I have matrix A and B. I want to set all values in A to 0, where B is false (has 0 as value). How can I do this the best way?
A and B have the same size.
Use Boolean operator, assuming A and B are of same sizes, but they don't have to be of same datatype
A(~B) = 0
More info here.
find all indices of B with 0 and set those indices in A as 0
>>A(B == 0) = 0
To see what is exactly happening you can use this as well
size=size(im);
for x=1:size(1)
for y=1:size(2)
if B(x,y)==0;
A(x,y)=0;
end
end
end
Related
I have searched the forum and have not found enough info to help me solve this problem.
Consider the set (cell of vectors)
A = {[1],[1 2],[2],[1 2 3],[1 2 3 4],[1 3]}
I want to construct a matrix B that looks like
B = [1 1 0 1 1 1
0 1 0 1 1 0
0 1 1 1 1 0
0 0 0 1 1 0
0 0 0 0 1 0
0 0 0 1 1 1]
The matrix B specifies membership of vectors with respect to each other. That is, the first row looks at the first element in A, [1], and checks if it is a member of the other vectors, placing a 1 if it is a member and a 0 otherwise.
I can do this using two for loops: one over the elements of A, and another nested, for each element of A, that checks membership with respect to every other member of A.
I want to avoid using for loops. Is there a vectorized solution for obtaining B from A?
With cell arrays it's hard to avoid loops, or their cousin cellfun. This is how I would do it:
[ii, jj] = ndgrid(1:numel(A)); % indices of all possible pairs
result = cellfun(#(x,y) all(ismember(x,y)), A(ii), A(jj)); % see if all elements in the
% first are present in the second
Well you asked for it, so here's an almost* vectorized solution using bsxfun and permute -
lens = cellfun('length',A)
vals = [A{:}]
mask = bsxfun(#ge,lens,[1:max(vals)]')
a = nan(size(mask))
a(mask) = vals
matches = bsxfun(#eq,a,permute(a,[3,4,1,2]));
out = bsxfun(#eq,squeeze(sum(any(matches,3),1)),lens(:))
*: Almost because of the use of cellfun at the start with cellfun('length',A), but since its just getting the length of the cells there, so computationally would be negligible .
Also, please note that this approach would use a lot of memory resources, so might not be beneficial, but just respecting the requirements of a vectorized solution as much as possible!
Say i have a 2D matrix A:
A = [ 1 1 0 0
1 0 0 0
1 1 1 0];
A is not necessarily binary, or even integer (i.e., floats are possible). I want to remove any column that contains uniform valued elements. In the above example, i would get:
1 0
0 0
1 1
To make this fully general, i'd like to allow the user to select the dimension along which rows/columns/slices are removed (i.e., with a DIM option).
Any ideas?
You could try using the min and max functions, which allow you to use the dim argument.
For example
index = min(A,[],1)==max(A,[],1);
A(:,index)=[];
will remove the columns you want. It is straightforward to do the same for rows
index = min(A,[],2)==max(A,[],2);
A(index,:)=[];
One-liner:
B = A(:,range(A)~=0); %//columns
The other one-liner is not that nice, and ugly one-liners should not be written down. :-) But is basically the same solution as S..'s, except is way more expensive (requires stats toolbox).
Please note that "generality" of subscript-based solutions doesn't extend to N-dimensional arrays as easily, because subscripting in ND arrays without checking beforehand the number of dimensions is difficult. Also, for the 1D arrays the notion of "uniformity" is a bit odd along the singleton dimension (the result is always empty).
Besides the neat solution provided by #S.. there is this simple hack also for your example:
for ii = 1:size(A,2)
T(ii) = all(A(:,ii) == sum(A(:,ii))/numel(A(:,ii)));
end
A(:,~T)
ans =
1 0
0 0
1 1
As suggested by #gariepy the right side of the equation can be replaced with mean function.
for ii = 1:size(A,2)
T(ii) = all( A(:,ii) == mean(A(:,ii)) );
end
A(:,~T)
A(:,~all(A == repmat(A(1,:),[size(A,1) 1])))
Inspired by #S.. but only checks if every element of the column equals the first element of the column. Seems like a little less work for the processor than finding the min and the max, and checking for equality.
