x0 double]} cells from showing up in the first place?
Edit 2: Update to the above statement "your answer gives me the ability to delete the cells that have {[0x0 double]} in them (this is what I mean by "discounting")". The cellfun(#isempty... ) function makes the {[0x0 double]}cells go to {[0x0 cell]}, it does not remove them. In other words, size(timeData(:,:,9)) is the same before and after the command is performed. This is not what I want. I want size(timeData(:,:,9)) to be 5x1 no matter what n is for any other value of a.
Edit 3: I just realized that the most desired output would be the following:
timeData(:,:,9) = {[1.1;2.3;5.5;8.6;4.4]} % An n x 1 column matrix within
% the cell.
but I can work with this outcome or the outcome as described above.
Unfortunately, I don't understand the structure of your dataset, which is why I can't suggest a better assignment method. However, I'd like to point out an operation that can you help deal with your data after it's been created:
cellfun(#isempty,timeData);
What the above does is return a logical array the size of timeData, indicating which cells contain something "empty". Typically, an array of arbitrary datatype is considered "empty" when it has at least one dimension that is equal to 0.
How can you use it to your advantage?
%% Example 1: counting non-empty cells:
nData = sum(~cellfun(#isempty,timeData(:)));
%% Example 2: assigning empty cells in place of empty double arrays:
timeData(cellfun(#isempty,timeData)) = {{}};
I'd like to use fprintf to show code execution progress in the command window.
I've got a N x 1 array of structures, let's call it myStructure. Each element has the fields name and data. I'd like to print the name side by side with the number of data points, like such:
name1 number1
name2 number2
name3 number3
name4 number4
...
I can use repmat N times along with fprintf. The problem with that is that all the numbers have to come in between the names in a cell array C.
fprintf(repmat('%s\t%d',N,1),C{:})
I can use cellfun to get the names and number of datapoints.
names = {myStucture.name};
numpoints = cellfun(#numel,{myStructure.data});
However I'm not sure how to get this into a cell array with alternating elements for C to make the fprintf work.
Is there a way to do this? Is there a better way to get fprintf to behave as I desire?
You're very close. What I would do is change your cellfun call so that the output is a cell array instead of a numeric array. Use the 'UniformOutput' flag and set this to 0 or false.
When you're done, make a new cell array where both the name cell array and the size cell array are stacked on top of each other. You can then call fprintf once.
% Save the names in a cell array
A = {myStructure.name};
% Save the sizes in another cell array
B = cellfun(#numel, {myStructure.data}, 'UniformOutput', 0);
% Create a master cell array where the first row are the names
% and the second row are the sizes
out = [A; B];
% Print out the elements side-by-side
fprintf('%s\t%d\n', out{:});
The trick with the third line of code is that when you unroll the cell array using {:}, this creates a comma-separated list unrolled in column-major format, and so doing out{:} actually gives you:
A{1}, B{1}, A{2}, B{2}, ..., A{n}, B{n}
... which provides the interleaving you need. Therefore, providing this order into fprintf coincides with the format specifiers that are specified and thus gives you what you need. That's why it's important to stack the cell arrays so that each column gives the information you need.
Minor Note
Of course one should never forget that one of the easiest ways to tackle your problem is to just use a simple for loop. Even though for loops are considered bad practice, their performance has come a long way throughout MATLAB's evolution.
Simply put, just do this:
for ii = 1 : numel(myStructure)
fprintf('%s\t%d\n', myStructure(ii).name, numel(myStructure(ii).data));
end
The above code is arguably more readable in comparison to what we did above with cell arrays. You're accessing the structure directly rather than having to create intermediate variables for the purpose of calling fprintf once.
Example Run
Here's an example of this running. Using the data shown below:
clear myStructure;
myStructure(1).name = 'hello';
myStructure(1).data = rand(5,1);
myStructure(2).name = 'hi';
myStructure(2).data = zeros(3,3);
myStructure(3).name = 'huh';
myStructure(3).data = ones(6,4);
I get the following output after running the printing code:
hello 5
hi 9
huh 24
We can see that the sizes are correct as the first element in the structure is simply a random 5 element vector, the second element is a 3 x 3 = 9 zeroes matrix while the last element is a 6 x 4 = 24 ones matrix.
I have an Excel sheet containing 1838 records and I need to RANDOMLY split these records into 3 Excel Sheets. I am trying to use Matlab but I am quite new to it and I have just managed the following code:
[xlsn, xlst, raw] = xlsread('data.xls');
numrows = 1838;
randindex = ceil(3*rand(numrows, 1));
raw1 = raw(:,randindex==1);
raw2 = raw(:,randindex==2);
raw3 = raw(:,randindex==3);
Your general procedure will be to read the spreadsheet into some matlab variables, operate on those matrices such that you end up with three thirds and then write each third back out.
