Go Slices Length and Capacity

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Slices have both length and capacity. The length of a slice is the number of elements currently in the slice, while the capacity is the number of elements the slice can hold before needing to be reallocated.

When creating a slice using the built-in make() function, you can specify its length, and optionally its capacity. If the capacity is not explicitly specified, it will be the specified length.

var s = make([]int, 3, 5) // length 3, capacity 5

You can check the length of a slice with the built-in len() function:

var n = len(s) // n == 3

You can check the capacity with the built-in cap() function:

var c = cap(s) // c == 5

Elements created by make() are set to the zero value for the element type of the slice:

for idx, val := range s {
    fmt.Println(idx, val)
// output:
// 0 0
// 1 0
// 2 0

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You cannot access elements beyond the length of a slice, even if the index is within capacity:

var x = s[3] // panic: runtime error: index out of range

However, as long as the capacity exceeds the length, you can append new elements without reallocating:

var t = []int{3, 4}
s = append(s, t) // s is now []int{0, 0, 0, 3, 4}
n = len(s) // n == 5
c = cap(s) // c == 5

If you append to a slice which lacks the capacity to accept the new elements, the underlying array will be reallocated for you with sufficient capacity:

var u = []int{5, 6}
s = append(s, u) // s is now []int{0, 0, 0, 3, 4, 5, 6}
n = len(s) // n == 7
c = cap(s) // c > 5

It is, therefore, generally good practice to allocate sufficient capacity when first creating a slice, if you know how much space you'll need, to avoid unnecessary reallocations.

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