I have a Matlab script including a for loop which loos like the following:
for k = 1:10
c = myfun(k,a,b);
result{k} = c;
end
Right now, the problem is that during the for loop, sometimes myfun() may have errors and stop. After fixing the error in myfun(), how can I continue to run with the existing value of variables? The reason is that myfun() will take a very long time to get the result and the previous results are right.
For example, if a error happens when k == 4, then I save all the variables in the current workspace. I set a breakpoint at c = myfun(k,a,b); and restore the saved variables, but I find that in the next loop, k will be 2 instead of 5 as I want. Matlab is not allowed to modify the value of k during the for loop I think. I have tested this for a few times.
How can I continue the for loop with some existing data?
You cannot change your for loop iterator programmatically inside of the loop.
For example:
for ii = 1:3
disp(ii)
ii = 3;
end
Prints:
1
2
3
If you're going to be modifying code based on errors received, dbstop if error is not going to be beneficial because it will not reflect changes in your code until the debugger is exited and your code executed again (unless you execute manually in the debugger). If you're not modifying code you could potentially use a try/except clause to catch fixable issues.
If you're loading data for a later index and then restarting, you can change where your for loop begins, or use a while loop (if appropriate).
For example:
% Load data here
for ii = 3:3
disp(ii)
end
Prints 3.
Where the while interpretation would be:
% Load data here
ii = 3
while ii <= 3
disp(ii)
ii = ii + 1;
end
For the same result.
On solution can be first catch the exception likes the following and pass from them:
bug = [];
for k = 1:10
try
c = myfun(k,a,b);
result{k} = c;
catch
warning('some bug for the following values:');
display([k a b]);
bug = [bug; k a b];
result{k} = NaN;
end
end
Then iterate over bug to compute missing information after debugging. This solution works when your algorithm is not dependent on the previous value of the result (or is not recursive).
Related
I have some image files. I'm trying to perform some calculations using each file and if a certain condition is met, I'd like to go back to a particular line in the code and run it from there once more. But only just once again. Regardless of whether the if condition is satisfied or not satisfied the second time, I want to go to the next iteration. But, MATLAB doesn't seem to have a goto function and also, using goto implies bad programming so I thought I'd just iterate the for loop twice for a particular 'i' value which satisfies the if condition.
file = dir('*.jpg');
n = length(file);
for i = 1:n
*perform some operations on the 'i'th file*
if 'condition'
*run the for loop again for the 'i'th file instead of going to the 'i+1'th file*
i=i-1;
else
*go to next iteration*
end
end
I have tried to code this by changing the loop variable 'i' inside the loop to 'i-1' so that on the next iteration, the 'i'th loop will be repeated again but doing so is giving the wrong output, though I don't know if there is some other error in my code or if the changing of the loop variable internally is the cause of the problem. Any help on this is appreciated.
Replace the for loop with a while loop to have a bit more flexibility. The only difference is that you have to manually increment i, hence this also allows you to not increment i.
Given your new requirement, you can keep track of the number of attempts, and easily change this if needed:
file = dir('*.jpg');
n = length(file);
i = 1;
attempts = 1;
while i <= n
% perform code on i'th file
success = doSomething(); % set success true or false;
if success
% increment to go to next file
i = i + 1;
elseif ~success && attempts <= 2 % failed, but gave it only one try
% increment number of attempts, to prevent performing
attempts = attempts + 1;
else % failed, and max attempts reached, increment to go to next file
i = i + 1;
% reset number of attempts
attempts = 1;
end
end
Given the new requirement, added after rinkert's answer, the simplest approach becomes separating out code from your loop in a separate function:
function main_function
file = dir('*.jpg');
n = length(file);
for i = 1:n
some_operations(i);
if 'condition'
some_operations(i);
end
end
function some_operations(i)
% Here you can access file(i), since this function has access to the variables defined in main_function
*perform some operations on the 'i'th file*
end
end % This one is important, it makes some_operations part of main_function
I am trying to force a for loop to restart if a condition is not stratified. I can do it with while since I want the loop to run for a certain number of iterations. I tried to set iter=iter-1 inside the if statement but it did not work. Any suggestions?
