First Question:
Hello there!
I am trying to assign this vector
v(mod(v,2)~=0)=0
The operations is supposed to replace odd numbers in a vector with 0. I am trying to assign this vector to result variable in a function.
something goes wrong when I try this
function
function [result1,result2] = myfunction(v)
v(mod(v,2)==0)= 0;
result1 = v;
v(mod(v,2)~=0) = 0;
result2 = v;
return
QUESTION 2:
I am trying to figure out an alternative way to express the the function cell2struct in for-loop format
for example,
if we have a cell array with 2 dimensions containing food labels. Their names, caloric count and price, each in one column. Can we write a function that's can transfer the information in cells to a struct that contain each of the fields above?
Thanks
Question #1
You are setting all even numbers to 0, but then you are using this mutated result to search for odd numbers and setting them to 0. This will probably not give you what you intended as you are using a modified copy of the original vector, so it would be prudent that you keep a copy of the vector before doing each operation.
function [result1,result2] = myfunction(v)
vcopy = v; %// Make a copy
vcopy(mod(vcopy,2)==0)= 0; %// Find even numbers and set to 0
result1 = vcopy;
vcopy = v; %// Make another copy
vcopy(mod(vcopy,2)~=0) = 0; %// Find odd numbers and set them to 0.
result2 = vcopy;
return
Question #2
Yup. If you have a list of field names stored in f and their corresponding entities for each field stored in c, simply use a loop like so:
function [s] = my_cell2struct(c, f)
for idx = 1 : numel(f)
s.(f{idx}) = c{idx};
end
The above code does no error checking, so you need to make sure that the total number of elements in c matches those of f. Also, c and f must be cell arrays. Notice that s wasn't declared at all in the function. Also, using the dot operator combined with enclosing brackets and a string that goes inside the enclosing brackets allows you to dynamically create field names on the fly. As such, for each string in f, we access the corresponding value stored in c, and we create a field name that contains this value.
Here's a reproducible example from the MathWorks documentation:
c = {'tree',37.4,'birch'};
f = {'category','height','name'};
s = cell2struct(c, f, 2)
s =
category: 'tree'
height: 37.4000
name: 'birch'
Notice that I use cell2struct here from native MATLAB to produce the above structure. Doing the above for loop which is wrapped in a function called my_cell2struct, we get:
c = {'tree',37.4,'birch'};
f = {'category','height','name'};
s = my_cell2struct(c, f)
s =
category: 'tree'
height: 37.4000
name: 'birch'
Related
I find it impossible to write MATLAB code without creating a huge number of superfluous, single-use variables.
For example, suppose function foo returns three column vectors of exactly the same size. Say:
function [a, b, c] = foo(n)
a = rand(n, 1);
b = rand(n, 1);
c = rand(n, 1);
end
Now, suppose that bar is a function that expect as imput a cell array of size (1, 3).
function result = bar(triplet)
[x, y, z] = triplet{:};
result = x + y + z;
end
If I want to pass the results of foo(5), I can do it by creating three otherwise-useless variables:
[x, y, z] = foo(5);
result = bar({x, y, z});
Is there some function baz that would allow me to replace the two lines above with
result = bar(baz(foo(5)));
?
NB: the functions foo and bar above are meant only as examples. They're supposed to represent functions over which I have no control. IOW, modifying them is not an option.
You can replace the three variables by a cell array using a comma-separated list:
vars = cell(1,3); % initiallize cell array with as many elements as outputs of foo
[vars{:}] = foo(5); % comma-separated list: each cell acts as a variable that
% receives an output of foo
result = bar(vars);
Not possible. baz in baz(foo(5)) will only take the first output of foo, the other two would be ignored. The plain two-line variant is not that awkward. And this is not a common situation. You don't generally work with cell arrays where normal numerical arrays would do.
You could of course just write your own wrapper for foo that returns whatever you need (i.e. containing similar two lines), in case you need to use it frequently.
