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Fix doc. about linear from/to Cartesian indices #49343

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Apr 13, 2023
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Fix doc. about linear from/to Cartesian indices #49343

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4 changes: 2 additions & 2 deletions base/indices.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,7 @@ to implement indexing (and indexed assignment) with a single `Int` index;
all other indexing expressions — including multidimensional accesses — will
be recomputed to the linear index. For example, if `A` were a `2×3` custom
matrix with linear indexing, and we referenced `A[1, 3]`, this would be
recomputed to the equivalent linear index and call `A[5]` since `2*1 + 3 = 5`.
recomputed to the equivalent linear index and call `A[5]` since `1 + 2*(3 - 1) = 5`.

See also [`IndexCartesian`](@ref).
"""
Expand All @@ -53,7 +53,7 @@ to implement indexing (and indexed assignment) with exactly `N` `Int` indices;
all other indexing expressions — including linear indexing — will
be recomputed to the equivalent Cartesian location. For example, if `A` were a `2×3` custom
matrix with cartesian indexing, and we referenced `A[5]`, this would be
recomputed to the equivalent Cartesian index and call `A[1, 3]` since `5 = 2*1 + 3`.
recomputed to the equivalent Cartesian index and call `A[1, 3]` since `5 = 1 + 2*(3 - 1)`.

It is significantly more expensive to compute Cartesian indices from a linear index than it is
to go the other way. The former operation requires division — a very costly operation — whereas
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