In my code, I have lots of places where I invoke functions of the form
X = f(X)
and X can be a rather large matrix. In my special case, I have mostly calls like
X = feval(somefunc, X)
or
X = obj.myfunc(X)
It would be bad if, every time the function is called, there is new space allocated for X. Is MATLAB smart enough to deal with such function calls? Is there a way to tell?
The answer to this question helps would help very much with a design descision. I like to program in object oriented style and if MATLAB is not smart enough for this, it might pay off for me to add another member for X in the class, although I would rather not do this otherwise.
Whether a copy is made of the input arguments or not when calling a function in MATLAB depends upon what happens inside of the function.
MATLAB uses a system referred to as copy-on-write. This means that if you pass a large variable to a function as an input, as long as you do not modify the variable within that function, the variable will not be copied into the workspace of the function and the function will instead read the data from it's current location in memory.
function Y = func(X)
Y = X + 1;
end
If you are modifying the variable within the function, then a copy of the input variable is made and placed into the local workspace of the function.
function X = func(X)
X = X + 1;
end
There is more information on Loren's Mathworks blog.
An easy way to determine if a copy of the data is made or not is to use the undocumented format debug mode which will show you where the data for a given variable is stored in memory.
format debug
%// Create a variable a and show where debug info
a = [1,2]
%// Structure address = 141f567f0
%// m = 1
%// n = 2
%// pr = 7f9540b85e20
%// pi = 0
%// 1 2
%// Assign b = a but don't modify
b = a
%// Structure address = 141f567f0
%// m = 1
%// n = 2
%// pr = 7f9540b85e20 <= POINTER TO DATA REMAINED UNCHANGED
%// pi = 0
%// 1 2
%// Modify (Will create a new copy)
b = b + 1
%// Structure address = 141f55b40
%// m = 1
%// n = 2
%// pr = 7f953bcf1a20 <= POINTER TO DATA CHANGED (COPY)
%// pi = 0
%// 2 3
If you prefer you can use this little anonymous function I've created to inspect the memory location of any particular variable.
memoryLocation = #(x)regexp(evalc('disp(x)'), '(?<=pr\s*=\s*)[a-z0-9]*', 'match')
a = [1,2];
memoryLocation(a)
%// 7f9540b85e20
b = a;
memoryLocation(b)
%// 7f9540b85e20
b = b + 1;
memoryLocation(b)
%// 7f953bcf1a20
As a bit of a side-note, I would recommend against using feval throughout your code and instead just use the function names directly.
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I have a large data set (vector) I want to split up in to n smaller sets to look at later with other scripts. I.e.if n = 10 I want to turn one 1x80000000 double in to ten 1x8000000 doubles. My thoughts are turn the original in to a n by m matrix then save each row of the matrix in to it's own vector, as follows.
%data-n-splitter
n = 10 %number of sections
L = length(data);
Ls = L/n;
Ls = floor(Ls);
Counter = 1;
%converting vector to matrix
datamatrix = zeros(n,Ls);
for k = 1:n
datamatrix(k,:) = data(Counter:Counter+ Ls - 1);
Counter = Counter + Ls;
end
How do I make matlab loop this part of the code n times:
%save each row of matrix as seperate vector
P1 = datamatrix(1,:);
P2 = datamatrix(2,:);
P3 = datamatrix(3,:);
P4 = datamatrix(4,:);
P5 = datamatrix(5,:);
P6 = datamatrix(6,:);
P7 = datamatrix(7,:);
P8 = datamatrix(8,:);
P9 = datamatrix(9,:);
P10 = datamatrix(10,:);
Example answer that I'm hoping for:
for k = 1:n
P('n') = datamatrix(n,:);
end
I've seen some articles about using cell arrays but the scripts I'm passing the variables to aren't set up for this so I'd rather not go down that route if possible.
There are several options:
use a struct, which comes closest to what you are hoping for,
use a cell, more convenient looping but no access over meaningful names,
use a higher-dimension matrix (in your case it is only 2D, but the same applies for 3D or higher). This is the most memory-efficient option.
To round this off, you could also use a table, which is a hybrid of a struct and a cell as you can use both notations to access it. There is no other benefit.