So you've got the read covered with xlsread, that results in the two matrices xlsnum and xlstxt. I would suggest using the syntax
[~, ~, raw] = xlsread('data.xls');
In the xlsread help file (you can access this by typing doc xlsread into the command window) it says that the three output arguments hold the numeric cells, the text cells and the whole lot. This is because a matlab matrix can only hold one type of value and a spreadsheet will usually be expected to have text or numbers. The raw value will hold all of the values but in a 'cell array' instead, a different kind of matlab data type.
So then you will have a cell array valled raw. From here you want to do three things:
work out how many rows you have (I assume each record is a row) by using the size function and specifying the appropriate dimension (again check the help file to see how to do this)
create an index of random numbers between 1 and 3 inclusive, which you can use as a mask
randindex = ceil(3*rand(numrows, 1));
apply the mask to your cell array to extract the records matching each index
raw1 = raw(:,randindex==1); % do the same for the other two index values
write each cell back to a file
xlswrite('output1.xls', raw1);
You will probably have to fettle the arguments to get it to work the way you want but be sure to check the doc functionname page to get the syntax just right. Your main concern will be to get the indexing correct - matlab indexes row-first whereas spreadsheets tend to be column-first (e.g. cell A2 is column A and row 2, but matlab matrix element M(1,2) is the first row and the second column of matrix M, i.e. cell B1).
UPDATE: to split the file evenly is surprisingly more trouble: because we're using random numbers for the index it's not guaranteed to split evenly. So instead we can generate a vector of random floats and then pick out the lowest 33% of them to make index 1, the highest 33 to make index 3 and let the rest be 2.
randvec = rand(numrows, 1); % float between 0 and 1
pct33 = prctile(randvec,100/3); % value of 33rd percentile
pct67 = prctile(randvec,200/3); % value of 67th percentile
randindex = ones(numrows,1);
randindex(randvec>pct33) = 2;
randindex(randvec>pct67) = 3;
It probably still won't be absolutely even - 1838 isn't a multiple of 3. You can see how many members each group has this way
numel(find(randindex==1))
I have a vector CD1 (120-by-1) and I separate CD1 into 6 parts. For example, the first part is extracted from row 1 to row 20 in CD1, and second part is extracted from row 21 to row 40 in CD1, etc. For each part, I need to compute the means of the absolute values of second differences of the data.
for PartNo = 1:6
% extract data
Y(PartNo) = CD1(1 + 20*(PartNo-1):20*(PartNo),:);
% find the second difference
Z(PartNo) = Y(PartNo)(3:end) - Y(PartNo)(1:end-2);
% mean of absolute value
MEAN_ABS_2ND_DIFF_RESULT(PartNo) = mean(abs(Z));
end
However, the commands above produce the error:
()-indexing must appear last in an index expression for Line:2
Any ideas to change the code to have it do what I want?
This error is often encountered when Y is a cell-array. For cell arrays,
Y{1}(1:3)
is legal. Curly braces ({}) mean data extraction, so this means you are extracting the array stored in location 1 in the cell array, and then referencing the elements 1 through 3 of that array.
The notation
Y(1)(1:3)
is different in that it does not extract data, but it references the cell's location 1. This means the first part (Y(1)) returns a cell-array which, in your case, contains a single array. So you won't have direct access to the regular array as before.
It is an infamous limitation in Matlab that you cannot do indirect or double-referencing, which is in effect what you are doing here.
Hence the error.
Now, to resolve: I suspect replacing a few normal braces with curly ones will do the trick:
Y{PartNo} = CD1(1+20*(PartNo-1):20*PartNo,:); % extract data
Z{PartNo} = Y{PartNo}(3:end)-Y{PartNo}(1:end-2); % find the second difference
MEAN_ABS_2ND_DIFF_RESULT{PartNo} = mean(abs(Z{PartNo})); % mean of absolute value
I might suggest a different approach
Y = reshape(CD1, 20, 6);
Z = diff(y(1:2:end,:));
MEAN_ABS_2ND_DIFF_RESULT = mean(abs(Z));
This is not a valid statement in matlab:
Y(PartNo)(3:end)
You should either make Y two-dimensional and use this indexing
Y(PartNo, 3:end)
or extract vector parts and use them directly, if you use a loop like you have shown
for PartNo = 1:6
% extract data
Y = CD1(1 + 20*(PartNo-1):20*(PartNo),:);
% find the second difference
Z = Y(3:end) - Y(1:end-2);
% mean of absolute value
MEAN_ABS_2ND_DIFF_RESULT(PartNo) = mean(abs(Z));
end
Also, since CD1 is a vector, you do not need to index the second dimension. Drop the :
Y = CD1(1 + 20*(PartNo-1):20*(PartNo));
Finally, you do not need a loop. You can reshape the CD1 vector to a two-dimensional array Y of size 20x6, in which the columns are your parts, and work directly on the resulting matrix:
Y = reshape(CD1, 20, 6);
Z = Y(3:end,:)-Y(1:end-1,:);
MEAN_ABS_2ND_DIFF_RESULT = mean(abs(Z));