This is documentation for Mathematica 3, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.1)
 Documentation / Mathematica / Built-in Functions / Programming / Expression Structure  /
Part

  • expr [[ i ]] or Part[ expr , i ] gives the i part of expr.
  • expr [[- i ]] counts from the end.
  • expr [[0]] gives the head of expr.
  • expr [[ i , j , ... ]] or Part[ expr , i , j , ... ] is equivalent to expr [[ i ]] [[ j ]] ... .
  • expr [[ , , ... ]] gives a list of the parts , , ... of expr.
  • You can make an assignment like t [[ i ]] = value to modify part of an expression.
  • When expr is a list, expr [[ , , ... ]] gives a list of parts. In general, the head of expr is applied to the list of parts.
  • You can get a nested listing of parts from expr [[ , , ... ]]. Each part has one index from each list.
  • Notice that lists are used differently in Part than in functions like Extract, MapAt and Position.
  • expr [[ Range[ i , j ] ]] can be used to extract sequences of parts.
  • In StandardForm and InputForm, expr [[ spec ]] can be input as exprspec.
  • and can be entered as [[ and ]] or \[LeftDoubleBracket] and \[RightDoubleBracket].
  • See the Mathematica book: Section 1.2.4, Section 1.2.4, Section 1.4.6, Section 1.8.4, Section 1.8.4Section 2.1.4.
  • See also: First, Head, Last, Extract, Position, ReplacePart, MapAt, Take.
  • Related package: LinearAlgebra`MatrixManipulation`.

    Further Examples

    This extracts the third element from the list.

    In[1]:=

    Out[1]=

    This extracts a list of elements.

    In[2]:=

    Out[2]=

    The second input resets the third element of the list.

    In[3]:=

    Out[3]=

    In[4]:=

    Out[4]=

    In[5]:=

    Out[5]=

    In[6]:=

    This picks out parts 2 and 4 of the sum, leaving their sum.

    In[7]:=

    Out[7]=