Trying to create a slice in which the type is set dynamicaly based on a pointer to a specific type, so i made the following sample
func main() {
var chicken *Chicken
//create a slice of chickens
chickens:=GetaDynamiclyTypedSlice(chicken)
//this throws cannot range over chickens (type *[]interface {}) and i cant figure how to create a slice using my above chicken pointer
for _,chicken := range chickens{
fmt.Println(chicken)
}
}
type Chicken struct{
Weight float64
}
func GetaDynamiclyTypedSlice(ptrItemType interface{})*[]interface {}{
var collection []interface{}
itemtyp := reflect.TypeOf(ptrItemType).Elem()
for i:=0;i<1000;i++{
//create an item of the wanted type
item := reflect.New(itemtyp)
//set a random float to the weight value
item.Elem().FieldByName("Weight").SetFloat(rnd.ExpFloat64())
collection = append(collection,&item)
}
return &collection
}
what should i do to be able to use range on the returned slice?
how can i use the itemtyp as the type of my slice?
There are few problems with your code.
You're returning a pointer to a reflect.Value, 99% sure that's not what you're trying to achive.
You're not dereferencing the slice like Simon mentioned.
Slices are pointer types, if you're returning *[]interface{} for performance reasons, you're actually hurting not helping.
So let's rewrite the code and optimize it! (it's late night SO, time to party):
// pass the size to preallocate the slice, also return the correct slice type.
func GetaDynamiclyTypedSlice(ptrItemType interface{}, size int) (col []interface{}) {
col = make([]interface{}, size)
itemtyp := reflect.TypeOf(ptrItemType).Elem()
for i := range col { //prettier than for i := 0; etc etc
item := reflect.New(itemtyp)
item.Elem().FieldByName("Weight").SetFloat(rand.ExpFloat64())
col[i] = item.Interface() //this is the magic word, return the actual item, not reflect.Value
}
return
}
playground
You just need to dereference the pointer (so you're not iterating over a pointer - you're iterating over a slice):
for _, chicken := range *chickens {
// ...
}
Playground link: http://play.golang.org/p/NBv9sooqEV
Related
I have a slice of struct []student, and I want to modify its content with function.
type student struct {
name string
age int
}
students := []student{
{"Doraemon", 30},
{"King Kong", 25},
}
Thus, I decided to pass it as a pointer. May I know how to pass the slice as a reference to a function?
func addAge (s *[]student) error { //this code has error
//everyone add 2 years old
for i, e := range *s {
s[i].age = s[i].age + 2
}
//make the first student much older
s[0].age = s[0].age + 5
return nil
}
I keep playing with Go Playground, but it gives many complains, such as
cannot range over s (type *[]student)
invalid operation: s[i] (type *[]student does not support indexing)
invalid indirect of s
...
How to precisely pass the reference of a slice of struct to a function? How to range the slice of struct? And how to change the value of the struct (modify the same struct in THE slice)?
I keep getting error while playing with s *[]student, range *s, s []student, s *[]*student ... so hard to get it correct...
sorry for my NEWBIE question, still learning GO... trying hard
Slices are passed by reference, so as long as you are modifying the existing slice content you should not explicitly pass a pointer.
package main
import (
"fmt"
)
type student struct {
name string
age int
}
func main() {
students := []student{
{"Doraemon", 30},
{"King Kong", 25},
}
err := addAge (students)
fmt.Println(students)
if err != nil {
fmt.Println("error")
}
}
func addAge (s []student) error {
for i, _ := range s {
s[i].age = 3
}
return nil
}
Now, for your addAdditinalStudent function you should actually use the append function. Plus, have in mind
..., since the slice header is always updated by a call to
append, you need to save the returned slice after the call. In fact,
the compiler won't let you call append without saving the result.
Slices#append
// add student
students = append(students, student{"Test", 33})
Go Playground
in Go you can pass items by value ([]student) or by reference ([]*student). When you want to operate on the values of a struct{} you should pass it to a function with its reference (the pointer).
So you can do something like this:
type student struct {
name string
age int
}
func addTwoYearsToAll(students []*student){
for _, s := range students {
s.age += 2
}
}
This way you're working with the same exact items you build when appending to the slice. Playground example.
Also take a look at Are Golang function parameter passed as copy-on-write?
