How could I correlate the first (also second and third, etc) line of each function together? I meant I want to pick up the value of the first line of function 1, first line of function 2 and so on... Specially number of these lines (between two #...#) are not always equal.
Let's say the values between (# -#) is one matrix. I see that the problem is I need to break them down into different matrix and then equalize all these matrix to the same size by adding NaN values and then reconstruct them again. That is what I think but I dont know how to ask Matlab do these tasks.
Would you please give me a help?
Thank you very much !!
Related
What is the best way to reduce the number of rows of a matrix by half in matlab?
What is the following command doing?
mymatrix = mymatrix(1:2:end,:);
Is there any better way available?
Short answer this is taking every second row of the matrix mymatrix starting with the first one (all odd-rows) and yes that is probably the easiest way. Added clarification based on comment from #Sardar_Usama
Longer version
end is matlab internal command that refers to the end of the array in the given dimension. roughly equivalent to size(var,dim).
so actually what mymatrix(1:2:end,:) can be re-written to mymatrix(1:2:size(mymatrix,1),:). Now if you actually look at 1:2:size(mymatrix,1) these are the rows that you are selecting.1, 3, 5, etc. You can actually specify whichever rows you want there, here are some examples.
1:floor(end/2); % first 'half'
floor(end/2)+1:end; % second 'half'
1:3:end; % every third element
1:2:floor(end/2); % every second element in the first 'half'
Added floor() to avoid problems for odd number lengths. In that case 'half' is not exactly half but rather roughly half. Alternatively ceil() depending on how you would like to define half for odd lengths.
I'm currently working on implementing a gradient check function in which it requires to get certain index values from the result matrix. Could someone tell me how to get a group of values from the matrix?
To be specific, for a result matrx res with size M x N, I'll need to get element res(3,1), res(4,2), res(1,3), res(2,4)...
In my case, M is dimension and N is batch size and there's a label array whose size is 1xbatch_size, [3 4 1 2...]. So the desired values are res(label(:),1:batch_size). Since I'm trying to practice vectorization programming and it's better not using loop. Could someone tell me how to get a group of value without a iteration?
Cheers.
--------------------------UPDATE----------------------------------------------
The only idea I found is firstly building a 'mask matrix' then use the original result matrix to do element wise multiplication (technically called 'Hadamard product', see in wiki). After that just get non-zero element out and do the sum operation, the code in matlab should look like:
temp=Mask.*res;
desired_res=temp(temp~=0); %Note: the temp(temp~=0) extract non-zero elements in a 'column' fashion: it searches temp matrix column by column then put the non-zero number into container 'desired_res'.
In my case, what I wanna do next is simply sum(desired_res) so I don't need to consider the order of those non-zero elements in 'desired_res'.
Based on this idea above, creating mask matrix is the key aim. There are two methods to do this job.
Codes are shown below. In my case, use accumarray function to add '1' in certain location (which are stored in matrix 'subs') and add '0' to other space. This will give you a mask matrix size [rwo column]. The usage of full(sparse()) is similar. I made some comparisons on those two methods (repeat around 10 times), turns out full(sparse) is faster and their time costs magnitude is 10^-4. So small difference but in a large scale experiments, this matters. One benefit of using accumarray is that it could define the matrix size while full(sparse()) cannot. The full(sparse(subs, 1)) would create matrix with size [max(subs(:,1)), max(subs(:,2))]. Since in my case, this is sufficient for my requirement and I only know few of their usage. If you find out more, please share with us. Thanks.
The detailed description of those two functions could be found on matlab's official website. accumarray and full, sparse.
% assume we have a label vector
test_labels=ones(10000,1);
% method one, accumarray(subs,1,[row column])
tic
subs=zeros(10000,2);
subs(:,1)=test_labels;
subs(:,2)=1:10000;
k1=accumarray(subs,1,[10, 10000]);
t1=toc % to compare with method two to check which one is faster
%method two: full(sparse(),1)
tic
k2=full(sparse(test_labels,1:10000,1));
t2=toc
I have a vector with +16M data, and I've got to transform it in 101 matrices of 401x401 elements each. I know how to create such matrices independently (writing a loop for each one of them) but I think there must be some way to create all of them in using two or more loops. The problem is, I don't know exactly how to do this.
This is what I've tried so far:
data=load('file.dat');%This file contains 3 columns of data, I only need the first one
var=data(:,1);
p=401;%Size of the matrices
for n=0:400
mat1(n+1,:)=var(p*n+1:p*(n+1),:);
end
This code would create the first 401x401 matrix. By changing the indices, I could (individually) create the rest, but I would prefer to add another loop (or loops) to create them automatically instead of repeating this code a hundred times.
The function mat = vec2mat(vec,matcol) converts the vector vec into a matrix with matcol columns, creating one row at a time. If the length of vec is not a multiple of matcol, then extra zeros are placed in the last row of mat. The matrix mat has ceil(length(vec)/matcol) rows.
See also the manual entry.
I have results which are 6 columns long however have been printed as 2 then 3 beneath then 1 beneath that! There are hundreds of lines and matlab will not except the structure of the matrix as it is now. Is there any way to tell matlab i want the first 5 results in their own columns then continuing down the rows after that?
My results appear as follows:
0.5 0
0.59095535915335684063 -0.59095535915335395405 -5.89791913085569763
33e-08
... repeated alot
thansk so much, em xx
I would just do a format shortE before you process the output, this will give you everything in scientific notation with 4 digits after the decimal. That 'should' allow you to fit your columns all in one line, so you don't have to deal with the botched output.
In general you should not want the output to be in a too specific format, but suppose you have this matrix:
M =[0.5 0 0.59095535915335684063 -0.59095535915335395405 -5.89791913085569763 33e-08];
To make it an actual matrix I will repeat it a bit:
M = repmat(M,10,1);
Now you can ensure that all six columns will fit on a normal screen by using the format.
format short
Try help format to find more options. Now simply showing the matrix will put all columns next to eachother. If you want one column below, the trick is to reduce your windows width untill it can only hold five columns. Matlab will now print the last column below the first.
M % Simply show the matrix
% Now reduce your window size
M % Simply show it again
This should help you display the numbers in matlab, if you want to process them further you can consider to write them to a file instead. Try help xlswrite for a simple solution.
I got the following error while working with a MATLAB program:
Error using - Matrix dimensions must agree
I noticed that the sizes of the matrices I'm trying to subtract from each other were:
firstMatrix --> 425x356
secondMatrix --> 426x356
How can I make them of equal size and go ahead and do my subtraction process?
I tried reshape, but the number of elements here seem to have to be equal.
Thanks.
I think both answers are missing the key point. Blithely subtracting two arrays of different size forgets that those arrays are NOT just numbers. The numbers must mean something. Else, they are just meaningless.
As well, simply deleting a row from the beginning or end may well be wrong, or padding with zeros. Only you know what the numbers mean, and why those arrays are not the same size. So only you can decide what is the proper action.
It might be right to pad, delete, interpolate, do any of these things. Or you might realize there is a bug in your code that created these arrays.
Your matrices have a different number of elements, so there's no point using reshape here (since it maintains the total number of elements). You'll have to discard one of the lines in the larger matrix before doing the subtraction:
For instance, you can discard the last line:
firstMatrix - secondMatrix(1:end - 1, :)
or discard the first line:
firstMatrix - secondMatrix(2:end, :)
Alternatively, you can pad the smaller matrix with default values (e.g NaN or zeroes), as suggested in another answer.
You're missing a row in firstMatrix
So can try:
firstMatrix=[firstMatrix;zeros(1,356)];
This will add a row of zeros at end of firstMatrix making it of 426x356