MATLAB: Populate matrix columns based on a vector of index vector

column populating

I generate an 8×8 A matrix below. I also have an 8×1 vector B containing random integer numbers from 1 to 8. And, I have an 8×1 C vector containing numbers obtained below. How can I, for all rows of A, populate those columns of A starting at B to the end, with values of corresponding rows in C without going through a loop (i.e. vectorized).

A=rand(8);
B = randi([1 8], 8, 1);
C=-linspace(8,30,8)';

Best Answer

>> X = (1:5)>=B(:); % requires >=R2016b, for earlier versions replace >= with BSXFUN
>> [R,~] = find(X);
>> A(X) = C(R)
A =
  -10  -10  -10  -10  -10
    1    2   -9   -9   -9
    1    2    3   -5   -5
    1   -2   -2   -2   -2
    1    2    3    4   -1

Related Solutions

MATLAB: Is it possible to extract the values with a vector of indices for each row without using the for statement from the matrix

see sub2ind()

A = [1, 2, 3; 4, 5, 6; 7, 8, 9; 10, 11, 12]; % reference matrix
b = [2; 1; 1; 3]; % index for each row that I want to extract
idx = sub2ind(size(A), 1:size(A,1), b.');
A(idx)

Result

>> A(idx)
ans =
     2     4     7    12

MATLAB: How to modify a matrix to change the entries >=0 and multiply it by 5 AND to change the entries <0 and add 2 to those entries

Start with "If an entry in B is greater than or equal to zero": You can either run a loop over all elements of B or use "logical indexing":

% Linear indexing:
for k = 1:numel(B)
   if B(k) >= 0
     ...
   end
end

Or two indices for rows and columns:

for r = 1:size(B, 2)
  for c = 1:size(B, 1)
    if B(r, c) >= 0
       ...
    end
  end
end

Or "logical indexing":

index = (B >= 0);
disp(B(index))

Now try to use this to modify B. Use the mentioned terms to search in the documentation and in the internet.

Best Answer

Related Solutions

MATLAB: Is it possible to extract the values with a vector of indices for each row without using the for statement from the matrix

MATLAB: How to modify a matrix to change the entries >=0 and multiply it by 5 AND to change the entries <0 and add 2 to those entries

Related Question