func ParseportsFromFile(file string) (map[string]string, error) {
buf, err := ioutil.ReadFile(file)
if err != nil {
return nil, err
}
ret := [make(map[string]string)]
rows := strings.Split(string(buf), "\n")
for _, row := range rows {
kvs := strings.SplitN(row, "=", 2)
if len(kvs) == 2 {
ret[strings.TrimSpace(kvs[0])] = strings.TrimSpace(kvs[1])
}
}
return ret, nil
}
This function allows me to read a file like that :
user1=123
user1=321
user2=124
However, the data return is
map[user1:321 user2:124]
So that mean user1=123 has been overwritten with user1=321
How to avoid that ?
How to create an array like
map[user1:[123,321], user2: 124]
to avoid an item to overwrite another ?
Since go is strongly typed, it would be easier to make it right away a map of slices. See the http.Header type, for example. They had the same problem to solve when designing it.
In your case, this could look something like this:
result := make(map[string][]string)
for _, row := range rows {
parts := strings.Split(row, "=")
key := parts[0]
value := parts[1]
result[key] = append(result[key], value)
}
https://go.dev/play/p/5uRH-aQmATR
Otherwise, you need to use interface{} (any) so you can have both, string and []string, but the logic to get that done would be more complex, and using it would be also more complex since you always need to check what it is and do type assertion and so on. After all, I would not recommend it.
I have a Protobuf structure defined as so in my .proto file:
message Msg{
message SubMsg {
string SubVariable1 = 1;
int32 SubVariable2 = 2;
...
}
string Variable1 = 1;
repeated SubMsg Variable2 = 2;
...
}
I pull data into this structure using the https://godoc.org/google.golang.org/protobuf/encoding/protojson package when consuming data from a JSON API, as so:
Response, err := Client.Do(Request)
if err != nil {
log.Error(err)
}
DataByte, err := ioutil.ReadAll(Response.Body)
if err != nil {
log.Error(err)
}
DataProto := Msg{}
err = protojson.Unmarshal(DataByte, &DataProto)
if err != nil {
log.Error(err)
}
What I want to be able to do is to range over the elements of Variable2 to be able to access the SubVariables using the protoreflect API, for which I have tried both:
Array := DataProto.GetVariable2()
for i := range Array {
Element := Array[i]
}
and also:
DataProto.GetVariable2().ProtoReflect().Range(func(fd protoreflect.FieldDescriptor, v protoreflect.Value) {
…
return true})
The first of which fails with error message:
cannot range over DataProto.GetVariable2() (type *SubMsg)
despite the fact DataProto.GetVariable2() returns a variable of type []*Msg_SubMsg.
The second of which fails with:
DataProto.GetVariable2.ProtoReflect undefined (type []*SubMsg has no field or method ProtoReflect)
which suggests that DataProto.GetVariable2() does indeed return an array unlike what is suggested in the error returned in my first approach. This makes sense to me as the protoreflect API only allows this method to be called on a defined message, not an array of those messages. There therefore must be another way of accessing the elements of these arrays to be able to make use of the protoreflect API (for which I have been unsuccessful in finding and answer to on the web thus far).
Could someone help me make sense of these seemingly conflicting error messages? Has anyone had any success iterating over a Protobuf array themselves?
Thanks in advance.
You'll want to treat your Array variable as a List, which means you can't use Range() as in your second attempt. It's close though. Here is a functional example of iterating through and inspecting nested messages:
import (
"testing"
"google.golang.org/protobuf/reflect/protoreflect"
)
func TestVariable2(t *testing.T) {
pb := &Msg{
Variable2: []*Msg_SubMsg{
{
SubVariable1: "string",
SubVariable2: 1,
},
},
}
pbreflect := pb.ProtoReflect()
fd := pbreflect.Descriptor().Fields().ByJSONName("Variable2")
if !fd.IsList() {
t.Fatal("expected a list")
}
l := pbreflect.Get(fd).List()
for i := 0; i < l.Len(); i++ {
// should test that we are now inspecting a message type
li := l.Get(i).Message()
li.Range(func(lifd protoreflect.FieldDescriptor, liv protoreflect.Value) bool {
t.Logf("%v: %v", lifd.Name(), liv)
return true
})
}
}
Run with go test -v ./... if you want to see output
I have a []byte which I need to sort, in ascending order.
