Steve,
`@inbounds` is certainly tricky because of the lack of documentation, which
I think is slightly on purpose as this is meant to be for more advanced
usage.
The main insight to using `@inbounds` correctly is realizing that
`@inbounds expression` returns the value `nothing`. That's why your first
two examples don't seem to work. The value is indeed being calculated with
bounds checking off, but you're not assigning the value anywhere, so
`nothing` is the result of the expression. You can also do multi-line
turning off of bounds checking by using a `begin...end` block.
Try the following:
function sqrtfirst{T}(a::Array{T, 1})
@assert(size(a,1) >= 1)
@inbounds ans = sqrt(a[1])
return ans
end
function sqrtfirst{T}(a::Array{T, 1})
@assert(size(a,1) >= 1)
@inbounds begin
# do several getindex, setindex! operations
end
return ans
end
Hope that helps!
-Jacob
On Mon, Aug 4, 2014 at 1:45 PM, <[email protected]> wrote:
> Dear Julia users,
>
> The usage of the @inbounds macro is not explained the manual, and its
> syntax appears to be strange. Consider the three functions at the end of
> this posting. Only the third one works -- why?
>
> In general, I think @inbounds is broken. Besides the weird syntax, it has
> two other issues. First, there is no way to apply the macro to one
> subscript operation but not another in a long expression (as far as I
> know). Second, it is not extensible in the sense that if programmer A
> implements his/her own array-like structure with his/her own getindex and
> setindex operations, he/she might like to have two versions of
> getindex/setindex, one safe/slower and the other unsafe/faster, but there
> is no way for programmer A to detect whether user B, a user of his/her new
> array-like structure, has requested @inbounds or not.
>
> I would like to propose the following replacement for @inbounds, which
> solves all three problems. Instead of a macro, there should be two
> different subscript operations, say a[1] and a[$ 1 $], where the first is
> safe/slow and the second is unsafe/fast. The compiler will compile the
> first as getindex/setindex and the second as getindexUnsafe/setindexUnsafe.
>
> -- Steve Vavasis
>
>
>
> function sqrtfirst{T}(a::Array{T, 1})
> @assert(size(a,1) >= 1)
> @inbounds sqrt(a[1])
> end
>
> function sqrtfirst{T}(a::Array{T, 1})
> @assert(size(a,1) >= 1)
> return @inbounds sqrt(a[1])
> end
>
> function sqrtfirst{T}(a::Array{T, 1})
> @assert(size(a,1) >= 1)
> @inbounds return sqrt(a[1])
> end
>
>
>
>
