I am trying to sort the following one-dimensional array in descending order of values:
109 32 29 22 22 19 15 12 12
I know that in this case, the array is already in descending order but I am trying to figure out why when I use sort(A,'descend'), MATLAB returns a new order:
1 2 3 5 6 4 7 8 9
This is no longer in descending order. This is obviously causing an error in my program whenever the array is in the correct order.
That order you provided is indeed not correct but that is surely not MATLAB's fault. Since your array is already in descending order, the indices of sorting should simply be enumerated from 1 up to 9.
I highly suspect that the culprit is due to you not inputting your array properly. Furthermore, your error can't be reproduced.
This can be verified by putting this into MATLAB and examining the indices of sorting:
A = [109 32 29 22 22 19 15 12 12];
[Ai,ind] = sort(A,'descend');
Ai will be the same as the input A while ind will be enumerated from 1 up to 9 in ascending order.
Related
It would really help me reason about my MATLAB code if I didn't have to worry about accidental 2d operations. For instance, if I want to do element-wise multiplication of 1d arrays, but one is a row and another is a column, I end up with a 2d result.
>> a = 1:8;
>> a = a(:);
>> a .* cumsum(ones(8))
ans =
1 1 1 1 1 1 1 1
4 4 4 4 4 4 4 4
9 9 9 9 9 9 9 9
16 16 16 16 16 16 16 16
25 25 25 25 25 25 25 25
36 36 36 36 36 36 36 36
49 49 49 49 49 49 49 49
64 64 64 64 64 64 64 64
I'd like to prevent this type of thing, and likely other problems that I can't foresee, by keeping all my arrays 1d wherever I can. But every time I check the size() of vector, I get at least 2 elements back:
>> size(1:1:6)
ans =
1 6
>> size(linspace(0, 5, 10))
ans =
1 10
I've tried the suggestions at How to create single dimensional array in matlab? and some of the options here (PDF download), and I can't get a "truly" 1d array. How would you deal with this type of issue?
There is no such thing as 1D array. The documentation says (emphasis mine):
All MATLAB variables are multidimensional arrays, no matter what type of data. A matrix is a two-dimensional array often used for linear algebra.
You may use isvector, isrow and iscolumn to identify vectors, row vectors and column vectors respectively.
#Sardar has already said the last word. Another clue is ndims:
N = ndims(A) returns the number of dimensions in the array A. The
number of dimensions is always greater than or equal to 2. ...
But about your other question:
How would you deal with this type of issue?
There's not much you can do. Debug, find the mistake and fix it. If it's some one-time script, you are done. But if you are writing functions that may be used later, it's better to protect them from accepting arguments with unequal dimensions:
function myFunc(A, B)
if ndims(A)~=ndims(B) || any(size(A)~=size(B))
error('Matrix dimensions must agree.');
end
% ...
end
Or, if your function really needs them to be vectors:
function myFunc(A, B)
if ~isvector(A) || ~isvector(B) || any(size(A)~=size(B))
error('A and B must be vectors with same dimensions.');
end
% ...
end
You can also validate different attributes of arguments using validateattributes:
function myFunc(A, B)
validateattributes(A, {'numeric'},{'vector'}, 'myFunc', 'A')
validateattributes(B, {'numeric'},{'size', size(A)}, 'myFunc', 'B')
% ...
end
Edit:
Also, if the function only needs the inputs to be vectors and their orientation does not matter, you can modify them inside the function (thanks to #CrisLuengo for commenting).
function myFunc(A, B)
if ~isvector(A) || ~isvector(B) || length(A)~=length(B)
error('A and B must be vectors with the same length.');
end
A = A(:);
B = B(:);
% ...
end
However, this is not recommended when the output of the function is also a vector with the same size as the inputs. This is because the caller expects the output to be in the same orientation as the inputs, and if this is not the case, problems may arise.
How to remove elements from an array that have difference of 3 or less between subsequent elements? For example
A=[3 6 10 14 17 20]
to this
B=[3 10 14 20]
I believe I can use diff but in what context should I use it to achieve this?
