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ReplaceAt
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ReplaceAt

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ReplaceAt[expr,rules,n]

transforms expr by replacing the n^(th) element using rules.

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ReplaceAt[expr,rules,{i,j,…}]

replaces the part of expr at position {i,j,…}.

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ReplaceAt[expr,rules,{{i1,j1,…},{i2,j2,…},…}]

replaces parts at several positions.

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ReplaceAt[rules,pos]

represents an operator form of ReplaceAt that can be applied to an expression.

Details

  • ReplaceAt is typically used to change one or more elements of an expression, based on supplied replacement rules.
  • ReplaceAt[expr,rules,{i,j,…}] replaces the part expr[[i,j,…]] using rules.
  • ReplaceAt[expr,rules,{{i1,j1,…},{i2,j2,…},…}] replaces the parts expr[[i1,j1,…]], expr[[i2,j2,…]], … using rules.
  • The rules must be of the form lhs->rhs or lhs:>rhs, or a list of such rules.
  • A list of rules can be given. The rules are tried in order. The result of the first one that applies is returned. If none of the rules apply, the original expr is returned.
  • ReplaceAt[expr,patt:>new,pos] can be used to replace a part without first evaluating it. With a rule such as pattnew, new is evaluated separately for each expression that matches patt.
  • ReplaceAt applies replacement rules repeatedly to a particular part if that part is mentioned more than once in the list of positions.
  • The list of positions used by ReplaceAt is in the same form as is returned by the function Position.
  • ReplaceAt works on Association objects, using the same specification for keys as in Part.
  • If expr is a SparseArray object or structured array, ReplaceAt[expr,…] replaces parts in the corresponding ordinary array.
  • ReplaceAt[rules,pos][expr] is equivalent to ReplaceAt[expr,rules,pos].

Examples

open allclose all

Basic Examples  (9)Summary of the most common use cases

Replace a specified part of an expression using a rule:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-ks0bns
Out[1]=1

Replace multiple parts:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-neqqel
Out[1]=1

Replace a nested part:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-kj9jwr
Out[1]=1

Use negative positions to count from the end:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-fy0y63
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bdpqbj1k-phyfi1
Out[2]=2

Use pattern-based replacements:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-kmz0zs
Out[1]=1

Use multiple replacement rules:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-edxmuc
Out[1]=1

Replace the second element of all top-level parts (the “second column”):

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-kdu5vi
Out[1]=1

Replace parts of an association specified by Key:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-n44xg1
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bdpqbj1k-c247vp
Out[2]=2

Use the operator form of ReplaceAt:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-ix2au6
Out[1]=1

Scope  (6)Survey of the scope of standard use cases

A single element of the expression:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-dzjfkj
Out[1]=1

Negative numbers count from the end:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-qqersb
Out[1]=1

Replace parts at level 2:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-gm35ye
Out[1]=1

Replace several parts:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-ovc7ve
Out[1]=1

Replace several nested parts:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-ct6zfg
Out[1]=1

Use Span syntax:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-om4yfw
Out[1]=1

Generalizations & Extensions  (4)Generalized and extended use cases

ReplaceAt can be used on expressions with any head:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-gnffva
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bdpqbj1k-guxgmv
Out[2]=2

Key and integer part specifications in associations are different:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-dahzrw
Out[1]=1

Use part 0 to perform replacements in the head of an expression:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-lhwza
Out[1]=1

ReplaceAt can be used with sparse arrays:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-lftjy1
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bdpqbj1k-gwzo3v
Out[2]=2

Properties & Relations  (7)Properties of the function, and connections to other functions

ReplaceAt can use lists of parts returned by Position:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-qk7d71
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bdpqbj1k-op3n7j
Out[2]=2
In[3]:=3
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https://wolfram.com/xid/0bdpqbj1k-ekeeo1
Out[3]=3

These are the subexpressions returned by Extract:

In[4]:=4
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https://wolfram.com/xid/0bdpqbj1k-qrvb5r
Out[4]=4

ReplaceAt can use an Association for replacement rules:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-ocuhdb
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bdpqbj1k-qaffcq
Out[2]=2

ReplaceAt can use a Dispatch object for replacement rules:

In[1]:=1
✖
https://wolfram.com/xid/0bdpqbj1k-ebvdjm
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bdpqbj1k-gvoxso
Out[2]=2

ReplaceAt applies replacement rules repeatedly if a part is mentioned repeatedly:

In[1]:=1
✖
https://wolfram.com/xid/0bdpqbj1k-m9j2bx
Out[1]=1

ReplaceAt will not evaluate held replacements:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-er5wdp
Out[1]=1

Manipulate code in an unevaluated association:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-yyo4r
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bdpqbj1k-ei4kd
Out[2]=2

Evaluate upon retrieval:

In[3]:=3
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https://wolfram.com/xid/0bdpqbj1k-qe3obz
Out[3]=3

Use Unevaluated to perform replacements prior to evaluating expr:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-gsg3ya
Out[1]=1

Possible Issues  (1)Common pitfalls and unexpected behavior

Repeated replacement parts may produce unexpected results if the structure of the expression is expected to change during replacements:

In[1]:=1
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https://wolfram.com/xid/0bdpqbj1k-djzrtn
Out[1]=1

Compare to using Nest:

In[2]:=2
✖
https://wolfram.com/xid/0bdpqbj1k-h1ldkc
Out[2]=2

This behavior is consistent with ReplacePart and MapAt:

In[3]:=3
✖
https://wolfram.com/xid/0bdpqbj1k-oy7w6i
Out[3]=3
In[4]:=4
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https://wolfram.com/xid/0bdpqbj1k-l3telm
Out[4]=4
Wolfram Research (2022), ReplaceAt, Wolfram Language function, https://reference.wolfram.com/language/ref/ReplaceAt.html.
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Wolfram Research (2022), ReplaceAt, Wolfram Language function, https://reference.wolfram.com/language/ref/ReplaceAt.html.

Text

Wolfram Research (2022), ReplaceAt, Wolfram Language function, https://reference.wolfram.com/language/ref/ReplaceAt.html.

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Wolfram Research (2022), ReplaceAt, Wolfram Language function, https://reference.wolfram.com/language/ref/ReplaceAt.html.

CMS

Wolfram Language. 2022. "ReplaceAt." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/ReplaceAt.html.

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Wolfram Language. 2022. "ReplaceAt." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/ReplaceAt.html.

APA

Wolfram Language. (2022). ReplaceAt. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/ReplaceAt.html

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Wolfram Language. (2022). ReplaceAt. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/ReplaceAt.html

BibTeX

@misc{reference.wolfram_2025_replaceat, author="Wolfram Research", title="{ReplaceAt}", year="2022", howpublished="\url{https://reference.wolfram.com/language/ref/ReplaceAt.html}", note=[Accessed: 09-July-2025 ]}

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@misc{reference.wolfram_2025_replaceat, author="Wolfram Research", title="{ReplaceAt}", year="2022", howpublished="\url{https://reference.wolfram.com/language/ref/ReplaceAt.html}", note=[Accessed: 09-July-2025 ]}

BibLaTeX

@online{reference.wolfram_2025_replaceat, organization={Wolfram Research}, title={ReplaceAt}, year={2022}, url={https://reference.wolfram.com/language/ref/ReplaceAt.html}, note=[Accessed: 09-July-2025 ]}

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@online{reference.wolfram_2025_replaceat, organization={Wolfram Research}, title={ReplaceAt}, year={2022}, url={https://reference.wolfram.com/language/ref/ReplaceAt.html}, note=[Accessed: 09-July-2025 ]}