I have written the following:
func main() {
//inside main
fileInputBytes, err := ioutil.ReadFile("/tmp/test")
byteSize2 := len(fileInputBytes)
var inputFileByteSlice = fileInputBytes[0:]
var numberOfIndexes = math.Floor(float64(byteSize / indexingOffset))
for i := 1; i <= int(numberOfIndexes); i++ {
// adding i to the indexer insures that we use lookahed to ignore previously inserted indexing values
var v int = (i * indexingOffset) + i
Insert(&inputFileByteSlice, v+i, indexingByteValue)
fmt.Println(i)
}
}
//outside main
//variation of https://blog.golang.org/slices with pointers and such
func Insert(slice *[]byte, index int, value byte) {
// Grow the slice by one element.
(*slice) = (*slice)[0 : len(*slice)+1]
// Use copy to move the upper part of the slice out of the way and open a hole.
copy((*slice)[index+1:], (*slice)[index:])
// Store the new value.
(*slice)[index] = value
// Return the result.
}
The slice bounds out of range error is getting on my nerves. The length of the slice grows outside of the size and overflows, the reason I don't understand is that I thought the call to 'grow' the slice by one element(before copy) will dynamically allocate more space. Since that is not the case, can anyone offer me a better suggestion?
First of all, a slice is already a reference type. So you don't need to pass its pointer around if you are not going to change its capacity. So your main can be simplified as:
func main() {
fileInputBytes, err := ioutil.ReadFile("/tmp/test")
byteSize2 := len(fileInputBytes)
// No need to use pointer to slice. If you want a brand new slice
// that does not affect the original slice values, use copy()
inputFileByteArray := fileInputBytes
var numberOfIndexes = math.Floor(float64(byteSize / indexingOffset))
for i := 1; i <= int(numberOfIndexes); i++ {
var v int = (i * indexingOffset) + i
// Insert needs to return the newly updated slice reference
// which should be assigned in each iteration.
inputFileByteArray = Insert(inputFileByteArray, v+i, indexingByteValue)
fmt.Println(i)
}
}
Then, the Insert function can be simplified simply by using append along with copy and returning the newly created slice:
func Insert(slice []byte, index int, value byte) []byte {
if index >= len(slice) {
// add to the end of slice in case of index >= len(slice)
return append(slice, value)
}
tmp := make([]byte, len(slice[:index + 1]))
copy(tmp, slice[:index])
tmp[index] = value
return append(tmp, slice[index:]...)
}
This may not be the best implementation but it is simple enough. Example usage at: https://play.golang.org/p/Nuq4RX9XQD
Your function only works if the slice happens to have enough initial capacity. If you need more capacity, you can only "grow" the slice using the append function. You can still use the *[]byte pointer argument to modify the slice in place like so:
func Insert(slice *[]byte, index int, value byte) {
*slice = append(*slice, 0)
copy((*slice)[index+1:], (*slice)[index:])
(*slice)[index] = value
}
However, it's more customary to return a new slice value, and reassign it each time. This gives you a similar function signature to the builtin append.
func Insert(slice []byte, index int, value byte) []byte {
slice = append(slice, 0)
copy(slice[index+1:], slice[index:])
slice[index] = value
return slice
}
I am trying to create a generic function that can handle actions on slices in Go... for instance, append an item of any type to a slice of that same type. This is simply a generic purpose for a more complex solution, but overall the issue boils down to this example:
package main
type car struct {
make string
color string
}
type submarine struct {
name string
length int
}
func genericAppender(thingList interface{}, thing interface{}) []interface{} {
return append(thingList, thing)
}
func main() {
cars := make([]car, 0, 10)
cars[0] = car{make: "ford", color: "red"}
cars[1] = car{make: "chevy", color: "blue"}
subs := make([]submarine, 0, 10)
subs[0] = submarine{name: "sally", length: 100}
subs[1] = submarine{name: "matilda", length: 200}
newCar := car{make: "bmw", color: "white"}
genericAppender(&cars, newCar)
}
The code playground is at this location
The above errors as follows:
prog.go:14: first argument to append must be slice; have interface {}
After this change you're still getting a runtime error (index out of range) however the problem is that thingList is not of type []interface{} but rather interface{} so you can't append to it. Here's an updated version of your code on playground that does a type assertion to convert it to an []interface{} in line with the append. In reality you need to do that on a separate line and check for errors.