As nirvana-msu said, it is not possible to do the task without creating temporary variables. But it is possible to handle it within a function and even with varargin and varargout. Inspired by this answer on my question, you can define and use the following function:
function varargout = redirect(F1, F2, count, list, varargin)
output = cell(1, count);
[output{:}] = F2(varargin{:});
varargout = cell(1, max(nargout, 1));
[varargout{:}] = F1(output(list));
end
For instance, in your example you can write result = redirect(#bar, #foo, 3, [1:3], 5);
The issue is you are converting the cell triplet{} into an array[], so a conversion is the method you want. Although this method will perform the inverse transformation, I know of no method that will perform the transformation you want, likely due to the relative complexity of the cell data structure. You may have some luck further digging into the API.
EDIT: EBH kindly pointed out this method, which does what you are looking for.
EDIT2: The above method will not perform the action OP asked for. Leaving this up because the API often has great solutions that are hidden by bad names.
This is my code in Matlab: How could I get all values of all 5 images saved? This code only returns the last image! I tried using IM(l) but it gives me an error: In an assignment A(I) = B, the number of elements in B and I must be the same.
Amount_measurements = 5;
IM=zeros(2097152,1);
l=1;
for l=(1:Amount_measurements)
if l < 9
%index = double(0)+double(0)+double(l+1);
index = strcat(num2str(double(0)),num2str(double(0)),num2str(double(l+1)));
elseif l < 99
index = double(0)+double(l+1);
else
index = double(l+1);
end
file_name1='trial.nii.gz';
%disp(file_name1);
jesu=load_nii(file_name1);
[x,y,z] = meshgrid(1:256,1:256,1:256);
[lx,ly,lz] = meshgrid(1:2:256,1:2:256,1:2:256);
newImage = interp3(x,y,z,jesu.img,lx,ly,lz);
IM= newImage(:);
end
I want the values newImage(:) to be stored as IM1=newImage(:) IM2=newImage(:) IM3=newImage(:) IM4=newImage(:) so on... How could I go about with it?
Since you mentioned wanting a variable-length version of IM1=newImage(:) IM2=newImage(:) IM3=newImage(:) IM4=newImage(:), you're looking for a cell array. Try
IM{l} = newImage;
instead of
IM(l) = newImage(:);
The important difference is the use of braces rather than parentheses. Use a right-hand side ofnewImage(:) if you want to reshape into a vector, just newImage if you want to preserve it as a matrix.
By using IM(l) you're trying to add an entire column vector (newImage(:)) to a single element (the l-th element) in the array IM, that's why Matlab throws the error.
You should consider concatenation: since newImage(:) is a column-vector, replace
IM= newImage(:);
with
IM=[IM newImage(:)];
but at the top of the script you should also initialize IM as
IM=[];
At the end of the loop, the resulting IM will have Amount_measurements columns where 1 column = 1 newImage(:).
Note #1: this will only work if newImage(:) always has the same length.
Note #2: if you know a priori how long the vector newImage(:) is and, again, by assuming that its length never changes, you should consider preallocating the IM matrix by replacing IM=[]; with IM=zeros(X,Amount_measurements); where X is the number of elements in newImage(:). Finally, regarding the concatenation stage, you should replace IM=[IM newImage(:)]; with IM(:,l)=newImage(:).
Note #3: as instead, if the size of newImage(:) can change you cannot rely on preallocation and matrices, but you must use cell arrays: the last instruction in your loop should be IM{l}=newImage(:);.
I want to define an indexing variable at the beginning of a function to subsequently index several vectors. The index should point to a specific element (eg element nr 3) or to the last entry of the vectors.
I hoped to be able to make the indexing variable a string and evaluate it.
In case of a specific element it would be:
idx = '3';
vector1(eval(idx))
vector2(eval(idx))
This works. But for the last element the approach fails:
idx = 'end';
vector1(eval(idx))
vector2(eval(idx))
just as eval('end'), it throws the following error:
Error: Illegal use of reserved keyword "end".
Does anyone know a solution how to set up an indexing variable, so that it can index to a specific element or the last element of vectors with unknown size?
Thanks!
