How to add the value in specific range in a matrix?
I mean if i have a matrix
Columns 1 through 7
4 4 4 4 4 4 4
48 48 48 48 48 48 48
Columns 8 through 14
4 4 4 13 13 13 13
48 48 48 57 57 57 57
Columns 15 through 20
13 13 13 13 13 13
57 57 57 57 57 57
I want to sum all 4 values ,all 13 values,all 48 values,and all 57 values,so the result should be m=[40 130 480 570]
The easiest but stupid method is like this
a=sum(1,(1:10));
b=sum(1,(11:20));
c=sum(2,(1:10));
d=sum(2,(11:20));
m=[a b c d]
If i want to write a code with for-loop or while-loop to show the result i want.how do i write the code?
Or can i use the some method to write a code without loop to show this?
Though the solution of #phnx works fine, you can also use the other outputs of the unique function in combination with accumarray as described in the docs:
[C, ~, ic] = unique(a);
a_counts = accumarray(ic,1);
m = C.*a_counts
This will avoid the warning 'hist' is not recommended...
A simple two-line solution, with A as your original matrix, would be:
[a,b]=hist(A(:),unique(A(:)))
c = a .* b'
with a containing the number of occurances, b the unique elements and c the sums.
Related
for a=1:50; %numbers 1 through 50
for b=1:50;
c=sqrt(a^2+b^2);
if c<=50&c(rem(c,1)==0);%if display only if c<=50 and c=c/1 has remainder of 0
pyth=[a,b,c];%pythagorean matrix
disp(pyth)
else c(rem(c,1)~=0);%if remainder doesn't equal to 0, omit output
end
end
end
answer=
3 4 5
4 3 5
5 12 13
6 8 10
7 24 25
8 6 10
8 15 17
9 12 15
9 40 41
10 24 26
12 5 13
12 9 15
12 16 20
12 35 37
14 48 50
15 8 17
15 20 25
15 36 39
16 12 20
16 30 34
18 24 30
20 15 25
20 21 29
21 20 29
21 28 35
24 7 25
24 10 26
24 18 30
24 32 40
27 36 45
28 21 35
30 16 34
30 40 50
32 24 40
35 12 37
36 15 39
36 27 45
40 9 41
40 30 50
48 14 50
This problem involves the Pythagorean theorem but we cannot use the built in function so I had to write one myself. The problem is for example columns 1 & 2 from the first two rows have the same numbers. How do I code it so it only deletes one of the rows if the columns 1 and 2 have the same number combination? I've tried unique function but it doesn't really delete the combinations. I have read about deleting duplicates from previous posts but those have confused me even more. Any help on how to go about this problem will help me immensely!
Thank you
welcome to StackOverflow.
The problem in your code seems to be, that pyth only contains 3 values, [a, b, c]. The unique() funcion used in the next line has no effect in that case, because only one row is contained in pyth. another issue is, that the values idx and out are calculated in each loop cycle. This should be placed after the loops. An example code could look like this:
pyth = zeros(0,3);
for a=1:50
for b=1:50
c = sqrt(a^2 + b^2);
if c<=50 && rem(c,1)==0
abc_sorted = sort([a,b,c]);
pyth = [pyth; abc_sorted];
end
end
end
% do final sorting outside of the loop
[~,idx] = unique(pyth, 'rows', 'stable');
out = pyth(idx,:);
disp(out)
a few other tips for writing MATLAB code:
You do not need to end for or if/else stements with a semicolon
else statements cover any other case not included before, so they do not need a condition.
Some performance reommendations:
Due to the symmetry of a and b (a^2 + b^2 = b^2 + a^2) the b loop could be constrained to for b=1:a, which would roughly save you half of the loop cycles.
if you use && for contencation of scalar values, the second part is not evaluated, if the first part already fails (source).
Regards,
Chris
You can also linearize your algorithm (but we're still using bruteforce):
[X,Y] = meshgrid(1:50,1:50); %generate all the combination
C = (X(:).^2+Y(:).^2).^0.5; %sums of two square for every combination
ind = find(rem(C,1)==0 & C<=50); %get the index
res = unique([sort([X(ind),Y(ind)],2),C(ind)],'rows'); %check for uniqueness
Now you could really optimized your algorithm using math, you should read this question. It will be useful if n>>50.
I have a few multidimensional matrices of dimensions mxnxt, where each element in mxn is an individual sensor input, and t is time. What I want to do is analyse only the peak values for each element in mxn over t, so I would end up with a single 2D matrix of mxn containing only max values.
I know there are are ways to get a single overall max value, but is there a way to combine this with element-by-element operations like bsxfun so that it examines each individual element over t?
I'd be grateful for any help you can give because I'm really stuck at the moment. Thanks in advance!
