This is documentation for Mathematica 3, which was
based on an earlier version of the Wolfram Language.
 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]:=