MATLAB: How to replace the diagonal entries of a square matrix with entries from a vectore of equal length

matrix manipulation

I have an nxn matrix M, and a vector v of length n. I want to swap the diagonal of M with the vector v, without a for loop. Is that possible? Thanks!

Best Answer

M = M - diag(diag(M)) + diag(v)
or...
M = M + diag(v - diag(M))
In either case, I killed the diagonal, then added the diagonal I want back in.
Or, you can simply replace those elements. A lazy way to do that is:
M(find(eye(size(M))) = v;
There are lots of simple variations on the latter theme of course.
n = size(M,1);
M(1:(n+1):end) = v;
For example:
M = rand(5)
M =
      0.45054      0.82582      0.10665      0.86869      0.43141
     0.083821      0.53834       0.9619     0.084436      0.91065
      0.22898      0.99613    0.0046342      0.39978      0.18185
      0.91334     0.078176      0.77491      0.25987       0.2638
      0.15238      0.44268       0.8173      0.80007      0.14554
v = 1:5;
n = size(M,1);
M(1:(n+1):end) = v
M =
            1      0.82582      0.10665      0.86869      0.43141
     0.083821            2       0.9619     0.084436      0.91065
      0.22898      0.99613            3      0.39978      0.18185
      0.91334     0.078176      0.77491            4       0.2638
      0.15238      0.44268       0.8173      0.80007            5
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