Your issue is that you're trying to do two different things. In one case (defining idx as a scalar) you are accessing the same element in each vector. In the second case, you are asking MATLAB to potentially access a different element in each vector (unless you know for certain that each vector is the same length). You will have to treat each case differently. You could write a subfunction that does something like this
function element = return_element(vector, idx)
if idx <= 0
element = vector(end);
else
element = vector(idx);
end
end
You would call it like this:
idx = -1;
% lots of code
return_element(vector1,idx); % = vector1(end)
return_element(vector2,idx); % = vector2(end)
idx = 3;
% lots of code
return_element(vector1,idx); % = vector1(3)
return_element(vector2,idx); % = vector2(3)
Is there something inbuilt in MATLAB which functions similar to the SQL COALESCE function. I want that function to return the first 'existing' value from all the arguments.
For example,
clear A B; C=10; COALESCE(A,B,C)
should return value of C (because A and B are unassigned/don't exist).
I know it would be very easy to code, and I am just being lazy here. But, I would be surprised if MATLAB doesn't have a similar function.
As far as I know there is no built-in function for that. But you can easily write your own.
Note that it is not possible in Matlab to pass a variable that has not been defined prior of using it. Therefore your proposed call clear A B; C=10; COALESCE(A,B,C) is invalid and will throw an error. Instead we can define an empty variable using var=[].
The following code creates two empty variables A, B and and assigns C=10. Inside the function coalesce we assume at the beginning that all variables are empty. In the for-loop we return the first non-empty variable. In the version without for-loop we get the index of the first non-zero element and then return the corresponding content of the cell if a non-zero element exists.
If you want the function to be accessible from everywhere within Matlab, see the documentation here.
function testcoalesce
A = [];
B = [];
C = 10;
COALESCE(A,B)
COALESCE(A,B,C)
end
% with for-loop (much faster)
function out = COALESCE(varargin)
out = [];
for i = 1:length(varargin)
if ~isempty(varargin{i})
out = varargin{i};
return;
end
end
end
% without for-loop (slower)
function out = COALESCE(varargin)
out = [];
ind = find(cellfun('isempty', varargin)==0, 1);
if ~isempty(ind);
out = varargin{ind};
end
end
The output is as expected:
ans =
[]
ans =
10
Timing the two functions showed, that the first solution using the for-loop is approximately 48% faster than the function without loop.
(10 samples, 1'000'000 iterations, 3 variables & 20 variables)
I have a function called SpreadFinder which returns an array of structures:
[ spreads ] = SpreadFinder( pp, corr_thres, i_range )
The return object looks like this:
spreads =
1x4026 struct array with fields:
px
tuple
beta
The code is:
m = containers.Map(0, 0, 'uniformValues',false); m.remove(0);
parfor col = 1:8
spreads = SpreadFinder(pp, 0.8, i_ranges(:,col)');
m(col) = spreads;
end
When I run it, I have this error:
Subscripted assignment dimension mismatch.
I know the Map can be used to store an array of structures. (https://stackoverflow.com/a/2365526)
This code below works. So I don't think it's a problem of dimension...
m(1) = spreads(1:2);
m(2) = spreads(1:4);
The objective is to merge all the results into one map and possibly iterate over this map after the parallel task.
Can someone help me out?
Best,
IIRC, you cannot assign to a containers.Map inside a parfor because a containers.Map is not sliceable. If you want to assign elements to an array inside a parfor, it must be sliceable. What I mean by sliceable is that you are able to extract a subset of the values via the colon operator (i.e. 1:3, 2:4), etc. You cannot do that with a containers.Map unless you specifically use the values function, and from there you can specify a cell array of keys and get the values from there. Using () syntax only permits you to access one key at a time.
Therefore, what I would suggest you do is assign the outputs to a cell array, and use this cell array as values into your containers.Map. Something like this:
%// Create temporary cell array of values
c = cell(1,8);
%// Run parfor for each col
parfor col = 1:8
c{col} = SpreadFinder(pp, 0.8, i_ranges(:,col)');
end
%// Make a new containers.Map for these values
m = containers.Map(1:8, c, 'uniformvalues', false);
%clear c; %// Optional if you don't want the temporary cell array to stick around