This question already has answers here:
How can I index a MATLAB array returned by a function without first assigning it to a local variable?
(9 answers)
Matlab Error: ()-indexing must appear last in an index expression
(1 answer)
Closed 5 years ago.
I'm trying to realise the following Octave command in MATLAB:
M = eye(x)(y,:);
x is just a number (in my example 10) and y is a vector (here 8x1):
y = [1 3 4 5 7 10 9 10];
The Octave command would generate:
M =
1 0 0 0 0 0 0 0 0 0
0 0 1 0 0 0 0 0 0 0
0 0 0 1 0 0 0 0 0 0
0 0 0 0 1 0 0 0 0 0
0 0 0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 1 0
0 0 0 0 0 0 0 0 0 1
The ones are kept very near to the diagonal.
The nearest I came with MATLAB is with the following commands:
n = size(y,1);
Y = eye(n, x);
but it would generate something still different. If the difference between rows and columns gets bigger, it would be very different.
M =
1 0 0 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0 0 0
0 0 1 0 0 0 0 0 0 0
0 0 0 1 0 0 0 0 0 0
0 0 0 0 1 0 0 0 0 0
0 0 0 0 0 1 0 0 0 0
0 0 0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 1 0 0
How could I get the first matrix with MATLAB?
First you should find what this expression eye(x)(y,:) means. First create an identity matrix with the size of x by x, and then select rows with index in y. Therefore, the equivalent syntax would be:
E = eye(x);
M = E(y,:);
Related
This question already has answers here:
Creating Indicator Matrix
(6 answers)
Closed 5 years ago.
I have vector like V[5000,1] with values in range 1-10
How to get matrix W[5000,10]
V[1] W[1 0 0 0 0 0 0 0 0 0]
[2] W[0 1 0 0 0 0 0 0 0 0]
[10] W[0 0 0 0 0 0 0 0 0 1]
[7] W[0 0 0 0 0 0 1 0 0 0]
... ...
its like W have "1" in column indicated by vector V
Solution
You can use the following approach:
W = zeros(length(v),10);
W(sub2ind(size(W),1:length(v),v'))=1;
Results
v = [10,4,10,4,9]
W =
0 0 0 0 0 0 0 0 0 1
0 0 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 1
0 0 0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 1 0
I have a matrix looks like:
0 0 0 0 0
1 0 0 0 0
0 2 0 0 0
0 0 2 0 0
0 0 0 1 0
1 0 0 0 1
0 4 0 0 0
0 0 3 0 0
6 0 0 4 0
0 3 0 0 2
0 0 5 0 0
It is 11x5 matrix.
I want to interpolate between the values vertically for each column.
Any help ?
Thanks.
M =[0 0 0 0 0
1 0 0 0 0
0 2 0 0 0
0 0 2 0 0
0 0 0 1 0
1 0 0 0 1
0 4 0 0 0
0 0 3 0 0
6 0 0 4 0
0 3 0 0 2
0 0 5 0 0];
xi = 1:size(M,1)
for colIdx = 1:size(M,2)
col = M(:,colIdx);
x = xi(~~col); %// Note that ~~col is a logical vector of elements that are not equal to zero. i.e. it's the same as col ~= 0
y = col(~~col);
M(:,colIdx) = interp1(x,y,xi);
end
then if you want the outer points to be 0 add this line after the loop:
M(isnan(M)) = 0;
I want to access multiple indices of a matrix as shown below. So what I want is indices (1,3),(2,6),(3,7) to be set to one. However, as you can see the entire column is set to one. I can see what it is doing but is there a way to do what I want it to (in an elegant way - no loops).
a=zeros(3,10)
a(1:3,[3 6 7])=1
a =
0 0 1 0 0 1 1 0 0 0
0 0 1 0 0 1 1 0 0 0
0 0 1 0 0 1 1 0 0 0
I realise that you can do something along the lines of
x_idx=1:3, y_idx=[3 6 7];
idx=x_idx*size(a,2)+y_idx;
a(idx)=1;
but just wondering if there was a better, or proper way of doing this in Matlab
You can use sub2ind, which essentially is doing what you have mentioned in your post, but MATLAB has this built-in:
a = zeros(3,10);
a(sub2ind(size(a), 1:3, [3 6 7])) = 1
a =
0 0 1 0 0 0 0 0 0 0
0 0 0 0 0 1 0 0 0 0
0 0 0 0 0 0 1 0 0 0
Another way would be to create a logical sparse matrix, then use this to index into a:
a = zeros(3,10);
ind = logical(sparse(1:3, [3 6 7], true, size(a,1), size(a,2)));
a(ind) = 1
a =
0 0 1 0 0 0 0 0 0 0
0 0 0 0 0 1 0 0 0 0
0 0 0 0 0 0 1 0 0 0
I have one binary image so it has only 2 value like 0 and 1. After, I convert this into a padded image of different values, like the image will have curve shape. I took a 3 X 3 matrix of value and if i get curve shape then I padded the image with 1, or any number. I use 15 different types shape values like junction point, end point etc.
After, I give the values 1 to 15 - or the appropriate number according its shape. As such, I am getting an image like:
Figure
0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 1 1 1 0 0 0 0
0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0
I would like to count how many 1s there are in the image, followed by 2s, 3s, etc. up to 15. For example,
as shown in the figure, if the pad number was 5, the total number of pixels would be 3. If the pad number was 1, the total number of pixels would be 6.
Use histc:
>> im = [ 0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 5 0 0 0 0 0 0 0 1 1 1 0 0 0 0
0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 ]; %// data
>> values = 1:15; %// possible values
>> count = histc(im(:), values)
count =
6 %// number of 1's
0 %// number of 2's, etc
0
0
3
0
0
0
0
0
0
0
0
0
0
Or compute it manually with bsxfun:
>> count = sum(bsxfun(#eq, im(:), values(:).'), 1)
>> count =
6 0 0 0 3 0 0 0 0 0 0 0 0 0 0
I can also suggest using accumarray:
im = [0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 5 0 0 0 0 0 0 0 1 1 1 0 0 0 0
0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 ]; %// data - borrowed from Luis Mendo
counts = accumarray(im(:) + 1, 1);
counts(1) = []
counts =
6
0
0
0
3
Note we have to offset by 1 as accumarray starts indexing the output array at 1. Because you want to disregard the 0s, I simply take the counts result and remove the first entry. This result agrees with what you are seeking. The first element is how many 1s we have encountered, which is 6. The last element is how many 5s you have encountered, which is 3. Because 5 is the largest number encountered in your image, we can say that all symbols after 5 (6, 7, 8, ..., 15) have a count of 0.
This question already has an answer here:
Closed 10 years ago.
Possible Duplicate:
Find rows in matrix where entries match certain constraints?
In Matlab, i have a matrix (MxN) and I want to find the rows whose entry in a specific column is equal to a specified value. For example, I have a matrix as follow:
0 0 0 0 0 0 0 0 1 0
1 0 0 0 0 0 1 0 0 1
0 0 1 0 0 0 0 0 0 0
0 0 0 1 0 0 0 0 1 0
0 0 0 0 1 0 0 0 0 1
0 0 0 0 0 0 0 0 0 0
0 1 1 0 0 0 1 0 1 0
0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 1
I want to find the rows whose 7th element us equal to 1. In this example, the result matrix should contains rows 2 and 7.
if your matrix is called A, just do:
A(A(:,7)==1,:)
PS: I think that this question has already been answered a million times...