Skip to main content

undocumented - Internal`WithTimeStamps usage


I was digging inside the Iconize implementation a little bit and saw that there's a function BoxForm`BoxFormAutoLoad which loads the FormatValues for objects.


For the most part it's boring, but one interesting function that I saw in it was Internal`WithTimeStamps. A sample usage of BoxForm`BoxFormAutoLoad is:


BoxForm`BoxFormAutoLoad[MakeBoxes, 
 IconizedObject["asd"], 
 StandardForm, 
 "NotebookTools`Iconize`", 

 {{IconizedObject, _}},
 Hold[IconizedObject]
 ]

And tracking the WithTimeStamps call inside we see it's running:


Internal`WithTimestampsPreserved[
 {IconizedObject},
 Apply[
  (BoxForm`wasProtected = Unprotect[#1];
    (* Uninteresting Which block removed *);

    Protect[Evaluate[BoxForm`wasProtected]]) &,
  {{IconizedObject, _}},
  {1}
  ];
 DumpGet[System`Private`$SystemFileDir <> 
   System`Dump`fixfile["NotebookTools`Iconize`"] <> "x"];
 ]

Running this seems to do nothing interesting so I'm wondering why it exists and in what way there are "Timestamps" in the call.



Answer




It is an internal and undocumented function that can be used to prevent reevaluation of expressions whenever particular symbols contained in them change. This is a simple example:


Clear[a, b];
b = 1 + a

(* 1 + a *)

Internal`WithTimestampsPreserved[{a}, a = 1];
{a, b}

(* {1, 1 + a} *)


Although the value of a changed, the evaluator did not think that b might need reevaluation.


That is because we preserved the timestamp of a, i.e. forced it to remain the same.


Contrast with the normal behavior where updating a also changed its timestamp, so now b is out of date and must be reevaluated.


a = 1; {a, b}

(* {1, 2} *)

In some sense, it accomplishes the opposite of Update, which can be used to force reevaluation in certain situations.


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