Easiest way is to show you through excel:
Unsorted:
Sorted:
This example is with excel, but I would need to do the same thing in matlab with thousands of entries (with 2 rows if possible).
Here is my code so far:
%At are random numbers between 0 and 2, 6000 entries.
[sorted]=sort(At);
max=sorted(end);
min=sorted(1);
%need the position of the min and max
But this is only 1 row that's being sorted and it has no numbers in the second row, and no index. How would I add one and keep it following my first row?
Thank you!
I don't have access to Matlab, but try
[sorted, I] = sort(At);
Where I will be a corresponding vector of indices of At. See the Matlab Documentation for details.
You have a couple of options here. For the simple case where you just need the indices, the fourth form of sort listed in the docs already does this for you:
[sorted, indices] = sort(At);
In this case, At(indices) is the same as sorted.
If your "indices" are actually another distinct array, you can use sortrows:
toSort = [At(:) some_other_array(:)];
sorted = sortrows(toSort);
In this case sorted(:, 1) will be the sorted array from the first example and sorted(:, 2) will be the other array sorted according to At.
sortrows accepts a second parameter which tells you the column to sort by. This can be a single column or a list of columns, like in Excel. It can also provide a second output argument, the indices, just like regular sort.
Related
I have a matrix and I am interested in changing values that satisfy a certain condition inside that matrix differently, depending on where they are. Say I have a matrix smallPic. How do I obtain a matrix smallPicB with the same dimensions that changed all values that are above 50 in the first two columns to a 255, while those that are in the third and fourth column are changed to a 180?
I have this code which works, but it is pretty ugly and requires splitting the matrix and concatenating it again:
smallPic1=smallPic(:,1:2);smallPic1(smalllPic1>50)=255;
smallPic2=smallPic(:,3:4);smallPic2(smalllPic2>50)=180;
smallPicB = [smalllPic1 smalllPic2];
How would you combine the logical index with the scalar index in one command?
What doesn't work is this:
smallPic(:,smallPic(:,3:4)>50) = 180
Here, the second mention of smallPic inside the brackets does not allow indexing into the correct position of smallPic because it doesn't have the same dimensions as smallPic. So this command actually replaces values in the first two columns of smallPic that are in the same row as those values that are above 50 in the third and fourth column, instead of replacing the values in the third and fourth column themselves.
Any other suggestions?
It probably is not what you're looking for, but it can help if you have lots of assignments like that:
J = repmat(1:size(smallPic, 2), size(smallPic, 1), 1)
smallPic((J<3)&(smallPic>50))=255
smallPic((J>2)&(J<5)&(smallPic>50))=180
You can also call ismember function if column indices are not consecutive:
smallPic(ismember(J, [[1:2 5:6]])&(smallPic>50))=255
My goal is to create a random, 20 by 5 array of integers, sort them by increasing order from top to bottom and from left to right, and then calculate the mean in each of the resulting 20 rows. This gives me a 1 by 20 array of the means. I then have to find the column whose mean is closest to 0. Here is my code so far:
RandomArray= randi([-100 100],20,5);
NewArray=reshape(sort(RandomArray(:)),20,5);
MeanArray= mean(transpose(NewArray(:,:)))
X=min(abs(x-0))
How can I store the column number whose mean is closest to 0 into a variable? I'm only about a month into coding so this probably seems like a very simple problem. Thanks
You're almost there. All you need is a find:
RandomArray= randi([-100 100],20,5);
NewArray=reshape(sort(RandomArray(:)),20,5);
% MeanArray= mean(transpose(NewArray(:,:))) %// gives means per row, not column
ColNum = find(abs(mean(NewArray,1))==min(abs(mean(NewArray,1)))); %// gives you the column number of the minimum
MeanColumn = RandomArray(:,ColNum);
find will give you the index of the entry where abs(mean(NewArray)), i.e. the absolute values of the mean per column equals the minimum of that same array, thus the index where the mean of the column is closest to 0.
