Why does append modify passed slice

by

Performance is the big reason. Creating a new slice and copying all the elements over to it is expensive, so the slice code doesn’t copy without good reason. However, if you exceed the slice’s capacity, it grows by a suitable amount by copying the underlying slice. That means that the slice that’s returned from append may not be the same slice you passed in.

The preferred way to use is:

args = append(args, newarg)

If you take a subslice, the capacity stays the same but your view into the slice changes. That means the missing elements are still there but outside the bounds of the new slice.

This explains the odd output of your code. You’re printing the result of append each time but not storing that result, which means args isn’t the same as what you printed.

The initial args slice is 3 elements big. For each index i – which is to say for 0, 1 and 2 – you take a subslice args[:i] and append all the elements of the remainder of the array args[i+1:] to it. That means that for:

i    args[:i]     args[i+1:]...   Result         args
0    {}           {"3", "8"}     {"3", "8"}     {"3", "8", "8"}
1    {"3"}        {"8"}          {"3", "8"}     {"3", "8", "8"}
2    {"3", "8"}   {}             {"3", "8"}     {"3", "8", "8"}

tl;dr you should always save the result of append, and if you want to make a copy so you can change it then make a copy yourself.

More Related Contents:

  1. Create a generic channel
  2. How to use a module that is outside of “GOPATH” in another module?
  3. slice of struct != slice of interface it implements?
  4. How can I convert string to integer in golang
  5. How to dump goroutine stacktraces?
  6. Does Go compiler’s evaluation differ for constant expression and other expression
  7. Cyclic dependencies and interfaces
  8. Accessing struct fields inside a map value (without copying)
  9. Why do I get a “cannot assign” error when setting value to a struct as a value in a map? [duplicate]
  10. Can I concurrently write different slice elements
  11. Organize local code in packages using Go modules
  12. Golang Dependency Management Best Practice
  13. How can I pass a slice as a variadic input?
  14. In Go, how do I capture stdout of a function into a string?
  15. How to get Type representation from name via reflection?
  16. Access a map value using a variable key in a Go template
  17. Golang embedded struct type
  18. Terminating function execution if a context is cancelled
  19. How can I access a struct field with generics (type T has no field or method)?
  20. Conversion of time.Duration type microseconds value to milliseconds
  21. How do I reverse a slice in go?
  22. How to delete an element from a Slice in Golang
  23. Does the Go language have function/method overloading?
  24. Go: panic: runtime error: invalid memory address or nil pointer dereference
  25. How do I compare two functions for pointer equality in the latest Go weekly?
  26. Go struct tags with underscore before function names
  27. Closing channel of unknown length
  28. Testing a gRPC service
  29. Convert an integer to a byte array
  30. Force priority of go select statement

Leave a Comment