MATLAB: Maybe a colon operator

colonfor

It is possible write this for in a more fast code (maybe colon)?

for i = 2 : n-1
  A(i) = A(i-1)+A(i+1);
end

Best Answer

Vectorizing is possible and better. One approach:

A=randi([1 10],1,10)
n=numel(A);
i=2:n-1;
A(i)=A(i-1)+A(i+1)

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MATLAB: How to replace the following code, to get the matrix

You really just need to make a sparse diagonal matrix. This can very easily be done with spdiags. Use the last syntax: A = spdiags(B,d,m,n), which you would be calling as: A = spdiags(B,d,n^2,n^2);. The issue now is only in setting up B and d which really just requires you to replace a few values in each of the columns of B, but this can be done by finding the appropriate indices to replace (note that not all values in B will be used, so start your counting from the bottom as this function will omit unnecessary values at the top of the matrix B). Put your head to the grindstone and I'm sure you can find the patterns necessary to do this for arbitrary values of n, without loops or cell arrays. I will do it for just for the lowest diagonal:

n = 5; % Change it to 4 and verify the code works in both cases.
B = 0.298*ones(n^2,1); % Use the calculation of wherever 0.298 came from if you wish
d = -(n+1); % The diagonal where the 1st vector of B should appear
B(end-n:-n:1) = 0;
A = spdiags(B,d,n^2,n^2); % This has the lower diagonal
Q = full(A); % For visualizing the correct results.
open Q

Now, just add the columns for the other diagonals to B and add the diagonal identifier to d. Also, never use the line of code:

if true
   %code
end

true is always true so the code always runs.

MATLAB: How to sort an array in descending order without using the sort function

You can use bubble sort:

swapped = 1; 
while swapped
  swapped = 0; 
  for i=1:numel(A)-1
    if A(i+1) > A(i)
      temp = A(i); A(i) = A(i+1); A(i+1) = temp; swapped = 1; 
    end
  end
end

Best Answer

Related Solutions

MATLAB: How to replace the following code, to get the matrix

MATLAB: How to sort an array in descending order without using the sort function

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