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"Value" (Data Structure)

"Value"

represents a mutable expression value.

Details

  • A mutable value can be modified in any part of a program:
  • CreateDataStructure["Value",x]create a new "Value" with specified initial value x
    Typed[x,"Value"]give x the type "Value"
  • For a data structure of type "Value", the following operations can be used:
  • ds["Copy"]return a copy of dstime: O(1)
    ds["Get"]get the value stored in dstime: O(1)
    ds["Set",x]set the value of ds to xtime: O(1)
    ds["Visualization"]return a visualization of dstime: O(1)
  • The following functions are also supported:
  • dsi===dsjTrue, if dsi equals dsj
    FullForm[ds]the full form of ds
    Information[ds]information about ds
    InputForm[ds]the input form of ds
    Normal[ds]convert ds to a normal expression

Examples

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Basic Examples  (1)

A new "Value" can be created with CreateDataStructure:

Wolfram Language code: ds = CreateDataStructure["Value", f[1]]

Extract the value stored:

Wolfram Language code: ds["Get"]

Insert a new value to be stored:

Wolfram Language code: ds["Set", f[2]]

Confirm that the value has updated:

Wolfram Language code: ds["Get"]

Return an expression version of ds:

Wolfram Language code: Normal[ds]

A visualization of the data structure can be generated:

Wolfram Language code: ds["Visualization"]

Scope  (6)

Mutability  (1)

The "Value" data structure is mutable, so its value can be changed:

Wolfram Language code: ds = CreateDataStructure["Value", f[x]]

Change the value:

Wolfram Language code: ds["Set", g[y]]

Check the value:

Wolfram Language code: ds["Get"]

Change the value again:

Wolfram Language code: ds["Set", h[z]]

Check the value again:

Wolfram Language code: ds["Get"]

Information  (1)

A new "Value" can be created with CreateDataStructure:

Wolfram Language code: ds = CreateDataStructure["Value", f[1]]

Information about the data structure ds:

Wolfram Language code: Information[ds]

Operations  (4)

"Copy"  (1)

Create a new "Value" with an initial value:

Wolfram Language code: ds = CreateDataStructure["Value", f[x]]

Get a copy of the data structure:

Wolfram Language code: ds1 = ds["Copy"]

The contents of both data structures are the same:

Wolfram Language code: {Normal[ds], Normal[ds1]}

Modifying the copy will not affect the original data structure:

Wolfram Language code: ds1["Set", g[x]]; {Normal[ds], Normal[ds1]}

"Get"  (1)

Create a new "Value" with an initial value:

Wolfram Language code: ds = CreateDataStructure["Value", f[x]]

Get the stored value:

Wolfram Language code: ds["Get"]

"Set"  (1)

Create a new "Value" with an initial value:

Wolfram Language code: ds = CreateDataStructure["Value", f[x]]

Set the data structure to a new expression:

Wolfram Language code: ds["Set", g[x]]

Get the stored value:

Wolfram Language code: ds["Get"]

"Visualization"  (1)

Create a new "Value" with an initial value:

Wolfram Language code: ds = CreateDataStructure["Value", f[x]]

Visualize the contents:

Wolfram Language code: ds["Visualization"]

Applications  (1)

In-Place Update of an Expression  (1)

Define a function that updates a "Value" passed as an argument:

Wolfram Language code: accumulate[f_, val_] /; DataStructureQ[val, "Value"] := val["Set", f[val["Get"]]];

Create a "Value" holding an initial expression:

Wolfram Language code: v = CreateDataStructure["Value", x]

Update the stored expression through repeated function calls:

Wolfram Language code: accumulate[# ^ 2&, v]; accumulate[# + 1&, v];

Get the final stored value:

Wolfram Language code: v["Get"]

Properties & Relations  (4)

InputForm  (1)

InputForm returns the serialized contents of the "Value":

Wolfram Language code: InputForm[CreateDataStructure["Value", f[x]]]

This serialized form can be used to recreate the data structure:

Wolfram Language code: DataStructure["Value", {"Data" -> f[x]}]

Normal  (1)

Normal can be used to get the contents of a "Value":

Wolfram Language code: ds = CreateDataStructure["Value", f[x]]; Normal[ds]

The same can be achieved with the "Get" operation:

Wolfram Language code: ds["Get"]

SameQ  (1)

SameQ can be used to test whether two "Value" data structures have the same contents:

Wolfram Language code: ds1 = CreateDataStructure["Value", f[x]]; ds2 = CreateDataStructure["Value", f[x]]; ds1 === ds2

If one element is changed, the data structures are no longer the same:

Wolfram Language code: ds1["Set", g[x]]; ds1 === ds2

"Counter"  (1)

The "Counter" data structure can be considered as a specialized version for integers of the "Value" data structure.

Neat Examples  (1)

Memoized Evaluation with Explicit Cache  (1)

Use a "Value" to implement a manually controlled cache that can be reset or copied:

Wolfram Language code: cache = CreateDataStructure["Value", <||>]

Define a function that consults and updates the cache:

Wolfram Language code: cachedEval[f_, x_] := With[{c = cache["Get"]}, If[KeyExistsQ[c, x], c[x] , cache["Set", Append[c, x -> f[x]]][x] ] ]

Evaluate the function, populating the cache:

Wolfram Language code: cachedEval[# ^ 2&, 10]; cachedEval[# ^ 2&, 11];

Inspect the cache:

Wolfram Language code: cache["Get"]