I am trying to concatenate matrices station_1, station_2,.....station_10 from my matlab workspace and trying to concatenate all stations automatically using a loop and not calling them one by one like this
cat(1,station_1,station_2,station_3,station_4... ,station_5,station_6,station_7,station_8... ,station_9,station_10 )
Any ideas?
the code below is what i was trying to improve
for jj= 1 : 10 T= cat(1,eval(['station_', num2str(jj)])); MegaMat = cat(1,T) end
Reading your code I think at the end of your loop you will have T = station_10.
If you want to concatenate all of them you would do
T = []
for jj= 1:10
T = cat(1, T, eval(['station_', num2str(jj)]));
end
MegaMat = T;
Using eval is not a good practice. Instead of creating station_1 to station_10 you could create a cell array
station{1} = ...
station{2} = ...
Then you could iterate like
T = []
for jj = 1:length(station)
T = cat(1, T, station{jj});
end
If the number of arrays is big this will be slow due to memory reallocation and copy. In that case is more efficient to initialize T as a matrix of the final dimension and write slices.
Appendix:
There is an interesting notation trick pointed by #Cris Luengo in the comments, that is when you have a cell array station, use the notation [station{:}], I have to admit, this notation is new to me. The only caveat is that if you set the items station{i} = ... then you will have the matrices concatenated horizontally rather than vertically.
The answer from #Mateo V, is also good, probably with leas overhead since it calls eval only once. That approach can be refined giving a one linear solution, and to be honest It felt not very unreadable.
MegaMat = eval(['cat(1', num2str(1:10, ', station_%d'), ')']);
No need for loops:
str = num2str(1:10, 'station_%i,'); % returns string 'station_1, station_2, ..., station_10,'
str = str(1:end-1); % remove last comma
eval(['MegaMat = cat(1, ', str, ');'])
Related
I am trying to concatenate several structs. What I take from each struct depends on a function that requires a for loop. Here is my simplified array:
t = 1;
for t = 1:5 %this isn't the for loop I am asking about
a(t).data = t^2; %it just creates a simple struct with 5 data entries
end
Here I am doing concatenation manually:
A = [a(1:2).data a(1:3).data a(1:4).data a(1:5).data] %concatenation function
As you can see, the range (1:2), (1:3), (1:4), and (1:5) can be looped, which I attempt to do like this:
t = 2;
A = [for t = 2:5
a(1:t).data
end]
This results in an error "Illegal use of reserved keyword "for"."
How can I do a for loop within the concatenate function? Can I do loops within other functions in Matlab? Is there another way to do it, other than copy/pasting the line and changing 1 number manually?
You were close to getting it right! This will do what you want.
A = []; %% note: no need to initialize t, the for-loop takes care of that
for t = 2:5
A = [A a(1:t).data]
end
This seems strange though...you are concatenating the same elements over and over...in this example, you get the result:
A =
1 4 1 4 9 1 4 9 16 1 4 9 16 25
If what you really need is just the .data elements concatenated into a single array, then that is very simple:
A = [a.data]
A couple of notes about this: why are the brackets necessary? Because the expressions
a.data, a(1:t).data
don't return all the numbers in a single array, like many functions do. They return a separate answer for each element of the structure array. You can test this like so:
>> [b,c,d,e,f] = a.data
b =
1
c =
4
d =
9
e =
16
f =
25
Five different answers there. But MATLAB gives you a cheat -- the square brackets! Put an expression like a.data inside square brackets, and all of a sudden those separate answers are compressed into a single array. It's magic!
Another note: for very large arrays, the for-loop version here will be very slow. It would be better to allocate the memory for A ahead of time. In the for-loop here, MATLAB is dynamically resizing the array each time through, and that can be very slow if your for-loop has 1 million iterations. If it's less than 1000 or so, you won't notice it at all.
Finally, the reason that HBHB could not run your struct creating code at the top is that it doesn't work unless a is already defined in your workspace. If you initialize a like this:
%% t = 1; %% by the way, you don't need this, the t value is overwritten by the loop below
a = []; %% always initialize!
for t = 1:5 %this isn't the for loop I am asking about
a(t).data = t^2; %it just creates a simple struct with 5 data entries
end
then it runs for anyone the first time.