Is this what you want?
out = max(A,[],3); %// checking maximum values in 3rd dimension
Example:
A = randi(50,3,3,3); %// Random 3x3x3 dim matrix
out = max(A,[],3);
Results:
A(:,:,1) =
35 5 8
38 12 42
23 46 27
A(:,:,2) =
50 6 39
4 49 41
23 1 44
A(:,:,3) =
5 41 10
20 22 14
13 46 8
>> out
out =
50 41 39
38 49 42
23 46 44
You can call max() with the matrix and select the dimension (look the documentation) on which the operation will be calculated, e.g
M = max(A,[],3)
I have an array which looks similar to:
0 2 3 4 0 0 7 8 0 10
0 32 44 47 0 0 37 54 0 36
I wish to remove all
0
0
from this to get:
2 3 4 7 8 10
32 44 47 37 54 36
I've tried x(x == 0) = []
but I get:
x =
2 32 3 44 4 47 7 37 8 54 10 36
How can I remove all zero columns?
Here is a possible solution:
x(:,all(x==0))=[]
You had the right approach with x(x == 0) = [];. By doing this, you would remove the right amount of elements that can still form a 2D matrix and this actually gives you a vector of values that are non-zero. All you have to do is reshape the matrix back to its original form with 2 rows:
x(x == 0) = [];
y = reshape(x, 2, [])
y =
2 3 4 7 8 10
32 44 47 37 54 36
Another way is with any:
y = x(:,any(x,1));
In this case, we look for any columns that are non-zero and use these locations to index into x and extract out those corresponding columns.
Result:
y =
2 3 4 7 8 10
32 44 47 37 54 36
Another way which is more for academic purposes is to use unique. Assuming that your matrix has all positive values:
[~,~,id] = unique(x.', 'rows');
y = x(:, id ~= 1)
y =
2 3 4 7 8 10
32 44 47 37 54 36
We transpose x so that each column becomes a row, and we look for all unique rows. The reason why the matrix needs to have all positive values is because the third output of unique assigns unique ID to each unique row in sorted order. Therefore, if we have all positive values, then a row of all zeroes would be assigned an ID of 1. Using this array, we search for IDs that were not assigned a value of 1, and use those to index into x to extract out the necessary columns.
You could also use sum.
Sum over the columns and any column with zeros only will be zeros after the summation as well.
sum(x,1)
ans =
0 34 47 51 0 0 44 62 0 46
x(:,sum(x,1)>0)
ans =
2 3 4 7 8 10
32 44 47 37 54 36
Also by reshaping nonzeros(x) as follows:
reshape(nonzeros(x), size(x,1), [])
ans =
2 3 4 7 8 10
32 44 47 37 54 36
I have an MxN matrix and I want a column vector v, using the vector s that tells me for each row in the matrix what column I will take.
Here's an example:
Matrix =
[ 4 13 93 20 42;
31 18 94 64 02;
7 44 24 91 15;
11 20 43 38 31;
21 42 72 60 99;
13 81 31 87 50;
32 22 83 24 04]
s = [4 4 5 4 4 4 3].'
And the desired output is:
v = [20 64 15 38 60 87 83].'
I thought using the expression
Matrix(:,s)
would've work but it doesn't. Is there a solution without using for loops to access the rows separately?
It's not pretty, and there might be better solutions, but you can use the function sub2ind like this:
M(sub2ind(size(M),1:numel(s),s'))
You can also do it with linear indexing, here is an example:
M=M'; s=s';
M([0:size(M,1):numel(M)-1]+s)
I have an excel_1 with 4 columns (A, B, C, score) with different combinations. I have another excel_2 with 3 columns (A, B, C). I would like according A, B, C and find out score. I'm trying using excel index and match function, but i still can not figure out. without coding, it really make me trouble to match it one by one...May i know how to write code in Matlab?
Example:
excel_1
99 5 35 12
99 2 32 14
97 5 13 94
...
excel_2
97 5 13
99 2 32
...
After execute the code,
Result:
excel_2
97 5 13 94
99 2 32 14
...
Thanks a lot...
A = xlsread('excel_1.xlsx');
B = xlsread('excel_2.xlsx');
[~,J] = ismember(B,A(:,1:size(B,2)),'rows');
if any(J)
result = A(J,:);
end
??? Subscript indices must either be real positive integers or logicals.
Error in ==> Untitled at 6
result = A(J,:);
Solved. can not exist impossible between B and A.
May be you can try this.
[~,J] = ismember(excel_2,excel_1(:,1:size(excel_2,2)),'rows');
if any(J)
result = excel_1(J,:);
end
Then
result =
97 5 13 94
99 2 32 14