Go Slices: What Determines a Slice's Length and Capacity?

Understanding Slice Capacity and Length in Go

In Go, slices are an essential data structure for working with sequences of values. While the tutorial provides an introduction to slices, it can sometimes leave beginners with questions about their capacity and length.

Case Study: Investigating Slices

Consider the following code snippet:

func main() {
  a := make([]int, 5)
  b := make([]int, 0, 5)
  c := b[:2]
  d := c[2:5]
}

Question 1: Why does c appear as [0,0] with length 2?

c is a slice of b, which initially contains zeroed values. When you slice b with [:2], you create a new slice c that shares the same backing array as b. This means that the first two elements of c are identical to the first two elements of b, which are both zeroed out.

Question 2: Why is the capacity of d 3?

Slicing a slice also shares its backing array. When you slice c with [2:5], you create a new slice d that shares the backing array of c. Since c has two elements, its backing array cannot contain more than two elements. Therefore, the capacity of d is limited to the remaining space in the backing array, which is 5-2 = 3.

In-depth Explanation of Slices

In Go, slices have three important properties: length, capacity, and pointer. Length represents the number of elements in the slice, while capacity indicates the maximum number of elements that the backing array can hold. The pointer points to the first element in the backing array.

When you create a slice using make, you specify the length and capacity. If the capacity is greater than the length, the slice can grow without reallocating a new backing array. When you slice another slice, the resulting slice shares the same backing array and pointer as the original slice, which influences its capacity and length.

Conclusion

Understanding the interplay between slice length, capacity, and pointer is crucial for mastering slices in Go. By leveraging slice slices and manipulating capacities, you can optimize your code for performance and memory usage.

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