Say I have an array stack of dimensions NxMxR. I would like to use logical indexing on a given dimension to update its value, without the use of a temporary variable.
Example using the temporary variable:
for k=1:R
temp=stack(:,:,k);
temp(temp<max(max(temp))/2)=NaN;
cropped(:,:,k)=temp;
end
Note that it would be trivial if the condition was the same for the entire array, but it varies along the R dimension. bsxfun would be appropriate if I just wanted to subtract the threshold value in a syntax like:
cropped = bsxfun(#minus, stack, max(max(stack, [], 3))/2);
I find the use of the temporary variable clumsy and can't find a proper way of defining the function for bsxfun to do what the loop does.
Does anyone has a suggestion?
stack(bsxfun(#minus, stack, max(max(stack))/2) < 0) = NaN;
Another bsxfun implementation -
t1 = reshape(stack,size(stack,1)*size(stack,2),[])
t1(bsxfun(#lt,t1,max(t1)/2))=NaN
cropped = reshape(t1,size(stack))
One more -
t1 = max(reshape(stack,size(stack,1)*size(stack,2),[]))
stack(bsxfun(#lt,stack,permute(t1/2,[3 1 2])))=NaN
And more that is dangerously close to Luis's answer -
stack(bsxfun(#lt,stack,max(max(stack)/2)))=NaN
Related
Is there a way to create an array of sets in Matlab.
Eg: I have:
a = ones(10,1);
b = zeros(10,1);
I need c such that c = [(1,0); (1,0); ...], i.e. each set in c has first element from a and 2nd element from b with the corresponding index.
Also is there some way I can check if an unknown set (x,y) is in c.
Can you all please help me out? I am a Matlab noob. Thanks!
There are not sets in your understanding in MATLAB (I assume that you are thinking of tuples on Python...) But there are cells in MATLAB. That is a data type that can store pretty much anything (you may think of pointers if you are familiar with the concept). It is indicated by using { }.
Knowing this, you could come up with a cell of arrays and check them using cellfun
% create a cell of numeric arrays
C = {[1,0],[0,2],[99,-1]}
% check which input is equal to the array [1,0]
lg = cellfun(#(x)isequal(x,[1,0]),C)
Note that you access the address of a cell with () and the content of a cell with {}. [] always indicate arrays of something. We come to this in a moment.
OK, this was the part that you asked for; now there is a bonus:
That you use the term set makes me feel that they always have the same size. So why not create an array of arrays (or better an array of vectors, which is a matrix) and check this matrix column-wise?
% array of vectors (there is a way with less brackets but this way is clearer):
M = [[1;0],[0;2],[99;-1]]
% check at which column (direction ",1") all rows are equal to the proposed vector:
lg = all(M == [0;2],1)
This way is a bit clearer, better in terms of memory and faster.
Note that both variables lg are arrays of logicals. You can use them directly to index the original variable, i.e. M(:,lg) and C{lg} returns the set that you are looking for.
If you would like to get logical value regarding if p is in C, maybe you can try the code below
any(sum((C-p).^2,2)==0)
or
any(all(C-p==0,2))
Example
C = [1,2;
3,-1;
1,1;
-2,5];
p1 = [1,2];
p2 = [1,-2];
>> any(sum((C-p1).^2,2)==0) # indicating p1 is in C
ans = 1
>> any(sum((C-p2).^2,2)==0) # indicating p2 is not in C
ans = 0
>> any(all(C-p1==0,2))
ans = 1
>> any(all(C-p2==0,2))
ans = 0
I need to create an array filled within a range in Matlab
e.g.
from=2
to=6
increment=1
result
[2,3,4,5,6]
e.g.
from=15
to=25
increment=2
result
[15,17,19,21,23,25]
Obviously I can create a loop to perform this action from scratch but I wondering if there is a coincise and efficent way to do this with built-in matlab commands since seems a very common operation
EDIT
If I use linspace the operation is weird since the spacing between the points is (x2-x1)/(n-1).
This can be handled simply by the : operator in the following notation
array = from:increment:to
Note that the increment defaults to 1 if written with only one colon seperator
array = from:to
Example
array1 = 2:6 %Produces [2,3,4,5,6]
array2 = 15:2:25 %Produces [15,17,19,21,23,25]
I'd like to replace a specific number of elements of my cell to zero without using for. For example to replace elements of row 2 in example cell a below: How should I proceed possibly using cellfun?
a=cell(2,3);
cellfun(#(x)(zeros(a{x}(2,:))),a);
It gives the error "Bad cell reference operation".
what if I'd like to make row 2 empty again?
Thanks in advance for any help
The action you want to perform requires an assignment within a function. The only way to achieve this is using eval, which is considered bad practice.
A loop is therefore the best remaining option, if you want to keep everything in one script:
A = {randn(2,3),randn(2,3)};
for ii = 1:numel(A)
A{ii}(2,:) = 0;
end
If you don't bother using multiple files, you can put the assignment in a function:
function [ out ] = setZero( cellarray, rowidx )
out = cellarray;
out(rowidx,:) = 0;
end
and use it as follows:
A = cellfun(#(x) setZero(x,2),A ,'uni',0)
You need to find a transformation that turns a given matrix A to a matrix where the second row is all-zero. Here are three alternatives
A=cellfun(#(x) [x(1,:); zeros(size(x(2,:))); x(3:end,:)], A, 'uni', 0)
and
A=cellfun(#(x) diag(1:size(x,1)~=2)*x, A, 'uni', 0)
and
A=cellfun(#(x) bsxfun(#times, (1:size(x,1))' ~= 2, x), A, 'uni', 0)
The first one is the most robust one because it will handle the cases that your matrix has NaN elements. The second and third alternatives simply multiply the second row by zero. The second achieves this by multiplying it with a diagonal matrix where all diagonal elements are 1 except element (2,2) which is zero. The third alternative achieves this using bsxfun.
