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I can do this:
let a: [f32; 3] = [0.0, 1.0, 2.0];
But why doesn't this work?
let a: [f32; _] = [0.0, 1.0, 2.0];
It seems to me that the length is redundant and trivial to infer. Is there a way to avoid having to specify it explicitly? (And without having to append f32 to all the literals.)
523
The length of an array is the number of elements present in the array. We use the len() function to obtain this value.
Arrays are collections of same-type-data values stored in contiguous memory locations. In Rust, arrays are created using square brackets [] and their size needs to be known at compile time. An array whose size is not defined is called a slice.
As far as I know all arrays are copied on move.
The below-given surtaxes can be used to declare and initialize an array in Rust: Syntax1: let variable_name = [value_1,value_2,value_3]; Syntax2: let variable_name:[dataType;array_size] = [value_1,value_2,value_3]; Syntax3: let variable_name:[dataType;array_size] = [default_value_for_elements,array_size];
_ can only be used in two contexts: in patterns, to match a value to ignore, and as a placeholder for a type. In array types, the length is not a type, but an expression, and _ cannot be used in expressions.
What you can do, though, is append f32 to only one of the literals and omit the type completely. Since all the items of an array must have the same type, the compiler will infer the correct element type for the array.
let a = [0.0f32, 1.0, 2.0];
Since 1.39 it's possible using a simple macro
macro_rules! arr {
($id: ident $name: ident: [$ty: ty; _] = $value: expr) => {
$id $name: [$ty; $value.len()] = $value;
}
}
Usage
arr!(static BYTES: [u8; _] = *b"foo");
arr!(let floats: [f32; _] = [0., 1.]);
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