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Remove

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

Remove[s1, s2, ...]

removes the named symbols completely, deleting their definitions from the symbol table. Remove[{s1, s2, ...}] accepts a list of specs.

Notes Remove has attribute HoldAll; symbols with attribute Protected are not affected.

Examples (10)

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (3)

In[1]:= f[x_] := x^2; SetAttributes[f, Listable]; Attributes[f]
Out[1]= {Listable}

In[2]:= ClearAll[f]; {Attributes[f], DownValues[f]}
Out[2]= {{}, {}}

In[3]:= x = 2; Remove[x]; x
Out[3]= x

Applications (7)

A user symbol

In[4]:= tmpvar = 7
Out[4]= 7

In[5]:= Remove[tmpvar]

A fresh, undefined symbol

In[6]:= tmpvar
Out[6]= tmpvar

The entry is gone from the symbol table

In[7]:= r = 7; Remove[r]; Names["r"]
Out[7]= {}

Default options go with it

In[8]:= Options[o2] = {opt -> 1}; Remove[o2]; Options[o2]
Out[8]= {}

Built-ins are Protected, so they are skipped

In[9]:= Remove[Plus]; 1 + 1
Out[9]= 2

Protected like every built-in

In[10]:= Attributes[Remove]
Out[10]= {HoldAll, Protected}

Implementation notes

Algorithm. builtin_remove (src/core.c) shares the core_apply_symbol_action walker with ClearAll: it visits each argument ā€” a symbol, a string, or a flat List of them ā€” and applies core_remove_one to every resolved name, returning Null. core_remove_one skips any Protected symbol (the guard that keeps Remove from ever deleting a builtin) and otherwise calls symtab_remove_symbol(name), deleting the symbol's definition from the symbol table entirely.

This is a stronger erase than ClearAll: where ClearAll empties a symbol but leaves the entry in place, Remove deletes the entry, so the name no longer appears in the symbol table until it is next referenced (at which point a fresh, undefined symbol is created). Remove is itself Protected so it cannot be removed.

Attributes & limits. Attributes[Remove] is {HoldAll, Protected}, and Protected alone is what keeps it, and every other builtin, from being removed. Its arguments arrive unevaluated, and non-symbol/non-string specs are ignored. A removed symbol's registered default options go with it, whereas Clear and ClearAll keep Options[s].

  • ClearAll and Remove both have attributes {HoldAll, Protected}. Both hold their arguments, so they operate on the symbol, not its current value.
  • Neither affects symbols with the attribute Protected. This is what prevents Remove/ClearAll from ever deleting or wiping a built-in.
  • ClearAll, unlike Clear, also removes attributes and the usage message.
  • Both return Null.

Attributes: HoldAll, Protected.

References

See also: ClearAll, Clear

Notes & additional examples

Notes

  • Remove[s] deletes the symbol s from the symbol table entirely. This is stronger than ClearAll, which empties a symbol but keeps its entry and its default options.
  • After removal the name no longer exists, and the next reference to it creates a fresh, undefined symbol. The result is Null.
  • Arguments may be symbols, strings, or a flat list of them.
  • A Protected symbol is skipped, so Remove never deletes a built-in. Attributes[Remove] is {HoldAll, Protected}.
  • See also: ClearAll, Clear, Names.