This is to demonstrate that you can achieve this without for loops however a simple for loop is much more readable.
Consider the following code snippet
for i = 1:100
Yi= x(i:i + 3); % i in Yi is not an index but subscript,
% x is some array having sufficient values
i = i + 3
end
Basically I want that each time the for loop runs the subscript changes from 1 to 2, 3, ..., 100. SO in effect after 100 iterations I will be having 100 arrays, starting with Y1 to Y100.
What could be the simplest way to implement this in MATLAB?
UPDATE
This is to be run 15 times
Y1 = 64;
fft_x = 2 * abs(Y1(5));
For simplicity I have taken constant inputs.
Now I am trying to use cell based on Marc's answer:
Y1 = cell(15,1);
fft_x = cell(15,1);
for i = 1:15
Y1{i,1} = 64;
fft_x{i,1} = 2 * abs(Y1(5));
end
I think I need to do some changes in abs(). Please suggest.
It is impossible to make variably-named variables in matlab. The common solution is to use a cell array for Y:
Y=cell(100,1);
for i =1:100
Y{i,1}= x(i:i+3);
i=i+3;
end
Note that the line i=i+3 inside the for-loop has no effect. You can just remove it.
Y=cell(100,1);
for i =1:100
Y{i,1}= x(i:i+3);
end
It is possible to make variably-named variables in matlab. If you really want this do something like this:
for i = 1:4:100
eval(['Y', num2str((i+3)/4), '=x(i:i+3);']);
end
How you organize your indexing depends on what you plan to do with x of course...
Yes, you can dynamically name variables. However, it's almost never a good idea and there are much better/safer/faster alternatives, e.g. cell arrays as demonstrated by #Marc Claesen.
Look at the assignin function (and the related eval). You could do what asked for with:
for i = 1:100
assignin('caller',['Y' int2str(i)],rand(1,i))
end
Another related function is genvarname. Don't use these unless you really need them.
I often find myself wanting to collapse an n-dimensional matrix across one dimension using a custom function, and can't figure out if there is a concise incantation I can use to do this.
For example, when parsing an image, I often want to do something like this. (Note! Illustrative example only. I know about rgb2gray for this specific case.)
img = imread('whatever.jpg');
s = size(img);
for i=1:s(1)
for j=1:s(2)
bw_img(i,j) = mean(img(i,j,:));
end
end
I would love to express this as something like:
bw = on(color, 3, #mean);
or
bw(:,:,1) = mean(color);
Is there a short way to do this?
EDIT: Apparently mean already does this; I want to be able to do this for any function I've written. E.g.,
...
filtered_img(i,j) = reddish_tint(img(i,j,:));
...
where
function out = reddish_tint(in)
out = in(1) * 0.5 + in(2) * 0.25 + in(3) * 0.25;
end
Many basic MATLAB functions, like MEAN, MAX, MIN, SUM, etc., are designed to operate across a specific dimension:
bw = mean(img,3); %# Mean across dimension 3
You can also take advantage of the fact that MATLAB arithmetic operators are designed to operate in an element-wise fashion on matrices. For example, the operation in your function reddish_tint can be applied to all pixels of your image with this single line:
filtered_img = 0.5.*img(:,:,1)+0.25.*img(:,:,2)+0.25.*img(:,:,3);
To handle a more general case where you want to apply a function to an arbitrary dimension of an N-dimensional matrix, you will probably want to write your function such that it accepts an additional input argument for which dimension to operate over (like the above-mentioned MATLAB functions do) and then uses some simple logic (i.e. if-else statements) and element-wise matrix operations to apply its computations to the proper dimension of the matrix.
Although I would not suggest using it, there is a quick-and-dirty solution, but it's rather ugly and computationally more expensive. You can use the function NUM2CELL to collect values along a dimension of your array into cells of a cell array, then apply your function to each cell using the function CELLFUN:
cellArray = num2cell(img,3); %# Collect values in dimension 3 into cells
filtered_img = cellfun(#reddish_tint,cellArray); %# Apply function to each cell
I wrote a helper function called 'vecfun' that might be useful for this, if it's what you're trying to achieve?
link
You could use BSXFUN for at least some of your tasks. It performs an element-wise operation among two arrays by expanding the size 1 - dimensions to match the size in the other array. The 'reddish tint' function would become
reddish_image = bsxfun(#times,img,cat(3,0.5,0.25,0.25));
filtered_img = sum(reddish_image,3);
All the above statement requires in order to work is that the third dimension of img has size 1 or 3. Number and size of the other dimensions can be chosen freely.
If you are consistently trying to apply a function to a vector comprised by the 3 dimension in a block of images, I recommend using a pair reshapes, for instance:
Img = rand(480,640,3);
sz = size(Img);
output = reshape(myFavoriteFunction(reshape(Img,[prod(sz(1:2)),sz(3)])'),sz);
This way you can swap in any function that operates on matrices along their first dimension.
edit.
The above code will crash if you input an image which has only one layer: The function below can fix it.
function o = nLayerImage2MatrixOfPixels(i)
%function o = nLayerImage2MatrixOfPixels(i)
s = size(i);
if(length(s) == 2)
s3 = 1;
else
s3 = s(3);
end
o = reshape(i,[s(1)*s(2),s(3)])';
Well, if you are only concerned with multiplying vectors together you could just use the dot product, like this:
bw(:,:,1)*[0.3;0.2;0.5]
taking care that the shapes of your vectors conform.