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I make a list of 5 empty $3 \times 3$ matrices via

 Table[m[i]=IdentityMatrix[3]-IdentityMatrix[3],{i,1,5}]

However when I want to change an entry via

 m[2][[3,4]]=5

I get an error which reads:

Set::setps: m[2] in the part assignment is not a symbol. >>

Any ideas? Thanks

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    $\begingroup$ Why not do something like m = ConstantArray[0, {5, 3, 3}] and then do m[[2, 3, 4]] = 1? $\endgroup$
    – J. M.'s eventual burnout ♦
    Mar 30, 2017 at 5:37
  • $\begingroup$ You mean instead of explicitly indexing the matrices, placing them in a list and identifying them there. That does make the way I call the matrices a little more intuitive, I don't have the single brackets and then the double brackets. However, I'm curious why indexing the matrices directly messes with the assignment I make. $\endgroup$
    – Mike
    Mar 30, 2017 at 5:40
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    $\begingroup$ The docs for the error message just says "Part assignments are implemented only for parts of the value of a symbol." So since m[2] is not a symbol itself, but a reference to one of the DownValues of the symbol m, this is what you get. $\endgroup$
    – Marius Ladegård Meyer
    Mar 30, 2017 at 5:42
  • $\begingroup$ Also, note the difference between Do and Table, we don't use Table just for side effects ;) $\endgroup$
    – Marius Ladegård Meyer
    Mar 30, 2017 at 5:43
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    $\begingroup$ You can do e.g. m[2] = ReplacePart[m[2], {3, 4} -> 5] but that copies the whole of m[2] so I would prefer @J.M. 's suggestion. $\endgroup$
    – Marius Ladegård Meyer
    Mar 30, 2017 at 5:49

1 Answer 1

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As the comments read, the problem is that m[2] is not a symbol but a reference to a value of the symbol m, so to change the value there I have to actually replace it via:

 m[2]=ReplacePart[m[2],{4,5}->5].

Which copies the entirety of m[2] and doesn't look all that pretty.

J.M.'s suggestion is to just make a list of the matrices and refer to them from there.

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