I'm using the code
k = 0;
while k<3
k = k+1;
a = 5^k;
disp(a);
end
however, when the result outputs it only gives me the answer of one iteration. I'm wondering what the difference is to the computer when you use this code instead:
clear, clc
k = 0;
while k<3
k = k+1;
a(k) = 5^k;
end
disp(a)
Why does the first code sample output only 125, while the second one outputs 5, 25, and 125?
In the first code, variable a is scalar.
So, Matlab erases and re-writes value into variable a in every iteration.
But, in case of second code, as you defined array index k at variable a, Matlab understands your variable a(k) as array variable. And, in every iteration, Matlab writes the assigned value 5^k on corresponding array point.
Related
I am trying to set up a parfor nested loop in MatLab R2016a as below.
N = size(A,1);
M = size(v,1);
in = zeros(N*M,1);
parfor i=1:N
for j=1:M
k = (i-1)*M+j;
if sqrt(sum((A(i,:)-v(j,:)).^2))<=tol
in(k) = i;
end
end
end
However, I am getting the following error Valid indices for 'in' are restricted in PARFOR loops. Is there some way I can correct this because both arrays A and v are considerably large, over 40,000 rows for A and 8,000 v? The variable tol is 0.0959.
The problem is that MATLAB doesn't recognize that the variable k is slicing the matrix in correctly. The solution should be to index in using i and j separately:
N = size(A,1);
M = size(v,1);
in = zeros(M,N);
parfor i=1:N
for j=1:M
if sqrt(sum((A(i,:)-v(j,:)).^2))<=tol
in(j,i) = i;
end
end
end
in = in(:); % reshape to a column vector, as the output in the question's code
The other alternative, but it requires more intermediate memory, is to compute this without a loop at all:
A = reshape(A,1,N,[]);
v = reshape(v,M,1,[]);
in = sum(bsxfun(#minus,A,v).^2,3) < tol*tol;
in = in(:);
(Or something similar to that, I have not run this code... Please let me know, or fix the post, if there is a typo or other mistake.)
N = size(A,1);
M = size(v,1);
in = cell(N,1);
parfor i=1:N
s=v;
p=zeros(1:M,1);
for j=1:M
k = (i-1)*M+j;
if sqrt(sum((A(i,:)-s(j,:)).^2))<=tol
p(k) = i;
end
end
in{i}=single(p);
end
in=cell2mat(in);
in=reshape(in,[N*M,1]);
sometime matlab doesnt recognize a variable in parfor loop as "sliced variable"
, a sliced variable is a variable that has a reference out of parfor loop and each of its element only accessed by a single worker (in parfor parallel workers)
so you could use a temporary variable and collect results after the parfor loop,
NOTE 1: It is better to vectorise code in the older versions because loops didn't use to be as good as they are now in R2017 referring to (this).
NOTE 2: if most elements of "in" are zero try using "sparse matrix" which could save a lot of memory;
total newbie here. I'm having problems looping a function that I've created. I'm having some problems copying the code over but I'll give a general idea of it:
function[X]=Test(A,B,C,D)
other parts of the code
.
.
.
X = linsolve(K,L)
end
where K,L are other matrices I derived from the 4 variables A,B,C,D
The problem is whenever I execute the function Test(1,2,3,4), I can only get one answer out. I'm trying to loop this process for one variable, keep the other 3 variables constant.
For example, I want to get answers for A = 1:10, while B = 2, C = 3, D = 4
I've tried the following method and they did not work:
Function[X] = Test(A,B,C,D)
for A = 1:10
other parts of the code...
X=linsolve(K,L)
end
Whenever I keyed in the command Test(1,2,3,4), it only gave me the output of Test(10,2,3,4)
Then I read somewhere that you have to call the function from somewhere else, so I edited the Test function to be Function[X] = Test(B,C,D) and left A out where it can be assigned in another script eg:
global A
for A = 1:10
Test(A,2,3,4)
end
But this gives an error as well, as Test function requires A to be defined. As such I'm a little lost and can't seem to find any information on how can this be done. Would appreciate all the help I can get.
Cheers guys
I think this is what you're looking for:
A=1:10; B=2; C=3; D=4;
%Do pre-allocation for X according to the dimensions of your output
for iter = 1:length(A)
X(:,:,iter)= Test(A(iter),B,C,D);
end
X
where
function [X]=Test(A,B,C,D)
%other parts of the code
X = linsolve(K,L)
end
Try this:
function X = Test(A,B,C,D)
% allocate output (it is faster than changing the size in every loop)
X = {};
% loop for each position in A
for i = 1:numel(A);
%in the other parts you have to use A(i) instead of just A
... other parts of code
%overwrite the value in X at position i
X{i} = linsolve(K,L);
end
end
and run it with Test(1:10,2,3,4)
To answer what went wrong before:
When you loop with 'for A=1:10' you overwrite the A that was passed to the function (so the function will ignore the A that you passed it) and in each loop you overwrite the X calculated in the previous loop (that is why you can only see the answer for A=10).
