如何在Julia中重新计算向量的Eltype

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Let's say I have a vector v = Any[1,2,3,4]

And I would like to recompute its eltype in such a way that typeof(v) = Vector{Int}

Is it possible to accomplish this without having to manually concatenate each of the elements in v?

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oqui
oqui

You can't "retype" the existing v, just create a copy of it with the more concrete type1.

转换次数

假设您已经（静态地）知道结果类型，则有多种选择。最易读的（和IMO，惯用的）是

Vector{Int}(v)

我认为这相当于

convert(Vector{Int}, v)

或者：

convert.(Int, v)

转换成什么

If you don't know what the "common type" would be, there are multiple options how to get one that matches. In general, typejoin can be used to find a least upper bound:

mapreduce(typeof, typejoin, v)

The result will most likely be abstract, e.g. Real for an array of Ints and Float64s. So, for numeric types, you might be better off with promote_type:

mapreduce(typeof, promote_type, v2)

This at least gives you Float64 for mixed Ints and Float64s.

但是，实际上并不建议所有这些方法，因为它可能很脆弱，令人惊讶，并且类型不稳定。

1For certain combinations of types, reinterpret will work and return a view with a different type, but this is only possible for bits types, which Any is not. For converting Any[1,2,3] to Int[1,2,3] copying is fundamentally necessary because the two arrays have different layouts in memory: the former is an array of pointers to individually allocated integers objects, whereas the latter stores the Int values inline in contiguous memory.

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