I want want to do some string calculation using Matlab, and then stored the value in an matrix.
For numerical study, I often predefined the dimensions in Matlab using zeros to create a 4*4 array.
a = zeros(4)
Now I want to do the same thing for the symbolic matrix. Obviously zeros didn't work at this time.
I tried to copy the official tutorial at this page http://www.mathworks.com/help/symbolic/sym.html
a = sym('0' ,4) % error
Still didn't work.
Now I have do use the ugly code like this
a = sym('[0 0 0 0; 0 0 0 0; 0 0 0 0; 0 0 0 0]');
Since I will use iterations, and dimension of the matrix grows every time. This method is not convenient.
Do you have any ideas? Thanks a lot!
Num = sym(Num) converts a number or a numeric matrix Num to symbolic form.
a=sym(zeros(4,4))
Can't try but suspect that the variables get initialized as zero by default.
For example when using
a = sym('a' ,[2 2])
I have an array that starts of with zeros and continues into other numbers
I would like to delete the columns in the array that start off with zero but keep the other numbers
example of an column array below:
x= [0 0 0 0 0 2 4 6 8 0 1 2];
Answer of column array would look like
x= 2 4 6 8 0 1 2
I'm using octave 3.4.2/matlab
Thanks
Here is the code:
x = x(find(x~=0, 1):end);
or
x(1:find(x~=0,1)-1) = [];
The find command should work for this.
Assuming your vector is x:
find(x ~= 0)
Will return all indices where x is non-zero. Just grab the first index and go from there to delete all values from 1 to index.
Logical indexing will work just fine in this case: i.e.,
y = x(:,x(1,:)~=0)
will do the job for you. The inner logical comparison, x(1,:)~=0 returns true for every column whose first element is not zero. The indexing operation, x(:,...) selects only those columns for which the logical comparison returned true.
I have one matrix called targets (1X4000); column 1 to 2000 contains double value 0 and column 2001 to 4000 contains double value 1
a)
i want to create a matrix called targets_1 where i want to check if the value is 0 then make the entry 1 so at the end of the day i must have a matrix with :column 1 to 2000 with value 1 and column 2001:4000 with value zero
b)
Same situation as above but this time i want to check if the value is 1 then make the entry 1 and if it is zero then make the entry zero; at the end; my new matrix targets_2 contains values: column 1 to 2000 with value zero and column 2001:4000 with value 1
i know how to use the strcmp function to make such checking with strings, but problem is that my original matrix is double and i dont know if there is such function like
setosaCmp = strcmp('setosa',species);
which could work with double (numbers); any help would be appreciated
Your question isn't very clear. It sounds like the following would satisfy your description:
targets_1 = 1 - targets;
targets_2 = targets;
targets1 = double(targets == 0);
targets2 = targets;
I'm basing this answer purely on the fact that you've mentioned setosaCmp = strcmp('setosa', species);. From this I'm guessing that
You have Statistics Toolbox, as setosa is a species of iris from the Fisher Iris dataset widely used in Statistics Toolbox demos, and
You have a variable containing class labels, and you'd like to construct some class indicator variables (i.e. a new variable for each class label, each of which is 1 when the item is in that class, and 0 when it's not).
Is that right? If not, please ignore me.
If I'm right, then I think the command you're looking for is dummyvar from Statistics Toolbox. Try this:
>> classLabels = [1, 2, 1, 2, 3, 1, 3];
>> dummyvar(classLabels)
ans =
1 0 0
0 1 0
1 0 0
0 1 0
0 0 1
1 0 0
0 0 1