Is Go\'s `len()` Function Always O(1) for Strings and Slices?

Understanding the O(1) Complexity of len() for Strings and Slices in Go

The len() function is widely used in Go to determine the length of sequences such as strings and slices. This functionality raises the question: does len() operate in constant time (O(1)) for both types?

Understanding Slice Length

Slices are essentially views into underlying arrays. Each slice header contains three fields: pointer to the array, length, and capacity. The length field indicates the number of elements in the slice. Retrieving this value involves a simple field lookup, making len() an O(1) operation.

String Internals and Length

Strings in Go are immutable sequences of UTF-8-encoded bytes. The string header consists of a pointer to the string data (array of bytes) and a length. This structure allows len() to retrieve the length field directly, resulting in an O(1) operation.

Builtin.go Context

The builtin.go file mentioned in the question provides documentation for Go's predeclared identifiers. However, it does not contain the actual implementations of these functions. Instead, it provides descriptions allowing godoc (Go documentation tool) to present documentation for special identifiers in the language.

Conclusion

Both strings and slices in Go have O(1) complexity for len() operations. Strings, while seemingly complex, benefit from an internal structure that provides direct access to the length field, contributing to its constant-time efficiency.

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