https://play.golang.org/p/YMed0VDZrv
So to put some code here;
func genericAppender(thingList interface{}, thing interface{}) []interface{} {
return append(thingList.([]interface{}), thing)
}
will solve the basic problem you're facing. As noted, you still get runtime errors when indexing into the slice. Also, you could change the argument to avoid this by making it;
func genericAppender(thingList []interface{}, thing interface{}) []interface{} {
return append(thingList, thing)
}
Here's a complete example of the second type; https://play.golang.org/p/dIuW_UG7XY
Note I also corrected the runtime error. When you use make with 3 args they are, in this order, type, length, capacity. This means the length of the array is 0 so when you try to assign to indexes 0 and 1 it was causing a panic for IndexOutoFRange. Instead I removed the middle argument so it's make([]interface{}, 10) meaning the length is initially set to 10 so you can assign to those indexes.
In the answer above if you do the following then it throws error. This is what the original question was about:
//genericAppender(subs, newCar). // Throws "cannot use subs (type []submarine) as type []interface {} in argument to genericAppender"
The trick is to convert your slice of specific type into a generic []interface{}.
func convertToGeneric(thingList interface{}) []interface{} {
input := reflect.ValueOf(thingList)
length := input.Len()
out := make([]interface{},length)
for i:=0 ;i < length; i++ {
out[i] = input.Index(i).Interface()
}
return out
}
This you can call the function like this:
genericAppender(convertToGeneric(subs), newCar)
You can check modified working code here: https://play.golang.org/p/0_Zmme3c8lT
With Go 1.19 (Q4 2022), no need for interface, or "convert your slice of specific type into a generic []interface{}"
CL 363434 comes with a new slices packages:
// Package slices defines various functions useful with slices of any type.
// Unless otherwise specified, these functions all apply to the elements
// of a slice at index 0 <= i < len(s).
package slices
import "constraints"
// Grow increases the slice's capacity, if necessary, to guarantee space for
// another n elements. After Grow(n), at least n elements can be appended
// to the slice without another allocation. If n is negative or too large to
// allocate the memory, Grow panics.
func Grow[S ~[]T, T any](s S, n int) S {
return append(s, make(S, n)...)[:len(s)]
}
// Equal reports whether two slices are equal: the same length and all
// elements equal. If the lengths are different, Equal returns false.
// Otherwise, the elements are compared in index order, and the
// comparison stops at the first unequal pair.
// Floating point NaNs are not considered equal.
func Equal[T comparable](s1, s2 []T) bool {
if len(s1) != len(s2) {
return false
}
for i, v1 := range s1 {
v2 := s2[i]
if v1 != v2 {
return false
}
}
return true
}
// ...
Ian Lance Taylor confirms in issue 45955:
This package is now available at golang.org/x/exp/slices.
Per this thread, it will not be put into standard library until the 1.19 release.
We may of course adjust it based on anything we learn about having it in x/exp.
I have custom types Int64Array, Channel and ChannelList like:
type Int64Array []int64
func (ia *Int64Array) Scan(src interface{}) error {
rawArray := string(src.([]byte))
if rawArray == "{}" {
*ia = []int64{}
} else {
matches := pgArrayPat.FindStringSubmatch(rawArray)
if len(matches) > 1 {
for _, item := range strings.Split(matches[1], ",") {
i, _ := strconv.ParseInt(item, 10, 64)
*ia = append(*ia, i)
}
}
}
return nil
}
func (ia Int64Array) Value() (driver.Value, error) {
var items []string
for _, item := range ia {
items = append(items, strconv.FormatInt(int64(item), 10))
}
return fmt.Sprintf("{%s}", strings.Join(items, ",")), nil
}
type Channel int64
type ChannelList []Channel
How can I embed Int64Array to ChannelList such that I can call Scan and Value methods on it? I tried the following:
type ChannelList []Channel {
Int64Array
}
but I'm getting syntax error. What's important is to make sure ChannelList items are of type Channel, if this isn't possible via embedding I might just create stand-alone functions to be called by both ChannelList and Int64Array.
An anonymous (or embedded field) is found in a struct (see struct type), not in a type alias (or "type declaration").
You cannot embed a type declaration within another type declaration.
Plus, as illustrated by the answers to "Go: using a pointer to array", you shouldn't be using pointers to slice, use directly the slice themselves (passed by value).
Wessie kindly points out in the comments that (ia *Int64Array) Scan() uses pointer to a slice in order to mutate the underlying array referenced by said slice.
I would prefer returning another slice instead of mutating the existing one.
