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Suppose I have a zero matrix Amat and a list of values ltest that contains the values of the matrix Amat for the some of its diagonal elements:

L = 10000;
Amat = 0. IdentityMatrix[L];
ltest = {{-0.70710, 1329}, {-0.25, 5607}, {-0.3535, 1261}, {-0.25, 4393}, {-0.5, 4358},
{-0.3511, 1009}, {-1., 3815}, {-0.25, 3784}, {-0.133, 3667}, {-0.88, 909}};

Now in order to pass the second column of ltest to Amat I can do:

For[i = 1, i <= Length[ltest], i++, Amat[[ltest[[All, 2]][[i]], ltest[[All, 2]][[i]]]] = ltest[[All, 1]][[i]]]

Is there a shorter way without using For?

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    $\begingroup$ Have you already tried looking at SparseArray[]? $\endgroup$ – J. M.'s ennui♦ Jan 26 at 12:18
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L = 10000;
ltest = {{-0.70710, 1329}, {-0.25, 5607}, {-0.3535, 1261}, {-0.25, 
    4393}, {-0.5, 4358}, {-0.3511, 1009}, {-1., 3815}, {-0.25, 
    3784}, {-0.133, 3667}, {-0.88, 909}};
Amat = SparseArray[{#2, #2} -> #1 & @@@ ltest, {L, L}, 0]

And Amat can be used as an ordinary matrix.

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  • $\begingroup$ thanks for the answer! $\endgroup$ – geom Jan 26 at 13:05
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    $\begingroup$ Shouldn't it be Amat = SparseArray[{#[[2]], #[[2]]} -> #[[1]] & /@ ltest, {L, L}, 0]? $\endgroup$ – MelaGo Jan 26 at 13:28
  • $\begingroup$ @MelaGo Yes you are right.@SneezeFor16Min please edit your post $\endgroup$ – geom Jan 26 at 14:01
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    $\begingroup$ @MelaGo Thanks for pointing that out! I misread OP's code... Answer updated. $\endgroup$ – SneezeFor16Min Jan 26 at 15:44
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With L and ltest as given in the OP, here is another way to use SparseArray[]:

DiagonalMatrix[SparseArray[Rule @@@ Reverse[ltest, 2], L]]

DiagonalMatrix[] is smart enough to return a SparseArray[] when fed a diagonal represented as a SparseArray[].

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  • $\begingroup$ Smart! This is ~1.6x faster than using straight-forward Rules for reasons I don't know. $\endgroup$ – SneezeFor16Min Jan 26 at 17:11
  • $\begingroup$ can it be used if the elements of Amat are not diagonal, i.e ltest = {{-0.70710, 5, 1329}, {-0.25, 31,5607}, {-0.3535, 411,1261}, {-0.25,706, 4393}, {-0.5,58, 4358}, {-0.3511, 2871,1009}, {-1.,7415, 3815}, {-0.25,1254, 3784}, {-0.133, 33,3667}, {-0.88, 17,909}}; where the last two elements are the i,j indices of Amat? $\endgroup$ – geom Jan 26 at 17:45
  • $\begingroup$ @geom, then you use SparseArray[] directly, as I originally advised you in my comment to your question. Try it out, and if you get stuck, ask a new question. $\endgroup$ – J. M.'s ennui♦ Jan 26 at 17:56
  • $\begingroup$ @J.M. thanks, no need to ask a new question, SparseArray[] works fine $\endgroup$ – geom Jan 26 at 18:03

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