As an appendix to gariepy's answer:
The matrix concatenation
A = [A k];
as a way of appending to it is actually pretty slow. You end up reassigning N elements every time you concatenate to an N size vector. If all you're doing is adding elements to the end of it, it is better to use the following syntax
A(end+1) = k;
In MATLAB this is optimized such that on average you only need to reassign about 80% of the elements in a matrix. This might not seam much, but for 10k elements this adds up to ~ an order of magnitude of difference in time (at least for me).
Bare in mind that this works only in MATLAB 2012b and higher as described in this thead: Octave/Matlab: Adding new elements to a vector
This is the code I used. tic/toc syntax is not the most accurate method for profiling in MATLAB, but it illustrates the point.
close all; clear all; clc;
t_cnc = []; t_app = [];
N = 1000;
for n = 1:N;
% Concatenate
tic;
A = [];
for k = 1:n;
A = [A k];
end
t_cnc(end+1) = toc;
% Append
tic;
A = [];
for k = 1:n;
A(end+1) = k;
end
t_app(end+1) = toc;
end
t_cnc = t_cnc*1000; t_app = t_app*1000; % Convert to ms
% Fit a straight line on a log scale
P1 = polyfit(log(1:N),log(t_cnc),1); P_cnc = #(x) exp(P1(2)).*x.^P1(1);
P2 = polyfit(log(1:N),log(t_app),1); P_app = #(x) exp(P2(2)).*x.^P2(1);
% Plot and save
loglog(1:N,t_cnc,'.',1:N,P_cnc(1:N),'k--',...
1:N,t_app,'.',1:N,P_app(1:N),'k--');
grid on;
xlabel('log(N)');
ylabel('log(Elapsed time / ms)');
title('Concatenate vs. Append in MATLAB 2014b');
legend('A = [A k]',['O(N^{',num2str(P1(1)),'})'],...
'A(end+1) = k',['O(N^{',num2str(P2(1)),'})'],...
'Location','northwest');
saveas(gcf,'Cnc_vs_App_test.png');
Given the code below:
% Generate some random data
n = 10;
A = cell(n, 1);
for i=1:n
A{i} = timeseries;
A{i}.Data = rand(100, 1);
A{i}.Time = 1:100;
end
I would like to make the sum of Data(end) without explicitly writing a for loop. Is there a smart way to select Data(end) in all cells in a single line? A{:}.Data(end) does not work.
You can do it with cellfun but that is essentially just a for loop wrapped up:
cellfun(#(x) x.Data(end), A)
I prefer Dan's answer, but for reference, I'll post an alternative using arrayfun. This is also just a for loop wrapped up to save keystrokes, but not necessarily time.
sum(arrayfun(#(n) A{n}.Data(end), 1:numel(A)))
You can also extract all of the Data fields into a single matrix, which might be worth it if you're planning on doing multiple operations on it:
A2 = [A{:}];
A3 = [A2.Data];
sum(A3(end,:))
Suppose I have n .mat files and each are named as follows: a1, a2, ..., an
And within each of these mat files there is a variable called: var (nxn matrix)
I would like to create a matrix: A = [a1.var a2.var, ..., an.var] without writing it all out because there are many .mat files
A for-loop comes to mind, something like this:
A = []
for i = 1:n
[B] = ['a',num2str(i),'.mat',var];
A = [A B]
end
but this doesn't seem to work or even for the most simple case where I have variables that aren't stored as a(i) but rather 'a1', 'a2' etc.
Thank you very much!
load and concatenate 'var' from each of 'a(#).mat':
n = 10;
for i = n:-1:1 % 1
file_i = sprintf('a%d.mat', i); % 2
t = load(file_i, 'var');
varsCell{i} = t.var; % 3
end
A = [varsCell{:}]; % concatenate each 'var' in one step.
Here are some comment on the above code. All the memory-related stuff isn't very important here, but it's good to keep in mind during larger projects.
1)
In MATLAB, it is rarely a good idea or necessary to grow variables during a for loop. Each time an element is added, MATLAB must find and allocate a new block of RAM. This can really slow things down, especially for long loops or large variables. When possible, pre-allocate your variables (A = zeros(n,n*n)). Alternatively, it sometimes works to count backwards in the loop. MATLAB pre-allocates the whole array, since you're effectively telling it the final size.
