I am trying to build a submatrix by iterating on the rows of the last column in the main matrix which is of size (50000, 21), and if the value inside is greater than 1000 then the whole corresponding y(i,:) row will be added to the new submatrix and add to it more 33 consecutive rows from the main matrix.
I wrote this code below but it is returning only a 33x21 size and I think i am iterating through the j index but not the i index:
for i=1:50000
if y(i,21)>1000
for j=1:33
n(j,:)=y(j+i,:)
j=j+1
end
i=i+1
end
end
What am I doing wrong?
try this:
k=1;
for i=1:50000
if y(i,21)>1000
for j=0:33 %watch this out i am not sure how many rows you want to keep and if these involve the one with the last element greater than 1000 or not..%
n(k,:)=y(j+i,:);
j=j+1;
k=k+1;
end
i=i+1;
end
end
Problem is that every new set of 33 rows overwrites the previous one.The above code i think will do the trick.
this is a vectorized verison that needs no loop:
%find elements that are greater than 1000
lo = find(y(:,21) > 1000);
%indeices of 33th row after the lo,
%since it may take a value that is greater than the size of matrix we limit this value by min
up = min(size(y,1), lo + 33);
% create indices based on ranges
%see http://stackoverflow.com/a/39434045/6579744
index=cumsum(accumarray(cumsum([1;up(:)-lo(:)+1]),[lo(:);0]-[0;up(:)]-1)+1);
index= index(1:end-1);
n = y(index, :)
Related
I have a 64 X 64 matrix that I need to find the column-wise mean values for.
However, instead of dividing by the total number of elements in each column (i.e. 64), I need to divide by the total number of non-zeros in the matrix.
I managed to get it to work for a single column as shown below. For reference, the function that generates my matrix is titled fmu2(i,j).
q = 0;
for i = 1:64
if fmu2(i,1) ~= 0;
q = q + 1;
end
end
for i = 1:64
mv = (1/q).*sum(fmu2(i,1));
end
This works for generating the "mean" value of the first column. However, I'm having trouble looping this procedure so that I will get the mean for each column. I tried doing a nested for loop, but it just calculated the mean for the entire 64 X 64 matrix instead of one column at a time. Here's what I tried:
q = 0;
for i = 1:64
for j = 1:64
if fmu2(i,j) ~= 0;
q = q +1;
end
end
end
for i = 1:64
for j = 1:64
mv = (1/q).*sum(fmu2(i,j));
end
end
Like I said, this just gave me one value for the entire matrix instead of 64 individual "means" for each column. Any help would be appreciated.
For one thing, do not call the function that generates your matrix in each iteration of a loop. This is extremely inefficient and will cause major problems if your function is complex enough to have side effects. Store the return value in a variable once, and refer to that variable from then on.
Secondly, you do not need any loops here at all. The total number of nonzeros is given by the nnz function (short for number of non-zeros). The sum function accepts an optional dimension argument, so you can just tell it to sum along the columns instead of along the rows or the whole matrix.
m = fmu2(i,1)
averages = sum(m, 1) / nnz(m)
averages will be a 64-element array with an average for each column, since sum(m, 1) is a 64 element sum along each column and nnz(m) is a scalar.
One of the great things about MATLAB is that it provides vectorized implementations of just about everything. If you do it right, you should almost never have to use an explicit loop to do any mathematical operations at all.
If you want the column-wise mean of non-zero elements you can do the following
m = randi([0,5], 5, 5); % some data
avg = sum(m,1) ./ sum(m~=0,1);
This is a column-wise sum of values, divided by the column-wise number of elements not equal to 0. The result is a row vector where each element is the average of the corresponding column in m.
Note this is very flexible, you could use any condition in place of ~=0.
I want to initiate a mxn binary matrix. The summation of each column of the matrix equals to a given value s(j), for j=1..n. In addition, the summation of each row of the matrix should be within the range of given bounds: lhs is dl(i) and rhs is du(i), for i=1..m.
