# Evaluating an expression a second time returns wrong result

I have the following code:

Sigma = {{1, 1}, {1, 1}};

sol =
Sum[ReplacePart[Apply[KroneckerProduct, Table[id, {i, 1, 2}]], {i -> Sigma}], {i, 1, 1}]

id = IdentityMatrix[2];

sol // Dimensions

{4, 4}

sol2 =
Sum[ReplacePart[Apply[KroneckerProduct, Table[id, {i, 1, 2}]], {i -> Sigma}], {i, 1, 1}]

sol2 // Dimensions

 {4}


As you see, when I evaluate the Sum[...] expression a second time, the dimension of the result changes and gives the wrong result. The result must be a 4 by 4 matrix. If I had put the id before sol, then I would have gotten the wrong result on the first evaluation.

How can I resolve this problem?

I think the problem is with

ReplacePart[Apply[KroneckerProduct, Table[id, {i, 1, 2}]], {i -> Sigma}]


Before assigning id the position i is clear because we have something like {id, id} so ReplacePart can change the position i which contains id with Sigma. However, after assigning id, the position i is not clear anymore. I cannot find a solution to this problem.

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Try ClearAll[sol, sol2, id, sigma]; before all your code; and move the definition of id before sol =... –  kglr Jun 4 '14 at 13:29
@kguler. It didn't work. –  yashar Jun 4 '14 at 13:35
@yashar - it worked perfectly with me after following kgulers advice. Also, you should use ":=" (SetDelayed) instead of just "=". –  eldo Jun 4 '14 at 13:38
@yashar I get 4 for both dimensions and also sol==sol2 gives True (btw, all this on MMA V 9.0.1.0 Windows 8 64bit) –  kglr Jun 4 '14 at 13:44
@kguler. The dimension 4 is the wrong result. The result of the calculation must be a matrix. –  yashar Jun 4 '14 at 13:45