The second try should work if you have created a file named Test.m with the function X = (A,B,C,D) as the first code in the file. Although the global assignment is unnecessary. In fact I would strongly recommend you not to use global variables as it gets very messy very fast.
I am trying to write a function transform(A) which when given a matrix A returns a new matrix. The new matrix should be obtained according to following:
if A has more than one row then interchange the first and second row. After this square the elements in the first row.
So far thats what I have written:
function[Anew] = transform(A)
dimension = size(A);
if dimension(1) > 1 %if there is more than 1 row
A([1 2],:)=A([2 1],:);
end
A(1,:,:) = A(1,:,:).^2 %squares elements in the first row
end
I tested my function by invoking it in matlab.
I noticed that because i dont have a semi colon next to A(1,:,:) = A(1,:,:).^2
I still obtain the desired result but not as output of the function.
I obtain A =
instead of Anew =
If i put a semi colon next to A(1,:,:) = A(1,:,:).^2; then I dont get an output at all.
Could you tell me what is wrong and what should I change in my program to obtain output as Anew?
Thank you
To return a value from a function in Matlab, you must directly assign to it.
In your case, you need to assign to Anew at the end of the operation (you could also technically just use that variable all-together).
function [Output1, Output2] = SomeFunction( ... )
% Do Some Work
% Store Output
Output1 = Result1(:,2) %some output
Output2 = Result2(3:end, :) %some other result
end
function[Anew] = transform(A)
dimension = size(A);
Anew = A;
if dimension(1) > 1 %if there is more than 1 row
Anew ([1 2],:)=Anew([2 1],:);
end
Anew(1,:,:) = Anew(1,:,:).^2; %squares elements in the first row
end
I have a little parfor test script which gives the warning in the title.
The code is this
out = zeros(10, 1);
in = rand(5e8, 10);
tic
parfor i = 1:10
for j = 1:5e8
p = floor(rand(1,1)*5e8);
out(i) = out(i) + in(p, i);
end
end
toc
tot = sum(out)
the warning comes out on line 7 regarding how variable in is accessed.
I don't understand why, slicing should be trivial. Just send each column of in to each worker.
If I change the code to
out = zeros(10, 1);
in = rand(5e8, 10);
tic
parfor i = 1:10
a = in(:,i);
for j = 1:5e8
p = floor(rand(1,1)*5e8);
out(i) = out(i) + a(p);
end
end
toc
tot = sum(out)
the warning disappears but I don't like that assignment to a.
The code was explicitly designed to mess up the cache memory.
Unfortunately, as explained here http://www.mathworks.com/help/distcomp/advanced-topics.html#bq_of7_-1 , MATLAB does not understand how to slice in, hence the code analyser warning. You have to read that page fairly closely to understand why it cannot be sliced. The relevant paragraph is:
Form of Indexing. Within the list of indices for a sliced variable, one of these indices is of the form i, i+k, i-k, k+i, or k-i, where i
is the loop variable and k is a constant or a simple (nonindexed)
broadcast variable; and every other index is a scalar constant, a
simple broadcast variable, colon, or end.
The clause in bold type at the end is the relevant one - in your case, p does not match this constraint.
I am having trouble using struct arrays in Matlab's parfor loop. The following code has 2 problems I do not understand:
s=struct('a',{},'b',{});
if matlabpool('size')==0
matlabpool open local 2
end
for j = 1:2
parfor k=1:4
fprintf('[%d,%d]\n',k,j)
s(j,k).a = k;
s(j,k).b = j;
end
end
matlabpool close
It fails with an error Error using parallel_function (line 589)
Insufficient number of outputs from right hand side of equal sign to satisfy assignment.
On output, variable s is a vector, not an array (as it should be, even if the code breaks before finishing).
EDIT the problem is solved if I initialize the struct arrays to the correct size, by:
s=struct('a',cell(2,4),'b',cell(2,4));
However, I would still be happy to get insights about the problem (e.g is it rally a bug, as suggested by Oleg Komarov)
It was originally working fine for me but then I don't know what happens. In general you need to be careful with parfor loops and there are ample documentation on how to align everything. Two different words of advice.
First and more importantly, the parfor loop is on the outside loop:
function s = foo
s=struct('a',{},'b',{});
parfor j = 1:2
for k=1:4
fprintf('[%d,%d]\n',k,j)
s(j,k).a = k;
s(j,k).b = j;
end
end
Two, Matlab gets very picky about writing the main exit variable (i.e. the variable contained in the parfor loop which is indexed to the loop, in your case, s). You first want to create a dummy variable that holds all the innerloop information, and then writes to it once at the end of the loops. Example:
function s = khal
s=struct('a',{},'b',{});
parfor j = 1:2
dummy=struct('a',{},'b',{});
for k=1:4
fprintf('[%d,%d]\n',k,j)
dummy(k).a = k;
dummy(k).b = j;
end
s(j,:) = dummy;
end
You don't have a problem here, but it can get complicated in other instances