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Mathematics Wolfram Language

Why inserting a command into a procedure yields error

Ehud Behar

Posted 2 years ago

I have a square matrix `m` to which I want to assign new values into its diagonal. For example, m={ {20, 24, 33, 36, 34}, {39, 13, 15, 30, 30}, {18, 17, 16, 28, 27}, {39, 35, 33, 32, 37}, {30, 21, 22, 10, 23} } I want to change the diagonal elements into some negative numbers: Table[ m[[i, i]] = RandomInteger[-10, Length[m]]〚i〛, {i, 1, Length[m]}] That works just fine. Now I want to write a function `f1` that performs this manipulation, and this is where I got stuck: f1[q_] := Table[ q[[i, i]] = RandomInteger[{-10, 0}, Length[q]]〚i〛, {i, 1, Length[q]}] f1[m] gives Set::setps: {{20,24,33,36,34},{39,13,15,30,30},{18,17,16,28,27},{39,35,33,32,37},{30,21,22,10,23}} in the part assignment is not a symbol. Set::setps: {{20,24,33,36,34},{39,13,15,30,30},{18,17,16,28,27},{39,35,33,32,37},{30,21,22,10,23}} in the part assignment is not a symbol. Set::setps: {{20,24,33,36,34},{39,13,15,30,30},{18,17,16,28,27},{39,35,33,32,37},{30,21,22,10,23}} in the part assignment is not a symbol. General::stop: Further output of Set::setps will be suppressed during this calculation. Why is that?

I have a square matrix m to which I want to assign new values into its diagonal. For example,

m={
  {20, 24, 33, 36, 34},
  {39, 13, 15, 30, 30},
  {18, 17, 16, 28, 27},
  {39, 35, 33, 32, 37},
  {30, 21, 22, 10, 23}
}

I want to change the diagonal elements into some negative numbers:

Table[
 m[[i, i]] = RandomInteger[-10, Length[m]]〚i〛,
 {i, 1, Length[m]}]

That works just fine.

Now I want to write a function f1 that performs this manipulation, and this is where I got stuck:

f1[q_] := Table[
  q[[i, i]] = RandomInteger[{-10, 0}, Length[q]]〚i〛,
  {i, 1, Length[q]}]

f1[m]

gives

Set::setps: {{20,24,33,36,34},{39,13,15,30,30},{18,17,16,28,27},{39,35,33,32,37},{30,21,22,10,23}} in the part assignment is not a symbol.

Set::setps: {{20,24,33,36,34},{39,13,15,30,30},{18,17,16,28,27},{39,35,33,32,37},{30,21,22,10,23}} in the part assignment is not a symbol.

Set::setps: {{20,24,33,36,34},{39,13,15,30,30},{18,17,16,28,27},{39,35,33,32,37},{30,21,22,10,23}} in the part assignment is not a symbol.

General::stop: Further output of Set::setps will be suppressed during this calculation.

Why is that?

POSTED BY: Ehud Behar

7 Replies

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Robert Nowak

Robert Nowak, IMS Nanofabrication

Posted 2 years ago

Hi Ehud What you are trying is called side effect programming which is considered as bad style and should be generally avoided in any programming. In partikular you are trying to pass a parameter by reference which is not the "Mathematica Way" . It is better to generate a new object. ReplacePart[m, {{i_, i_}} :> RandomInteger[-10]] m=ReplacePart[m, {{i_, i_}} :> RandomInteger[-10]] f1[q_] := ReplacePart[q, {{i_, i_}} :> RandomInteger[-10]] m = f1[m]

POSTED BY: Robert Nowak

Ehud Behar

Posted 2 years ago

Totally agree with you. Thanks a lot for pointing that out and providing this nice solution.

POSTED BY: Ehud Behar

Hans Milton

Posted 2 years ago

I will try to give a condensed explanation. By default a function argument is evaluated before it is passed on to the functions body. So the argument q becomes an explicit 5x5 matrix when it is seen as the left hand side of the assignment in `Table`. And the assignment `lhs[[x]]=rhs` works only if `lhs` is a symbol. If not it gives the error you saw. `Unevaluated` prevents evaluation of the argument. So q becomes the symbol m.

POSTED BY: Hans Milton

Rohit Namjoshi

Posted 2 years ago

Another way SetAttributes[f1, HoldFirst]

POSTED BY: Rohit Namjoshi

Hans Milton

Posted 2 years ago

Yes Rohit, `Unevaluated` and the function attribute `HoldFirst` gives the same effect in this case. But one could argue that `Unevaluated` is more 'entry level' and easier to explain.