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ReplacePart
ReplacePart[
expr
,
new
,
n
] yields an expression in which the n
 part of expr is replaced by new.
ReplacePart[
expr
,
new
,
i
,
j
, ... 
] replaces the part at position 
i
,
j
, ...  .
ReplacePart[
expr
,
new
,
,
, ... 
,
,
, ... 
, ... 
] replaces parts at several positions by new.
ReplacePart[
expr
,
new
,
pos
,
npos
] replaces parts at positions pos in expr by parts at positions npos in new.
Example: ReplacePart[
a, b, c, d
, x, 3] .
The list of positions used by ReplacePart is in the same form as is returned by the function Position.
ReplacePart[
expr
,
Hold[
new
],
pos
,
1] can be used to replace a part without evaluating it.
If pos and npos both specify multiple parts, each part in pos is replaced by the corresponding part in npos.
See the Mathematica book: Section 1.8.6, Section 2.1.4.
See also: Part, Extract, MapAt, FlattenAt, Insert, Delete, Sequence, StringReplacePart.
Further Examples
This puts x into the third part of the expression.
In[1]:= 
Out[1]= 
A powerful application of ReplacePart is in the partial evaluation of a held expression. Here's a function that takes an expression and a list of positions, and evaluates in place the parts at the specified positions.
In[2]:= 
In this case, {1,2} specifies the second element of the held list, and {1,-1,1} the first part of the last element.
In[3]:= 
Out[3]= 
In[4]:= 
THIS IS DOCUMENTATION FOR AN OBSOLETE PRODUCT. SEE THE DOCUMENTATION CENTER FOR THE LATEST INFORMATION. | | | |
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