I want to create a matrix which has distinct rows selected from another matrix.
For Example, I have a 10x3 matrix A
A =
1 2 3
4 5 6
7 8 9
10 11 12
13 14 15
16 17 18
19 20 21
22 23 24
25 26 27
28 29 30
Now I want to create a new matrix B of size 2 X 3 from A in a iterative process in such a way that the matrix B should consist different rows in each iteration (max iteration = 5)
My Pseudo-code:
for j=1:5
create matrix 'B' by selecting 2 rows randomly from 'A', which should be different
end
You could use randperm to mess up the rows randomly and then take two rows in each iteration successively in order.
iterations = 4;
permu = randperm(size(A,1));
out = A(permu(1:iterations*2),:);
for ii = 1:iterations
B = out(2*ii - 1:2*ii,:)
end
Results:
B =
22 23 24
25 26 27
B =
1 2 3
13 14 15
B =
19 20 21
16 17 18
B =
7 8 9
10 11 12
Related
I have a matrix
A= [1 2 3 4; 5 6 7 8; 9 10 11 12; 13 14 15 16; 17 18 19 20]
I want to do some calculation on this matrix. But actually I do not need all the rows. So I have to discard some of the rows from the above matrix before doing a calculation. After discarding 3 rows, we will have a new matrix.
B= [1 2 3 4; 9 10 11 12; 17 18 19 20];
Now I have to use B to make some other calculations. So how can I discard some of the unwanted rows from a matrix in matlab? Any suggestion will be helpful. Thanks.
Try this: (Use when no. of rows to keep is lesser)
%// Input A
A = [1 2 3 4; 5 6 7 8; 9 10 11 12; 13 14 15 16; 17 18 19 20];
%// Rows (1-3,5) you wanted to keep
B = A([1:3, 5],:)
Output:
B =
1 2 3 4
5 6 7 8
9 10 11 12
17 18 19 20
Alternative: (Use when no. of rows to discard is lesser)
%// rows 2 and 3 discarded
A([2,3],:) = [];
Output:
>> A
A =
1 2 3 4
13 14 15 16
17 18 19 20
Note: Here (in the alternate method), the output replaces the original A. So you need to back up A if you need it afterwards. You could do this before discarding operation to backup Input matrix
%// Input A is backed up in B
B = A;
You can select the indices of the rows you want to keep:
A([1,3,5],:)
ans =
1 2 3 4
9 10 11 12
17 18 19 20
I made a matrix in Matlab, say,
A = magic(5);
A =
17 24 1 8 15
23 5 7 14 16
4 6 13 20 22
10 12 19 21 3
11 18 25 2 9
Now I found the indices I want using the find function as:
ind = find(A(:,5)>3 & A(:,4)>= 8);
ind =
1
2
3
Now if I want to get a subset of matrix A for those indices using B = A(ind) function, I only get the first column of the matrix:
B = A(ind)
B =
17
23
4
How can I get all the columns as subset??
Oops ... I got it
B = A(ind,:);
I got a data set in a matrix like the following (imported from Excel):
matrix =
Cat1 1 2 3 4
Cat2 9 10 11 12
Cat3 17 18 19 20
Cat1 5 6 7 8
Cat2 13 14 15 16
Cat3 21 22 23 24
I would like to reshape it into 3 vectors (one for every category) of the same size to do a stacked bar plot. Vectors should look like this after reshape operation (It would be nice if the vector had the name of the first column and the matrix could be of any size):
cat1 = [ 1 2 3 4 5 6 7 8]
cat2 = [ 9 10 11 12 13 14 15 16]
cat3 = [17 18 19 20 21 22 23 24]
I sincerely hope this is not duplicate. I couldn't produce a working solution with the help of the other reshape questions.
If your data is a matrix, you can manipulate the order of the rows when indexing, so you can do something like this:
rows = reshape(1:size(matrix, 1), n, []).';
res = reshape(matrix(rows, :).', [], n).';
The resulting matrix res is composed of the concatenated rows.
