I have a cell matrix of size 10000 X 3 in Matlab and I would like to remove rows with the same value in the first column.
That is, if row i and row j have the same value in the first column, I'd like to delete both rows.
I should also say that there can be more than two rows with the same value in the first column and in that case, I'd like to delete all these rows.
How do I do it?
Thanks!
You can use the functions histc, unique and logical indexing to achieve what you want. Here's a small example.
a=randi(10,5,3) %#generate a sample random matrix
a =
5 3 5
5 7 10
7 7 4
8 2 6
8 2 3
[uniqVals,uniqIndx]=unique(a(:,1)); %# get unique values and corresponding indices of the first column of a
count=histc(a(:,1),uniqVals); %# get the bin counts of the elements (i.e., find which are repeated)
b=a(uniqIndx(count==1),:)
b =
7 7 4
Only the row with the non-repeated element is selected. Since you said that you have a cell matrix, simply covert it to a matrix using cell2mat before doing this.
Related
I have a matrix A and a vector b. I don't know their sizes, the size varies because it is the output of another function. What I want to do is filter A by a column (let's say jth column) which has at least one value that is in b.
How do I do this without measuring the size of b and concatenating every filtered result. Right now, the code is like this (assume j is a given value)
bsize=size(b,1);
for i=1:bsize
if i==1
a=A(A(:,j)==b(i),:);
else
a=[a; A(A(:,j)==b(i),:)];
end
end
I want to code a faster solution.
I am adding a numerical example just to make it clear. Let's say
A=[2 4
7 14
11 13
15 14]
and b=[4 14]
What I'm trying to do is filter to obtain the A matrix whose values are 4 and 14 in the second column, the elements of b to obtain the following output.
A=[2 4
7 14
15 14]
In my data A has more than 12000 rows and b has more than 100 elements. It doesn't always have to be the second column, sometimes the column index changes but that's not the problem now.
Use the ismember function to create a logical index based on column j=2 of A and vector b, and use that index into the rows of A:
output = A(ismember(A(:,j), b), :);
This question already has answers here:
Building a matrix by merging the same row vector multiple times
(2 answers)
Closed 8 years ago.
I have a matrix myVel that is of size [1 501] meaning 1 row and 501 columns.
I want to extend this matrix so that the matrix will be of size [N 501], where N is an arbitrary number.
Each of the values in the columns need to be the same (meaning that all the values in the first column are all say, x and all of the values in the second column are say, y and so on).
This means that each row would consist of the same values.
How can I achieve this efficiently?
Divakar's solution is one way to do it, and the link he referenced shows some great ways to duplicate an array. That post, however, is asking to do it without the built-in function repmat, which is the easiest solution. Because there is no such restriction for you here, I will recommend this approach. Basically, you can use repmat to do this for you. You would keep the amount of columns the same, and you would duplicate for as many rows as you want. In other words:
myVelDup = repmat(myVel, N, 1);
Example:
myVel = [1 2 3 4 5 6];
N = 4;
myVelDup = repmat(myVel, N, 1);
Output:
>> myVel
myVel =
1 2 3 4 5 6
>> myVelDup
myVelDup =
1 2 3 4 5 6
1 2 3 4 5 6
1 2 3 4 5 6
1 2 3 4 5 6
In general, repmat is called in the following way:
out = repmat(in, M, N);
in would be a matrix or vector of values you want duplicated, and you would want to duplicate this M times horizontally (rows) and N times vertically (columns). As such, for your case, as you have an array, you will want to duplicate this N times vertically and so we set the first parameter to N. The second parameter, the columns stay the same so we specify this to be 1 as we don't want to have any duplications... and thus the call to repmat you see above.
For more information on repmat, check out this link: http://www.mathworks.com/help/matlab/ref/repmat.html
I have columns of values that decrease to ~1, after which all remaining values are NaNs. For example:
10 10
8 9
6 8
5 7
3 5
1 2
NaN 1
NaN NaN
What I need is the row number of each column immediately before the sequence of NaNs, or the last positive integer in the sequence. Row 6 and row 7 in the example above.
While a loop can probably be used to find the desired row of each column, I'm unable to identify the correct command to find this row. I attempted using find 'last' without success. Please advise the best way to achieve this. Thanks.
if your matrix is a, you can use
sum(isfinite(a))
ans =
6 7
this means that for the first col it is the 6th row, and for second col it is the 7th row that have the last non-NaN values.
this is alternatively the same: sum(~isnan(a))...
How do I just do a simple sort in matlab. I always have to use the excel link to import my data, sort it, then export back to matlab. This is annoying!!!
I have one matrix <10x10> and I want to sort the first column in descending order while keeping it's respective values on the second column. Matlab seems to just sort each column individually.
Example:
matrix a
5 4
8 9
0 6
7 3
matrix b (output)
0 6
5 4
7 3
8 9
The sortrows answer by #chaohuang is probably what you're looking for. However, it sorts based on all columns. If you only want to sort based on the first column, then you can do this:
% sort only the first column, return indices of the sort
[~,sorted_inds] = sort( a(:,1) );
% reorder the rows based on the sorted indices
b = a(sorted_inds,:);
Simply use b=sortrows(a); See here.
I am trying to copy a few elements from a matrix, but not a whole row, and not a single element.
For example, in the following matrix:
a = 1 2
3 4
5 6
7 8
9 0
How would I copy out just the following data?
b = 1
3
5
i.e. rows 1:3 in column 1 only... I know that you can remove an entire column like this:
b = a(:,1)
... and I appreciate that could just do this and then dump the last two rows, but I'd like to use more streamlined code as I am running a very resource-intensive solution.
Elements in a matrix in MATLAB are stored in column-major order. Which means, you could even use a single index and say:
b = a(1:3);
Since the first 3 elements ARE 1,3,5. Similarly, a(6) is 2, a(7) is 4 etc. Look at the sub2ind method to understand more:
http://www.mathworks.com/help/techdoc/ref/sub2ind.html
You are not "removing" the second column, you are referencing the other column.
You should read some of the Matlab docs, they provide some help about the syntax for accessing portions of matrices:
http://www.mathworks.com/help/techdoc/learn_matlab/f2-12841.html#f2-428