Skip to main content

variable definitions - Subscripts - Why do I see the error "only assignments to symbols are allowed" when using a Module and not otherwise?


When I type


Remove["Global`*"]
foo[] := Module[{Subscript[a, 1] = "x"}, 0]; 
foo[]

I get the expected error "only assignments to symbols are allowed". I understand the error.


enter image description here


But why I do not get the same error when I type the same assignment in the notebook?


Remove["Global`*"]

Subscript[a, 1] = "x"

No error and no beep.


enter image description here


What is the difference? I'm using version 8.04 on Windows.



Update


The non-localized trick shown by Szabolcs answer worked for me on a demo. Here is a screen shot of the result. What I was trying to do is to typeset some information about a solver, some matrix equation, and I thought why not take advantage of MatrixForm and Grid's nice way of typesetting things, and display the information right there in the demo screen as an option? This is easier than putting it in the text section below where it can hard to get to as I have so many such things.


Here is an example of the code and the result. You can see I did not localized these symbols in the module which builds this expression. I just started trying this idea, but so far I am happy that it worked. This below is only an example, not cleaned up, just wanted to see if the demo will work.


enter image description here




Answer



This is because only symbols can be localized by Module. It is not about assignment, but localization.


Subscript[a, 1] is not a symbol, but a compound expression, so:


Module[{Subscript[a, 1] = "x"}, 0] (* <-- not allowed *)

Module[{}, Subscript[a, 1] = "x"]  (* <-- allowed but not localized *)

I agree that the error you got may be a bit confusing.


A somewhat ugly workaround is Module[{Subscript}, Subscript[a, 1] = "x"] or you may try to use the Notation` package to create symbol names with subscripts in them. A word of warning though: in some cases, Module variables that have DownValues do not get destroyed when the Module finishes evaluating. For more information, see the end of the Module section in this answer by Leonid Shifrin, and the comments on that answer.


Comments

Post a Comment

Popular posts from this blog

mathematical optimization - Minimizing using indices, error: Part::pkspec1: The expression cannot be used as a part specification

I want to use Minimize where the variables to minimize are indices pointing into an array. Here a MWE that hopefully shows what my problem is. vars = u@# & /@ Range[3]; cons = Flatten@ { Table[(u[j] != #) & /@ vars[[j + 1 ;; -1]], {j, 1, 3 - 1}], 1 vec1 = {1, 2, 3}; vec2 = {1, 2, 3}; Minimize[{Total@((vec1[[#]] - vec2[[u[#]]])^2 & /@ Range[1, 3]), cons}, vars, Integers] The error I get: Part::pkspec1: The expression u[1] cannot be used as a part specification. >> Answer Ok, it seems that one can get around Mathematica trying to evaluate vec2[[u[1]]] too early by using the function Indexed[vec2,u[1]] . The working MWE would then look like the following: vars = u@# & /@ Range[3]; cons = Flatten@{ Table[(u[j] != #) & /@ vars[[j + 1 ;; -1]], {j, 1, 3 - 1}], 1 vec1 = {1, 2, 3}; vec2 = {1, 2, 3}; NMinimize[ {Total@((vec1[[#]] - Indexed[vec2, u[#]])^2 & /@ R...
Post a Comment
Read more

functions - Get leading series expansion term?

Given a function f[x] , I would like to have a function leadingSeries that returns just the leading term in the series around x=0 . For example: leadingSeries[(1/x + 2)/(4 + 1/x^2 + x)] x and leadingSeries[(1/x + 2 + (1 - 1/x^3)/4)/(4 + x)] -(1/(16 x^3)) Is there such a function in Mathematica? Or maybe one can implement it efficiently? EDIT I finally went with the following implementation, based on Carl Woll 's answer: lds[ex_,x_]:=( (ex/.x->(x+O[x]^2))/.SeriesData[U_,Z_,L_List,Mi_,Ma_,De_]:>SeriesData[U,Z,{L[[1]]},Mi,Mi+1,De]//Quiet//Normal) The advantage is, that this one also properly works with functions whose leading term is a constant: lds[Exp[x],x] 1 Answer Update 1 Updated to eliminate SeriesData and to not return additional terms Perhaps you could use: leadingSeries[expr_, x_] := Normal[expr /. x->(x+O[x]^2) /. a_List :> Take[a, 1]] Then for your examples: leadingSeries[(1/x + 2)/(4 + 1/x^2 + x), x] leadingSeries[Exp[x], x] leadingSeries[(1/x + 2 + (1 - 1/x...
Post a Comment
Read more

What is and isn't a valid variable specification for Manipulate?

I have an expression whose terms have arguments (representing subscripts), like this: myExpr = A[0] + V[1,T] I would like to put it inside a Manipulate to see its value as I move around the parameters. (The goal is eventually to plot it wrt one of the variables inside.) However, Mathematica complains when I set V[1,T] as a manipulated variable: Manipulate[Evaluate[myExpr], {A[0], 0, 1}, {V[1, T], 0, 1}] (*Manipulate::vsform: Manipulate argument {V[1,T],0,1} does not have the correct form for a variable specification. >> *) As a workaround, if I get rid of the symbol T inside the argument, it works fine: Manipulate[ Evaluate[myExpr /. T -> 15], {A[0], 0, 1}, {V[1, 15], 0, 1}] Why this behavior? Can anyone point me to the documentation that says what counts as a valid variable? And is there a way to get Manpiulate to accept an expression with a symbolic argument as a variable? Investigations I've done so far: I tried using variableQ from this answer , but it says V[1...
Post a Comment
Read more