That being said, the Golang Code Review does mention:
If the receiver is a struct, array or slice and any of its elements is a pointer to something that might be mutating, prefer a pointer receiver, as it will make the intention more clear to the reader.
I keep getting this error when trying to range over a slice pointer.
app/domain/repositories/class_repository.go:24: cannot range over classes (type *[]entities.Class)
What am I doing wrong?
Here is the struct:
package repositories
import (
"mobifit/app/domain/entities"
)
type ClassRepository struct {
*Repository
}
func (c *ClassRepository) ClassesForLastNDays(days int) *[]entities.Class {
classes := new([]entities.Class)
query := Select("*").
From("Class").
Where("VisibleAt > CURRENT_TIMESTAMP() - INTERVAL ? DAY").
OrderBy("ClassTypeId").
Sql()
c.Repository.Select(classes, query, days)
c.populateClassRelationships(classes)
return classes
}
func (c *ClassRepository) populateClassRelationships(classes *[]entities.Class) {
for i := range classes { <<<<<<<<<<< Here is the problem
class := classes[i]
// ClassType
c.Repository.GetById(class.ClassType, class.ClassTypeId)
//Instructor
c.Repository.GetById(class.Instructor, class.ClassType.InstructorId)
// Equipment
query := Select("E.*").
From("Equipment E").
Join("ClassEquipment CE on E.Id = CE.EquipmentId").
Where("CE.ClassId = ?").
Sql()
c.Repository.Select(class.Equipment, query, class.Id)
}
}
Here is the Class struct:
package entities
import (
"time"
)
type Class struct {
Id int
ClassTypeId int
VideoPath string
VideoSize int
Duration float64
CreatedAt time.Time
VisibleAt time.Time
NoLongerVisibleAt time.Time
// Relationships
ClassType ClassType
Instructor User
Equipment []Equipment
}
You're assuming the pointer to a slice will be automatically dereferenced for the iteration.
That's not the case and there's no reason for that because a slice is already a kind of pointer, rendering a pointer to a slice totally useless.
From Effective Go :
If a function takes a slice argument, changes it makes to the elements
of the slice will be visible to the caller, analogous to passing a
pointer to the underlying array.
Internally, a slice is made of
a pointer to the first element of the slice in the underlying array
the length of the slice
the capacity of the slice (the slice can usually be extended until the end of the array)
This structure is very small, rendering a pointer useless.
if you need to pull an individual element from the *slice, you have to dereference it first like this: (*slice)[0]. I pounded my head against *slice[0] for about 6 hours before I realized this. It has to do with the order of operations, and is not, IMO, a very elegant result.
I ended up writing some pointer receiver methods to do in-place modifications like append and pop in a more, to my mind, reasonable way - an example can be found here: https://play.golang.org/p/qZEYMcPHl4
From Effective Go:
If you're looping over an array, slice, string, or map, or reading
from a channel, a range clause can manage the loop.
You are attempting to iterate over a pointer to a slice which is a single value, not a collection therefore is not possible.
Change the argument to populateClassRelationships to be an slice, not a pointer to a slice. Or you could dereference the pointer:
func (c *ClassRepository) populateClassRelationships(classes *[]entities.Class) {
for i := range *classes { // dereferencing the pointer to get the actual slice
class := classes[i]
// ClassType
c.Repository.GetById(class.ClassType, class.ClassTypeId)
//Instructor
c.Repository.GetById(class.Instructor, class.ClassType.InstructorId)
// Equipment
query := Select("E.*").
From("Equipment E").
Join("ClassEquipment CE on E.Id = CE.EquipmentId").
Where("CE.ClassId = ?").
Sql()
c.Repository.Select(class.Equipment, query, class.Id)
}
}
You could dereference the pointer:
func (c *ClassRepository) populateClassRelationships(classes *[]entities.Class) {
for _, class := range *classes { // NOTE the * dereference
// ClassType
c.Repository.GetById(class.ClassType, class.ClassTypeId)
//Instructor
c.Repository.GetById(class.Instructor, class.ClassType.InstructorId)
// Equipment
query := Select("E.*").
From("Equipment E").
Join("ClassEquipment CE on E.Id = CE.EquipmentId").
Where("CE.ClassId = ?").
Sql()
c.Repository.Select(class.Equipment, query, class.Id)
}
}
I also changed the range clause as I don't think you're modifying classes.