R=2*10^3;
lamda= 0.00001;
h=100;
a = 9.6117;
b = 0.1581;
for iter=1:10
M=poissrnd(lamda*R^2);
xx=R*rand(1,M);
yy=R*rand(1,M);
zz=ones(1,M)*h;
BS=[xx' yy' zz'];
user=[0,0, 0];
s=pdist2(BS(:,1:2),user(1,1:2));
anga=atand(h./s);
PL=1./(1+(a*exp(b*(a-anga))));
berRV=binornd(1,PL);
if berRV(1)==1
% do something
else
% repeat
end
end
You can accomplish this by using a while loop, with a comparison to whether the number of needed results have been identified. See the comment about saving your found values, as you hadn't specified what needs to be done when the condition you're searching for is satisfied.
R=2*10^3;
lamda= 0.00001;
h=100;
a = 9.6117;
b = 0.1581;
total_results_found = 0;
needed_results_found = 10;
while total_results_found < needed_results_found
M=poissrnd(lamda*R^2);
xx=R*rand(1,M);
yy=R*rand(1,M);
zz=ones(1,M)*h;
BS=[xx' yy' zz'];
user=[0,0, 0];
s=pdist2(BS(:,1:2),user(1,1:2));
anga=atand(h./s);
PL=1./(1+(a*exp(b*(a-anga))));
berRV=binornd(1,PL);
if berRV(1)==1
% save the result here
% iterate the counter
total_results_found = total_results_found + 1;
end
end
The simplest approach here would be with a while loop inside the for loop:
for iter=1:10
berRV(1) = 0
while berRV(1)~=1
% original loop code here
end
% do something
end
[Sadly, MATLAB does not have a do...while loop, it would make the above a little cleaner.]
While the other two solutions are perfectly valid, I wanted to give you another solution, which I think is the closest to the logic you provided in the question.
for iter=1:10
while 1
% loop code here
if berRV(1) == 1
break
end
end
end
The idea is similar to the one Cris presents, namely that you repeat the body of the for-loop until some condition is met. The difference lies solely in how you terminate the while loop,
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'm running a while loop and I am running in to some problems.
I have the following piece of code:
Angle_int = 0.5; % Initial interpolation angle of attack
Clmax2d(1,1:length(Yle_wing)) = 3; % Dummy value
diff = 0; % Dummy value
while sum(diff < 0) > fix(tol*length(Yle_wing))
Angle_int = Angle_int + 0.5; % Interpolation angle increases with 0.5 with every iteration
for j = 1:length(Yle_wing)
CL3d = interp1(Angle,[cl_matrix(1,j) cl_matrix(2,j) cl_matrix(3,j)],Angle_int,'linear');
CL_3DD(:,j) = CL3d;
end
diff = (Clmax2d - CL_3DD); % Difference between Cl2d and Cl3d
Angle_stall = Angle_int;
CL_3D = CL_3DD;
end
For some reason, CL_3D = CL_3DD; and Angle_stall = Angle_int; seem to disappear when the while loop finishes. Hence, I cannot use their converged values ahead of the while loop since these variables are not recognized. I get the following error:
Undefined function or variable "CL_3D".
Hence, does someone knows what I am doing wrong? or any tips would be welcome as well.
Thanks in advance,
cheers
The error message:
Undefined function or variable "CL_3D".
is always because you're trying to use a variable or function that you haven't initialized yet. Often this happens in loops where you want to increment a counter, or compare values etc.
A common error is doing something like this without writing ii = 0 in front of the loop:
while ii < some_num
ii = ii + 1;
some_function();
end
With your dummy variables, you never enter the loop (unless tol < 0, which seems like an odd choice). You probably want to initialize diff = Inf or something like that. However, using diff as a variable name is not a good idea, since it's a builtin function.
You probably try to use CL_3D, when it's not yet initialized (somewhere else in your code, not in the part you posted). MATLAB tells you which line the error appears in, you should try using it!
Maybe initializing it like zeros(size(Clmax2d)); could work (it will definitely remove your error, but it might not give the desired behavior).
PS!
Using i and j as variables are not recommended as they represent the imaginary unit in MATLAB.
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