Now I can only randomly generate binary columns, in each of which the sum of ones equals to s(j) of that column, such as the following codes.
xij = zeros(m,n);
for j=1:n
randRows=randperm(m); %a row vector containing a random permutation of the integers from 1 to m inclusive.
rowsWithOne=randRows(1:sj(j)); %row indices having 1, and sum up to sj
xij(rowsWithOne,j)=1;
end
However, xij usually doesn't satisfy the horizontal constraints. I was thinking I should create a matrix first meets the row constraints lhs (lower bound) dl(i), then, column constraint s(j), and finally fill the offsets to meet rhs du(i), but I don't know how to implement it in Matlab. Is there any ideas to create xij?
Thanks in advance.
There are a couple things to first take into account, mainly if the problem can even be solved with the given constraints. First, you must check that the sum of s(j), sum(s) is greater than the sum of the lower bound constraints, sum(dl), and also less than the sum of the upper bound constraints, sum(du).
A simple if statement should be able to check this. After this has been done, the following code should be one solution to the problem, but given the nature of the problem, there will probably not be a unique solution.
%initialize counters
x = 1;
y = 1;
%make sure the lower bounds are first satisfied
for k = 1:nrows
while (dl(k) > 0)
if (s(y) > 0)
xij(k,y) = 1;
dl(k) = dl(k)-1;
du(k) = du(k)-1;
s(y) = s(y)-1;
end
y = y+1;
end
y = 1;
end
%make sure the columns all add to their specified values
for k = 1:ncols
while (s(k) > 0)
if (xij(x,k) == 0 && du(x) > 0)
xij(x,k) = 1;
du(x) = du(x)-1;
s(k) = s(k)-1;
end
x = x+1;
end
x = 1;
end
The first for loop adds 1s along the rows such that each row satisfies the minimum constraint. The next for loop adds 1s along the columns such that each column adds up to the desired value. The if statement in the first for loop ensures that a 1 isn't added if that column has already reached its desired value, and the if statement in the second for loop ensures that a 1 isn't added to a row that would put it over the maximum value of the row.
In order to keep track of the amount of ones, the du is lowered by 1 every time a 1 is added to the corresponding row, and likewise the desired value s(j) is subtracted by 1 every time a 1 is added to that column of the xij matrix.
%Create an initial xij matrix, each column of which consists of a random binary xij sequence, but sum of ones in each column equals to sj
Init_xij = zeros(nShift,nCombo);
for j=1:nCombo
randRows=randperm(nShift); %a row vector containing a random permutation of the integers from 1 to nShift inclusive.
rowsWithOne=randRows(1:sj(j)); %row indices having 1, and sum up to sj
Init_xij(rowsWithOne,j)=1;
end
%Adjust the initial xij matrix to make it feasible, satisfying horizontal
%LHS (dli) and RHS (dui) constraints
Init_xij_Feasible=Init_xij;
k=1;
while k
RowSum=sum(Init_xij_Feasible,2); %create a column vector containing the sum of each row
CheckLB=lt(RowSum,dli); %if RowSum <dli, true
CheckUB=gt(RowSum,dui); %if RowSum >dui, true
if ~any(CheckLB)&&~any(CheckUB) %if any element in CheckLB and CheckUB is zero
break,
else
[~,RowIdxMin]=min(RowSum);
[~,RowIdxMax]=max(RowSum);
ColIdx=find(Init_xij_Feasible(RowIdxMax,:) & ~Init_xij_Feasible(RowIdxMin,:),1); % returns the first 1 column index corresponding to the nonzero elements in row RowIdxMax and zero elements in row RowIdxMin.
%swap the min and max elements
[Init_xij_Feasible(RowIdxMin,ColIdx),Init_xij_Feasible(RowIdxMax,ColIdx)]=deal(Init_xij_Feasible(RowIdxMax,ColIdx),Init_xij_Feasible(RowIdxMin,ColIdx));
end
k=k+1;
end
I have an N-by-M-Matrix as input called GR wich consists of the following numbers: -3,0,2,4,7,10,12
And I have to return a vector. If M=1, then it should just return the input.
If M>1 It should remove the lowest number from the matrix and then calculate the mean of the remaining numbers.
However, if one of the numbers in the row is -3, it should return the value -3 in the output.