You can use diff and then compare this to 3. You can then construct a logical array (that includes the first value by default) and use this to index into A.
tokeep = [true, diff(A) > 3];
B = A(tokeep);
In your example though, the difference between 17 and 20 is 3 so that would remove 20.
I have an array in MATLAB containing elements such as
A=[12 13 14 15 30 31 32 33 58 59 60];
How can I identify breaks in values of data? For example, the above data exhibits breaks at elements 15 and 33. The elements are arranged in ascending order and have an increment of one. How can I identify the location of breaks of this pattern in an array? I have achieved this using a for and if statement (code below). Is there a better method to do so?
count=0;
for i=1:numel(A)-1
if(A(i+1)==A(i)+1)
continue;
else
count=count+1;
q(count)=i;
end
end
Good time to use diff and find those neighbouring differences that aren't equal to 1. However, this will return an array which is one less than the length of your input array because it finds pairwise differences up until the last element, so naturally there will be one less. As such, when you find the locations that aren't equal to 1, make sure you add 1 to the locations to account for this:
>> A=[12 13 14 15 30 31 32 33 58 59 60];
>> q = find(diff(A) ~= 1) + 1
q =
5 9
This tells us that locations 5 and 9 in your array is where the jump happens, and that's right for your example data.
However, if you want to find the locations before the jump happens, such as in your code, don't add 1 to the result:
>> q = find(diff(A) ~= 1)
q =
4 8
I have 2 arrays say
sum:
68
78
25
45
85
Index:
0
1
2
3
4
I did bubble sort on sum and got as:
25
45
68
78
85
Now, I need to know sort index array with respect to the sum array. So, my output of array should be:
2
3
0
1
4
How can I do that?
I'd recommend you sort the index array instead, and have the comparisons made using the values the indexes "point" to. So then you don't actually need to sort the "value" array just the "index" array.
So, here is a very simple solution that I never thought of and all credits goes to my project supervisor.. and it is.
Sort both the arrays under the same loop.. Thus when ever I will refer to any index, it will give me the sorted version.
I am having difficulties with making an array formula work the way I want it to work.
Out of a column of dates which is not sorted, I want it to extract values into a new column. The formula below identifies the required cells of a given month and year, but they appear in their original row rather than on top of the output range. Moreover, I want all ""/FALSE cells to be excluded from the output array.
=IF((MONTH($I$15:$I$1346)=1)*(YEAR($I$15:$I$1346)=2008),$I$15:$I$1346,"")
In fact, the $I$15:$I$1346 should be dynamic and go to the last filled range (I could make a named range for that)
Part two is to expand on that formula so that it calculates the data that is an two column offset of the data described above.
Is the above possible to build into one cell probably with a combination of IF, INDEX, SMALL and maybe others?
I'm not looking for a filter solution. Hope the above is clear enough and that you can help!
Here's a shortened sample layout:
A B C
1 Date Series_A Series_B
2 03/01/2011 45 20
3 04/01/2011 73 30
4 06/01/2011 95 40
5 08/01/2011 72 50
6 06/02/2011 5 13
7 09/02/2011 12 #N/A
8 05/02/2011 23 65
9 07/03/2011 12 65
Then I want three input cells for the year and and the month and series name (index/match, as there are many more columns with data). If it would be 2011, Feb and Series_A, I want it to calculate the average for that month. In this case it would be (5+12+23)/3. If it would be Feb-2011 and Series_B instead, which has an error, it should show (13+65)/2 rather than an error.
Aside from that I want a separate which will output an array with the data instead without 'holes' in between and with the right 'length'. Example for Feb-2011 in Column C:
A B C D
1 Date Series_A Desired Output Output based on f above
2 03/01/2011 45 5
3 04/01/2011 73 12
4 06/01/2011 95 23
5 08/01/2011 72
6 06/02/2011 5 5
7 09/02/2011 12 12
8 05/02/2011 23 23
9 07/03/2011 12
If I then run a =ISBLANK(C5) it should be true, rather than =""=C5
Hope the edit clarifies
I reached out to various platsforms to get an answer, and here you have one which is ok. Still doesn't fully answer part 1, but works nonetheless.
http://www.excelforum.com/excel-formulas-and-functions/905356-exclude-blank-false-cells-in-in-excel-array-if-formula-output.html