This solution holds for cell arrays as well, but you'll need an additional cell2mat to turn the result into a matrix.
Example
matrix = [1:4; 9:12; 17:20; 5:8; 13:16; 21:24];
n = 3;
rows = reshape(1:size(matrix, 1), n, []).';
res = reshape(matrix(rows, :).', [], n).';
The result is:
res =
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
17 18 19 20 21 22 23 24
EDIT:
Try the following:
%# dataset stored in a cell array
data = {
'Cat1' 1 2 3 4
'Cat2' 9 10 11 12
'Cat3' 17 18 19 20
'Cat1' 5 6 7 8
'Cat2' 13 14 15 16
'Cat3' 21 22 23 24
};
%# get all possible values of first column,
%# and map them to integer indices
[L,~,IDX] = unique(data(:,1));
%# for each possible "category"
groups = cell(max(IDX),1);
for i=1:max(IDX)
%# get the rows of numeric data matching current category
M = data(IDX==i, 2:end)';
%# flatten matrix into a vector and store in cell (row-major order)
groups{i} = [M{:}];
end
Now you can access the i-th "cat" vector as: groups{i}
>> [cat1,cat2,cat3] = deal(groups{:})
cat1 =
1 2 3 4 5 6 7 8
cat2 =
9 10 11 12 13 14 15 16
cat3 =
17 18 19 20 21 22 23 24
Note that the matching "cat" labels are stored in L{i} (the mapping keys)
I have a loop:
for i=1:size(A,1),
if A(i,4:6) == [0,0,3.4]
K = [K; A(i,:)];
end
end
and I would like to delete the last row in the matrix but I do not know what number row it will be. How do I delete the last row in the matrix in the loop? Or should I do it after the loop?
Why do you have loop? it is a one time action, not something you do several times.
check this out, I delete the last row:
>> a = magic(5);
>> a
a =
17 24 1 8 15
23 5 7 14 16
4 6 13 20 22
10 12 19 21 3
11 18 25 2 9
>> a = a(1:end-1,:);
>> a
a =
17 24 1 8 15
23 5 7 14 16
4 6 13 20 22
10 12 19 21 3
you can refer to last row by END keyword:
A= A(1:end-1, :)
We have the following case:
Q = [idxcell{:,1}];
Sort = sort(Q,'descend')
Sort =
Columns 1 through 13
23 23 22 22 20 19 18 18 18 18 17 17 17
Columns 14 through 26
15 15 14 14 13 13 13 12 12 12 11 10 9
Columns 27 through 39
9 9 8 8 8 8 8 7 7 7 7 7 7
Columns 40 through 52
7 6 6 6 5 4 4 3 3 3 3 2 2
Columns 53 through 64
2 2 2 2 2 2 2 1 1 1 1 1
How can we sort matrix Sort according to how many times its values are repeated?
Awaiting result should be:
repeatedSort = 2(9) 7(7) 1(5) 8(5) 3(4) 18(4) 6(3) 9(3) 12(3) 13(3) 17(3) 4(2) 14(2) 15(2) 22(2) 23(2) 5(1) 10(1) 11(1) 19(1) 20(1)
or
repeatedSort = 2 7 1 8 3 18 6 9 12 13 17 4 14 15 22 23 5 10 11 19 20
Thank you in advance.
You can use the TABULATE function from the Statistics Toolbox, then call SORTROWS to sort by the frequency.
Example:
x = randi(10, [20 1]); %# random values
t = tabulate(x); %# unique values and counts
t = t(find(t(:,2)),1:2); %# get rid of entries with zero count
t = sortrows(t, -2) %# sort according to frequency
the result, where first column are the unique values, second is their count:
t =
2 4 %# value 2 appeared four times
5 4 %# etc...
1 3
8 3
7 2
9 2
4 1
6 1
Here's one way of doing it:
d = randi(10,1,30); %Some fake data
n = histc(d,1:10);
[y,ii] = sort(n,'descend');
disp(ii) % ii is now sorted according to frequency