Now, how to do this? The simplest (and best) solution first: create a 2D matrix with reshape
Ary = 1:10; % I shrank your 1x80000000 array to 1x10 but you'll get the idea
%% create new structure
Mat = reshape(Ary,5,2);
%% access new structure (looping over columns)
for i = 1:size(Ary,2)
% access columns through slicing
ary_sct = Mat(:,i);
% do something
end
Pro: memory efficient (requires the same amount of memory as the initial array); easy looping
Con: only works if you can slice the initial array evenly
Next: create a cell
Ary = 1:10;
n = 2; % number of sections
L = floor(length(Ary)/n);
% allocate memory
C = cell(1,n);
%% create new structure
for i = 1:n
% access the content of a cell with {}
C{i} = Ary((i-1)*L+1:i*L);
end
%% access new structure (looping over entries)
for i = 1:length(C)
% access the content of a cell with {}
ary_sct = C{i};
% do something
end
Pro: You can store anything in a cell. Every data type and -- what is often more important -- of any dimension
Con: The accessing the content (through {}) or accessing the element (through ()) is a bit annoying if your are a beginner; each element require a memory overhead of about 60 bytes as those are pointers, which need to store the information where and on what they are pointing.
Next: use a struct
Ary = 1:10;
n = 2; % number of sections
L = floor(length(Ary)/n);
% create empty struct
S = struct();
%% create new structure
for i = 1:n
% create fieldname (must start with a character!)
fld = num2str(i,'F%d');
% write to field (note the brackets)
S.(fld) = Ary((i-1)*L+1:i*L);
end
%% access new structure (looping over fieldnames)
% get all field names
FlNms = fieldnames(S);
for i = 1:length(FldNames)
% access field names (this is a cell!)
fld = FldNms{i};
% access struct
ary_sct = S.(fld);
% do something
end
Pro: Field names are convenient to keep the overview of your data
Con: accessing field names in a loop is a bit tedious; each element require a memory overhead of about 60 bytes as those are pointers, which need to store the information where and on what they are pointing.
I would like to name variable (type double) in the following way:
k0 = D(1,1);
k1 = D(2,2);
k2 = D(3,3);
k3 = D(4,4);
k4 = D(5,5);
k5 = D(6,6);
k6 = D(7,7);
k7 = D(8,8);
...
up to k99 automatically using for loop. So I see that I should use array or cell instead of double variable using eval as it is slow. But if I should use array or cell instead of double variable, I have to start at k{1} or k(1), which loses the meaning as I want exactly that k0 refers to D(1,1), i.e. the number in my variable is 1 less. How do I create meaningful cell name like k{0}?
Also, say I have an array A. There are also some times i need meaningful variable name, such as
c111 = A(1)*A(1)*A(1)
c222 = A(2)*A(2)*A(2)
c333 = A(3)*A(3)*A(3)
How can I create c{111} efficiently using for loop?
Use structures:
D = rand(21);
c = 1;
for k = -10:10
if k<0
s.(['k_' num2str(abs(k))]) = D(c,c);
else
s.(['k' num2str(k)]) = D(c,c);
end
c = c+1;
end
This will give you a structure like:
s =
k_10: 0.51785
k_9: 0.90121
k_8: 0.40746
k_7: 0.092989
.
.
k_1: 0.75522
k0: 0.55257
k1: 0.28708
.
.
k9: 0.94182
k10: 0.2124
and don't use eval...
Answer to 1st Question:-
D=randn(100); % A matrix of random elements of size 8x8
for m=0:99
assignin('base', ['k' num2str(m)], D(m+1,m+1))
end
Answer to 2nd Question:-
A=randn(1,3); % An array of 3 random elements
for n=1:3
assignin('base', ['c' num2str(111*n)], A(n)^3)
end
Comments:-
You've stated that you need variables like k0,k1,k2,... and c111,c222,c333 but you're asking how to create k{0}, k{1},k{2},... and c{111},c{222},c{333}. As far as your need is concerned, I have given answer to it. Regarding the latter, k{0} is never possible and c{111},c{222},c{333},... don't make good sense without using any of the first 0:100 values and then 112:221 values and so on. Although you can do it using:
A=rand(1,3); % An array of 3 random elements
c{333} = 0 ; % Pre-allocation
for p=1:3 % Since you want to use a 'for loop'
c{111*p} = A(p)^3;
end
And regarding the requirement that you made in the comment in these words "I also have some variable using negative index", you can never have variables in the negative index. If you mean you want to create variables with names like k-1, k-2,... etc, it is not possible. An alternate way is to use k_1, k_2,... etc but then as you said in the question "k0 refers to D(1,1), i.e. the number in my variable is 1 less". It means k_1 will refer to D(0,0) and so on which is again an invalid thing for MATLAB.
