How to initialize a slice with both initial value and capacity?

php editor Baicao will introduce how to use both the initial value and capacity during slice initialization to improve the efficiency of the code. In PHP, a slice is a dynamic array that can grow or shrink dynamically as needed. Usually, when initializing a slice, we can specify an initial value, but we cannot directly specify the capacity. However, there is a trick to initialize a slice using both initial value and capacity, let’s take a look!

Question content

Part of the code is as follows:

const estimatedsize = 10000
var init_arr = [...]int{1,1,2,3,5,8,13}
var fibbo []int = make([]int, len(init_arr), estimatedsize)    
}

SlicefibboThere is no reserved function in c like vector to reserve estimatedsize for the slice to reduce the problem when expanding the capacity Copy time. So I can only assign estimatedsize to capacity at initialization time. But neither of the following is legal.

var fibbo []int = init_arr[::estimatedsize]
var fibbo []int = make([]int{1,1,2,3,5,8,13}, len(init_arr), estimatedsize)

Is there any solution to this problem? Or maybe it's clumsy code and the following is the only way?

var fibbo []int = make([]int, len(init_arr), ESTIMATEDSIZE) 
for i:= range init_arr {
    fibbo[i] = init_arr[i]
}

Solution

No, this is not possible. This is part of the go design philosophy that guides this design decision: "Don't hide complexity."

Your "clumsy" code is actually the preferred and idiomatic way of doing this in go, with one small change:

var fibbo = make([]int, len(init_arr), estimatedsize) 
for i := range init_arr {
    fibbo[i] = init_arr[i]
}

The only change I made (besides the format) was to remove the type from the var declaration, as that was completely redundant. in other words:

-var fibbo []int = make([]int, len(init_arr), estimatedsize) 
+var fibbo = make([]int, len(init_arr), estimatedsize)

As pointed out in the comments, if you don't make any other changes to the input, you can also use the built-in copy to eliminate the for loop:

var fibbo = make([]int, len(init_arr), ESTIMATEDSIZE)
copy(fibbo, init_arr)

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