2)
Equivalent to file_i = ['a',num2str(i),'.mat'] in this case, sprintf can be clearer and more powerful.
3)
Store each 'var' in a cell array. This is a balance between allocating all the needed memory and the complication of indexing into the correct places of a preallocated array. Internally, the cell array is a list of pointers to the location of each loaded 'var' matrix.
to create a test set...
generate 'n' matrices of n*n random doubles
save each as 'a(#).mat' in current directory
for i = 1:n
var = rand(n);
save(sprintf('a%d.mat',i), 'var');
end
Code
%%// The final result, A would have size nX(nXn)
A = zeros(n,n*n); %%// Pre-allocation for better performance
for k =1:n
load(strcat('a',num2str(k),'.mat'))
A(1:n,(k-1)*n+1:(k-1)*n+n) = var;
end
I have the following piece of code:
for query = queryFiles
queryImage = imread(strcat('Queries/', query));
queryImage = im2single(rgb2gray(queryImage));
[qf,qd] = vl_covdet(queryImage, opts{:}) ;
for databaseEntry = databaseFiles
entryImage = imread(databaseEntry.name);
entryImage = im2single(rgb2gray(entryImage));
[df,dd] = vl_covdet(entryImage, opts{:}) ;
[matches, H] = matchFeatures(qf,qf,df,dd) ;
result = [result; query, databaseEntry, length(matches)];
end
end
It is my understanding that it should work as a Java/C++ for(query:queryFiles), however the query appears to be a copy of the queryFiles. How do I iterate through this vector normally?
I managed to sort the problem out. It was mainly to my MATLAB ignorance. I wasn't aware of cell arrays and that's the reason I had this problem. That and the required transposition.
From your code it appears that queryFiles is a numeric vector. Maybe it's a column vector? In that case you should convert it into a row:
for query = queryFiles.'
This is because the for loop in Matlab picks a column at each iteration. If your vector is a single column, it picks the whole vector in just one iteration.
In MATLAB, the for construct expects a row vector as input:
for ii = 1:5
will work (loops 5 times with ii = 1, 2, ...)
x = 1:5;
for ii = x
works the same way
However, when you have something other than a row vector, you would simply get a copy (or a column of data at a time).
To help you better, you need to tell us what the data type of queryFiles is. I am guessing it might be a cell array of strings since you are concatenating with a file path (look at fullfile function for the "right" way to do this). If so, then a "safe" approach is:
for ii = 1:numel(queryFiles)
query = queryFiles{ii}; % or queryFiles(ii)
It is often helpful to know what loop number you are in, and in this case ii provides that count for you. This approach is robust even when you don't know ahead of time what the shape of queryFiles is.
Here is how you can loop over all elements in queryFiles, this works for scalars, row vectors, column vectors and even high dimensional matrices:
for query = queryFiles(:)'
% Do stuff
end
Is queryFiles a cell array? The safest way to do this is to use an index:
for i = 1:numel(queryFiles)
query = queryFiles{i};
...
end
How would I generate a vector like
x1,x2,x3,x4,...,xn
the problem is concatenate ','and a 'x' char
n=100
A = (1:n);
This is a slight improvement on #Jonas's answer. SPRINTF will do the repeating for you avoiding the need for a mask:
>> n = 5;
>> out = sprintf('x%u,', 1:n);
>> out(end) = []
out =
x1,x2,x3,x4,x5
To generate the string 'x1,x2' etc, you can create a mask for SPRINTF using REPMAT like so:
n = 5;
mask = repmat('x%i,',1,n);
out = sprintf(mask,1:n);
out = out(1:end-1)
out =
x1,x2,x3,x4,x5
Note that in case you actually want to create a vector containing the strings 'x1','x2' etc, you'd use ARRAYFUN to generate a cell array:
out = arrayfun(#(x)sprintf('x%i',x),1:n,'uniformOutput',false)
out =
'x1' 'x2' 'x3' 'x4' 'x5'
The better answer is, don't do it. While you CAN do so, this will likely cause more heartache for you in the future than you want. Having hundreds of such variables floating around is silly, when you can use an array to index the same data. Thus perhaps x{1}, x{2}, ....