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FlattenAt[list,n]

flattens out a sublist that appears as the n th element of list. If n is negative, the position is counted from the end.

FlattenAt[expr,{i,j,…}]

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

FlattenAt[expr,{{i1,j1,…},{i2,j2,…},…}]

flattens out parts of expr at several positions.

FlattenAt[pos]

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

Details
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Examples  
Basic Examples  
Scope  
Properties & Relations  
Possible Issues  
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FlattenAt

FlattenAt[list,n]

flattens out a sublist that appears as the n th element of list. If n is negative, the position is counted from the end.

FlattenAt[expr,{i,j,…}]

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

FlattenAt[expr,{{i1,j1,…},{i2,j2,…},…}]

flattens out parts of expr at several positions.

FlattenAt[pos]

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

Details

Examples

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Basic Examples  (2)

Flatten at position 2:

Wolfram Language code: FlattenAt[{a, {b, c}, {d, e}, {f}}, 2]

Flatten at positions 2 and 4:

Wolfram Language code: FlattenAt[{a, {b, c}, {d, e}, {f}}, {{2}, {4}}]

Scope  (3)

FlattenAt can splice the arguments of any expression, not just lists:

Wolfram Language code: FlattenAt[f[g[1, 2], g[3, 4], g[5]], {2}]

Consider a matrix:

Wolfram Language code: mat = Array[a, {2, 2}]

Flatten the first element (row) of the matrix:

Wolfram Language code: FlattenAt[mat, 1]

Equivalently:

Wolfram Language code: FlattenAt[mat, {1}]

Flatten at position 1,2 (the element in the first row, second column):

Wolfram Language code: FlattenAt[mat, {1, 2}]

Flatten at positions 1 and 2 (the rows) of the matrix:

Wolfram Language code: FlattenAt[mat, {{1}, {2}}]

Create an operator form of FlattenAt:

Wolfram Language code: op = FlattenAt[1]

Apply the operator to different expressions:

Wolfram Language code: op[{{1, 2}, {3, 4}}]
Wolfram Language code: op[{f[1, 2], g[3, 4]}]

Properties & Relations  (2)

For lists, FlattenAt[list,pos] is equivalent to MapAt[Splice,list,pos]:

Wolfram Language code: FlattenAt[{a, {b, {c}}, d, {e}}, {{2, 2}, {4}}]
Wolfram Language code: MapAt[Splice, {a, {b, {c}}, d, {e}}, {{2, 2}, {4}}]

For a general expression, FlattenAt[expr,pos] is equivalent to MapAt[Delete[0],expr,pos]:

Wolfram Language code: FlattenAt[f[a, g[b, c], d, {e}], {{2}, {4}}]
Wolfram Language code: MapAt[Delete[0], f[a, g[b, c], d, {e}], {{2}, {4}}]

Possible Issues  (1)

FlattenAt removes the head of the expression at the specified position:

Wolfram Language code: FlattenAt[{1, {{2}, {3}}, 4}, {2}]

Use MapAt with Flatten to remove inner heads from a specified position:

Wolfram Language code: MapAt[Flatten, {1, {{2}, {3}}, 4}, {2}]
Wolfram Research (1991), FlattenAt, Wolfram Language function, https://reference.wolfram.com/language/ref/FlattenAt.html.

Text

Wolfram Research (1991), FlattenAt, Wolfram Language function, https://reference.wolfram.com/language/ref/FlattenAt.html.

CMS

Wolfram Language. 1991. "FlattenAt." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/FlattenAt.html.

APA

Wolfram Language. (1991). FlattenAt. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/FlattenAt.html

BibTeX

@misc{reference.wolfram_2026_flattenat, author="Wolfram Research", title="{FlattenAt}", year="1991", howpublished="\url{https://reference.wolfram.com/language/ref/FlattenAt.html}", note=[Accessed: 13-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_flattenat, organization={Wolfram Research}, title={FlattenAt}, year={1991}, url={https://reference.wolfram.com/language/ref/FlattenAt.html}, note=[Accessed: 13-June-2026]}