• ← PrependTo
• → Rest

ReplacePart

WMA link

ReplacePart[expr, i -> new]

replaces part i in expr with new.

ReplacePart[expr, {{i, j} -> $e_1$, {k, l} -> $e_2$}]

replaces parts i and j with $e_1$, and parts k and l with $e_2$.

• • ReplacePart[{a, b, c}, 1 -> t]

• • ReplacePart[{{a, b}, {c, d}}, {2, 1} -> t]

• • ReplacePart[{{a, b}, {c, d}}, {{2, 1} -> t, {1, 1} -> t}]

• • ReplacePart[{a, b, c}, {{1}, {2}} -> t]

Delayed rules are evaluated once for each replacement:

• • n = 1;

• • ReplacePart[{a, b, c, d}, {{1}, {3}} :> n++]

Non-existing parts are simply ignored:

• • ReplacePart[{a, b, c}, 4 -> t]

You can replace heads by replacing part 0:

• • ReplacePart[{a, b, c}, 0 -> Times]

(This is equivalent to Apply.)

Negative part numbers count from the end:

• • ReplacePart[{a, b, c}, -1 -> t]