Note that you don't need your MeanArray, as it transposes (which can be done by NewArray.', and then gives the mean per column, i.e. your old rows. I chucked everything in the find statement.
As suggested in the comment by Matthias W. it's faster to use the second output of min directly instead of a find:
RandomArray= randi([-100 100],20,5);
NewArray=reshape(sort(RandomArray(:)),20,5);
% MeanArray= mean(transpose(NewArray(:,:))) %// gives means per row, not column
[~,ColNum] = min(abs(mean(NewArray,1)));
MeanColumn = RandomArray(:,ColNum);
i've got a cell array full of numbers, with 44 rows and different column length in each row
how could i calculate the number of columns in each row?(the columns which their contents are not empty)
i've used 2 different ways which both of them where wrong
the 1st one:
%a is the cell array
s=length(a)
it gives 44 which is the number of rows
the 2nd one
[row, columms]=size(a)
but it doesn't work either cause the number of columns is different in each row.
at least i mean the number of columns which are not empty
for example i need the number of columns in row one which it is 43(a{1 1:43}) but it gives the number of columns for each elements like a{1,1} which is 384 or a{1,2},a{1,3} and so on
You need to access each member of the cell array separately, you are looking for the size of the data contained in the cell - the cell is the container. Two methods
for loop:
cell_content_lengths=zeros(1,length(a));
for v=1:length(a)
cell_content_lengths(v)=length(a{v});
end
cellfun:
cell_content_lengths=cellfun(#length,a);
Any empty cells will just have length 0. To extend the for-loop to matrices is trivial, and you can extend the cellfun part to cells containing matrix by using something like this, if you are interested:
cell_content_sizes=cell2mat(cellfun(#length,a,'uniformoutput',false));
(Note for the above, each element of a needs to have the same dimension, otherwise it will give errors about concatenating different size matrices)
EDIT
Based on your comment I think I understand what you are looking for:
non_empty_cols = sum(~cellfun(#isempty,a),2);
With thanks to #MZimmerman6 who understood it before me.
So what you're really asking, is "How many non-empty elements are in each row of my cell array?"
filledCells = ~cellfun(#isempty,a);
columns = sum(filledCells,2);
I have a 161*32 matrix (labelled "indpic") in MATLAB and I'm trying to find the frequency of a given number appearing in a row. So I think that I need to analyse each row separately for each value, but I'm incredibly unsure about how to go about this (I'm only new to MATLAB). This also means I'm incredibly useless with loops and whatnot as well.
Any help would be greatly appreciated!
If you want to count the number of times a specific number appears in each row, you can do this:
sum(indpic == val, 2)
where indpic is your matrix (e.g image) and val is the desired value to be counted.
Explanation: checking equality of each element with the value produces a boolean matrix with "1"s at the locations of the counted value. Summing each row (i.e summing along the 2nd dimension results in the desired column vector, where each element being equal to the number of times val is repeated in the corresponding row).
If you want to count how many times each value is repeated in your image, this is called a histogram, and you can use the histc command to achieve that. For example:
histc(indpic, 1:256)
counts how many times each value from 1 to 256 appears in image indpic.
Like this,
sum(indpic(rownum,:) == 7)
obviously change 7 to whatever.
You can just write
length(find(indpic(row_num,:)==some_value))
and it will give you the number of elements equal to "some_value" in the "row_num"th row in matrix "indpic"
I want to reference an index of values inside the 5th column of an array like blitzer.
E.g. I want to access, say, all values of blitzer(:,5) where blitzer(:,4) < 10. This outputs an index of values. So maybe I could set blitzer5 = blitzer(:,5), and then call blitzer5(blitzer(:,4) < 10).
But is there a quick way to do this without having to create an entirely new vector? Ideally I'd like to call blitzer(:,5)[blitzer(:,4) < 10]. If so, how?
You can give vertical index from the matrix itself:
blitzer(blitzer(:,4) < 10,5)
This will give you elements from the 5-th column, where the corresponding elements in the 4-th row are less than 10.
If you want to reference the 8th element in the 5th column of an array, you can write
theElement = blitzer(8,5)