Go Slice Expressions

Go allows operation on slices to extract part of its elements using a beautiful syntax. Go calls it Slice Expressions.

var s []Type
s[low:high]

Here is how the official Go langange specs define Slice Expressions.

For a string, array, pointer to array, or slice a, the primary expression a[low : high] constructs a substring or slice. The indices low and high select which elements of operand a appear in the result. The result has indices starting at 0 and length equal to high - low.

The way I like to think of it is as an exclusive range represented by this formula

a[low, high-1]

Lets see some examples.

Examples

func main() {
  s := []int{0,1,2,3,4,5,6,7,8,9}
  
fmt.Println("slice s =", s)
fmt.Println("s[2:6] ",s[2:6])
fmt.Println("s[2:] ", s[2:])
fmt.Println("s[:4] ", s[:4])
fmt.Println("s[:] ", s[:])

Outputs

slice s =[0 1 2 3 4 5 6 7 8 9]
s[2:6] [2,3,4,5]
s[2:] [2,3,4,5,6,7,8,9]
s[:4] [0,1,2,3]
s[:] [0,1,2,3,4,5,6,7,8,9]

s[2:6] means, extract elements of s starting from index 2 upto index 6 exclusive. s[2:] means , extract elements of s starting from index 2 till the end. s[:4] means, extract elements of s starting from index 0 (because no low index is given) upto index 4 exclusive. s[:] means, extract all elements s starting from index 0 till the end.

Important Notes

The slices created by this operation are only a view over original slice. This little detail got me in the past, where changes from the extracted slice reflect in the original slice.To avoid this, you can make use of copy function to reset internal array pointer. Remember slices are just a reference to an array object underneath.

copy(extract,s[2:]) // now `extract` is a different slice
fmt.Println(&s[2] == &extract[0] // false

Thinking of these slice expressions as an exclusive range can be confusing sometimes. Another way is to remember the following formula

For example, s2 := s[1:3] creates a new slice with length 3 - 1 = 2, so it will contain 2 elements: s[1] and s[2]

I got that trick from this SO answer.

While other langanges use functions to realize the same functionality on collections, Go makes it easy and fun by providing a builtin langange construct to accomplish these things.