My thoughts of the problem:
Is it possible to make a for loop?
for i=1:length(GR(:,1))
If length(GR(1,:))==1
GR=GR
end
If length(GR(1,:))>1
x=min(GR(i,:))=[] % for removing the lowest number in the row
GR=sum(x)/length(x(i,:))
I just don't have any Idea of how to detect if any of the numbers in the row is -3 and then return that value instead of calculating the mean and when I tried to delete the lowest number in the matrix using x=min(GR(i,:)) matlab gave me this error massage 'Deletion requires an existing variable.'
I put in a break function. As soon as it detects a -3 value it breaks from the loop. Same goes for the other function.
Note that it is an i,j (M*N) matrix. So you might need to change your loop.
for i=1:length(GR(:,1))
if GR(i,1)==-3
GR=-3
break
end
If length(GR(1,:))==1
GR=GR
break
end
If length(GR(1,:))>1
x=min(GR(i,:))=[] % for removing the lowest number in the row
GR=sum(x)/length(x(i,:))
end
end
you can use Nan's, nanmean, any, and dim argument in these functions:
% generate random matrix
M = randi(3);
N = randi(3);
nums = [-3,0,2,4,7,10,12];
GR = reshape(randsample(nums,N*M,true),[N M]);
% computation:
% find if GR has only one column
if size(GR,2) == 1
res = GR;
else
% find indexes of rows with -3 in them
idxs3 = any(GR == -3,2);
% the (column) index of the min. value in each row
[~,minCol] = min(GR,[],2);
% convert [row,col] index pair into linear index
minInd = sub2ind(size(GR),1:size(GR,1),minCol');
% set minimum value in each row to nan - to ignore it on averaging
GR(minInd) = nan;
% averaging each rows (except for the Nans)
res = nanmean(GR,2);
% set each row with (-3) in it to (-3)
res(idxs3) = -3;
end
disp(res)
I have written a matlab code and want to terminate the code after finding the first one in the matrix. BUT, it continues until the last one and then break works. What is the problem?
% The code is as follows:
for i=1:n
for j=1:m
if (bw(i,j)==1) % element with value one
p1(1)=i;p1(2)=j;
% fprintf('value of a: %d\n', i ,j)
sprintf('Found first one in the loop!!')
break; % terminate
end
end
end
It looks like you want to find the first 1 value in a matrix, searching in row-major order (increase column index first, then row index). You can do that without loops using the find function:
M = [0 1; 1 1]; %// example matrix
v = 1; %// value you want to find
[col, row] = find(M.'==v, 1);
Note that the matrix is transposed and the order of the two outputs of find is reversed. This is because Matlab finds elements in column-major order (increase row index first, then column index).
In the above example, with
M =
0 1
1 1
the result is
row =
1
col =
2
Since break exits only the inner loop (the one that runs with j) but not the outer loop (the one that runs with i), you can tackle the problem by using a simple boolean which you initially set as false.
When you break the inner loop, change it to true and then check (in the outer loop) if such boolean is true. If it is, also break the outer loop.
IWantToBreak=false;
for i=1:n
for j=1:m
if (bw(i,j)==1) % element with value one
p1(1)=i;p1(2)=j;
% fprintf('value of a: %d\n', i ,j)
sprintf('Found first one in the loop!!')
IWantToBreak=true;
break; % terminate
end
end
if IWantToBreak==true
break;
end
end
Can any body help me explain Line 7 of this code. What does this line
temp(min(temp,[],2) >=1 & max(temp,[],2)<=N,:)
do in the code?
Line 7 of this code is throwing me off
N=10;
H=-1;
J=0;
for i=1:N
for j=1:N
temp=[i-1,j;i+1,j;i,j-1;i,j+1];
ngh{i,j}=temp(min(temp,[],2) >=1 & max(temp,[],2)<=N,:);
end
end
That line is selecting the rows of temp which have all values between 1 and N, and assigning that submatrix to ngh{i,j}.
Note that
min(...,[],2) or max(...,[],2) gives the minimum or maximum of each row;
The logical comparison with 1 or N and the & operation result in a logical index vector, which is used to address the desired rows of temp (and all columns).