ReplaceAt¶

Applies transformation rules to the parts of an expression at the given position(s). The position is specified the same way as Position output, and each targeted part is transformed with the first matching rule (parts that match no rule are left unchanged).

Replace a single part:

$ wo 'ReplaceAt[{a, a, a, a}, a -> xx, 2]'
{a, xx, a, a}

Replace several parts at once:

$ wo 'ReplaceAt[{a, a, a, a}, a -> xx, {{1}, {4}}]'
{xx, a, a, xx}

Address a nested part:

$ wo 'ReplaceAt[{{a, a}, {a, a}}, a -> xx, {2, 1}]'
{{a, a}, {xx, a}}

Delayed rules evaluate per match:

$ wo 'ReplaceAt[{1, 2, 3, 4}, x_ :> 2 x - 1, {{2}, {4}}]'
{1, 3, 3, 7}

A list of rules uses the first matching rule at each position:

$ wo 'ReplaceAt[{a, b, c, d}, {a -> xx, _ -> yy}, {{1}, {2}, {4}}]'
{xx, yy, c, yy}

The operator form ReplaceAt[rules, pos] is a function:

$ wo 'ReplaceAt[a -> xx, 2][{a, a, a, a}]'
{a, xx, a, a}