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 1.8.4, Section 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]=






