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Extract rows from matrix and make a new matrix in MATLAB
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Closed 10 years ago.
I have a 6639x5 matrix in Matlab and I would like to select certain specific rows in a particular order( say 1st,11th,21st,31st rows... and subsequent additions of 10 until end) to form a new matrix.Any ideas?
Thank you,
Oti.
subset = a(1:10:end, :);
Selects every 10th row until the end, and all columns.
Example:
>> 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
>> a(1:2:end, :)
ans =
17 24 1 8 15
4 6 13 20 22
11 18 25 2 9
Related
I have a list of coordinates I would like to sample from a Matrix.
Is there any elegant way to do it?
Ideally, something that looks like:
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
r = 1:5; % row coordinates
c = 5:-1:1; % column coordinates
A(r,c)
ans =
15 14 13 12 11
Which is equivalent to
for k=1:length(r)
A(r(k), c(k))
end
I am sure someone has asked that, but I couldn't find it anywhere.
Applying #excaza comment I was able to solve this with:
rc_ids = sub2ind(size(A), r,c);
A(rc_ids)
Is there a MATLAB function to generate this matrix?:
[1 2 3 4 5 6 7 ... n;
2 3 4 5 6 7 8 ... n+1;
3 4 5 6 7 8 9 ... n+2;
...;
n n+1 n+2 ... 2*n-1];
Is there a name for it?
Thanks.
Yes indeed there's a name for that matrix. It's known as the Hankel matrix.
Use the hankel function in MATLAB:
out = hankel(1:n,n:2*n-1);
Example with n=10:
out =
1 2 3 4 5 6 7 8 9 10
2 3 4 5 6 7 8 9 10 11
3 4 5 6 7 8 9 10 11 12
4 5 6 7 8 9 10 11 12 13
5 6 7 8 9 10 11 12 13 14
6 7 8 9 10 11 12 13 14 15
7 8 9 10 11 12 13 14 15 16
8 9 10 11 12 13 14 15 16 17
9 10 11 12 13 14 15 16 17 18
10 11 12 13 14 15 16 17 18 19
Alternatively, you may be inclined to want a bsxfun based approach. That is certainly possible:
out = bsxfun(#plus, (1:n), (0:n-1).');
The reason why I wanted to show you this approach is because in your answer, you used repmat to generate the two matrices to add together to create the right result. You can replace the two repmat calls with bsxfun as it does the replication under the hood.
The above solution is for older MATLAB versions that did not have implicit broadcasting. For recent versions of MATLAB, you can simply do the above by:
out = (1:n) + (0:n-1).';
My standard approach is
repmat(1:n,n,1)+repmat((1:n)',1,n)-1
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
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