Recommendation:-
You really need to modify your code.
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');
I created a matfile in which I store data that are constantly overwritten by user behavior. This occurs in a function "test()".
n=1
while n < 5
myVal = double(Test704(1, 780, -1)) %Returns the user's behavior
if myVal == 1
n = n + 1 %"n" is the overwritten variable in the matfile
end
save('testSave1.mat') %The matfile
m = matfile('testSave1.mat')
end
Then, I want to display these data in another function (it is essential to have two separated functions) called "storageTest()". More particularly, storageTest() is a GUI function where I developped an uitable "t". So, I first call the function "test()" and give its output values as data of "t". Here is the code of the interesting part of "storageTest":
m = test()
d = [m.n]
t = uitable('Data',d, ...
'ColumnWidth',{50}, ...
'Position',[100 100 461 146]);
t.Position(3) = t.Extent(3);
t.Position(4) = t.Extent(4);
drawnow
This code executes only when "m = test()" running is over and displays me a tab in which I can see the final value of "n". However, I want my table to be displayed before and to see my value incrementing according to user's behavior.
I have searched on the web to solve my issue, but I cannot find any answer, is it possible to do such a thing?
Assuming I'm interpreting the question correctly, it should be fairly trivial to accomplish this if you initialize your table prior to calling test and then pass the handle to your table for test to update in the while loop:
For example:
function testGUI
% Initialize table
t = uitable('ColumnWidth',{50}, 'Position',[100 100 461 146]);
test(t)
function test(t)
n = 1;
while n < 5
n = n + 1;
t.Data = n;
pause(0.25); % Since we're just incrementing a number, a slight so we can actually see the change
end
When you run the above, you'll notice the data in your table iterating as expected.
excaza was a little faster in writing basically the same answer like me. As it looks a slightly different, I'll post it anyway.
function storagetest()
close all
f = figure;
data = [1];
t = uitable(f,'Data',data,'ColumnWidth',{50});
test()
end
function test()
% handle uitable
t = evalin('caller','t')
n = 1;
while n < 5
newVal = input('Enter a number:');
data = get(t,'Data');
set(t,'Data', [data; newVal]);
n = n + 1;
end
end
The "user behaviour" I imitated with the input function. The basic idea is to update your table from within test(). evalin you can use, if you don't want to pass parameters to test(), though passing the handle of the uitable directly is certainly the better option.
If you are working on a serious GUI project I highly recommend you reading this answer.
I have a simple but interesting question. i tired hard to google it but my google got upset and giving me the same results...
i wanted to know is it possible to Update a constant variable form workspace command..
A Simple Example:
function y =StupidQuestion
a = 10; % some value
b =[5,6,7;1,2,8]; % some value
y = b*a % some operation
I forget to tell you that we can do it with simulink block by using below command
set_param('obj', 'parameter1', value1, 'parameter2', value2, ...)
i Want to use the assigned value for 3 weeks and without any reason i wants to change my values [a,b] to other but through command windows. any Idea. Waiting for your interesting Reply...................
You can set defaults for the inputs:
function y = foo(a,b)
if nargin < 1 || isempty(a), a = 10; end
if nargin < 2 || isempty(b), b = [5,6,7;1,2,8]; end
y = b*a
end
You can call foo() without inputs (and it will use the defaults for a and b) or supply your own values as: foo(12), foo(12,[10,20]), foo([],[23,23]), etc...
A possible way is to save some variables in an external file. Note that in this case a and b are only in the function workspace (you won't see their values unless you load the contents of test.mat separately). I'm passing the filename in rather than hard-coding it in case you need to switch between multiple settings.
Personally I would prefer to have a human-readable data file, but the concept remains the same (you'd just need some parser function which returned values for a and b given a file).
a = 10; % some value
b =[5,6,7;1,2,8]; % some value
save('test.mat','a','b');
clear a b;
function y = savedvariables(filename)
load(filename);
y = b*a; % some operation
end
y = savedvariables('test.mat');