I get an object with the items and then iterate the array in order to create the object returned:
// unfortunately, for some obscure reason I can't change the data types of the caller and the object from the function call are different, although both are []byte underneath (...)
type ID []byte
// in another package:
type ByteInterface []byte
func (c *Store) GetAll() ByteInterface {
returnObj := make([]ByteInterface,0)
obj, err := GetData()
// err handling
for _, b := range obj.IDs {
returnObj = append(returnObj, ByteInterface(b))
}
return returnObj
}
So I'm asking myself if it is possible to do the append so that returnObj is sorted right away, or if I need to sort obj.ByteData upfront (or sort returnOjb afterwards).
On each iteration, do the following:
Grow the target slice (possibly reallocating it):
numElems := len(returnObj)
returnObj = append(returnObj, make([]byte, len(obj))...)
Use the standard approach for insertion to keep the destination sorted by finding a place to put each byte from the source slice, one by one:
for _, b := range obj {
i := sort.Search(numElems, func (i int) bool {
return returnObj[i] >= b
}
if i < numElems {
copy(returnObj[i+1:], returnObj[i:])
}
returnObj[i] = b
numElems++
}
(The call to copy should be optimized by copying less but this is left as an exercise for the reader.)
I have an array of strings, and I'd like to exclude values that start in foo_ OR are longer than 7 characters.
I can loop through each element, run the if statement, and add it to a slice along the way. But I was curious if there was an idiomatic or more golang-like way of accomplishing that.
Just for example, the same thing might be done in Ruby as
my_array.select! { |val| val !~ /^foo_/ && val.length <= 7 }
There is no one-liner as you have it in Ruby, but with a helper function you can make it almost as short.
Here's our helper function that loops over a slice, and selects and returns only the elements that meet a criteria captured by a function value:
func filter(ss []string, test func(string) bool) (ret []string) {
for _, s := range ss {
if test(s) {
ret = append(ret, s)
}
}
return
}
Starting with Go 1.18, we can write it generic so it will work with all types, not just string:
func filter[T any](ss []T, test func(T) bool) (ret []T) {
for _, s := range ss {
if test(s) {
ret = append(ret, s)
}
}
return
}
Using this helper function your task:
ss := []string{"foo_1", "asdf", "loooooooong", "nfoo_1", "foo_2"}
mytest := func(s string) bool { return !strings.HasPrefix(s, "foo_") && len(s) <= 7 }
s2 := filter(ss, mytest)
fmt.Println(s2)
Output (try it on the Go Playground, or the generic version: Go Playground):
[asdf nfoo_1]
Note:
If it is expected that many elements will be selected, it might be profitable to allocate a "big" ret slice beforehand, and use simple assignment instead of the append(). And before returning, slice the ret to have a length equal to the number of selected elements.
Note #2:
In my example I chose a test() function which tells if an element is to be returned. So I had to invert your "exclusion" condition. Obviously you may write the helper function to expect a tester function which tells what to exclude (and not what to include).
Have a look at robpike's filter library. This would allow you to do:
package main
import (
"fmt"
"strings"
"filter"
)
func isNoFoo7(a string) bool {
return ! strings.HasPrefix(a, "foo_") && len(a) <= 7
}
func main() {
a := []string{"test", "some_other_test", "foo_etc"}
result := Choose(a, isNoFoo7)
fmt.Println(result) // [test]
}
Interestingly enough the README.md by Rob:
I wanted to see how hard it was to implement this sort of thing in Go, with as nice an API as I could manage. It wasn't hard.
Having written it a couple of years ago, I haven't had occasion to use it once. Instead, I just use "for" loops.
You shouldn't use it either.
So the most idiomatic way according to Rob would be something like:
func main() {
a := []string{"test", "some_other_test", "foo_etc"}
nofoos := []string{}
for i := range a {
if(!strings.HasPrefix(a[i], "foo_") && len(a[i]) <= 7) {
nofoos = append(nofoos, a[i])
}
}
fmt.Println(nofoos) // [test]
}
This style is very similar, if not identical, to the approach any C-family language takes.
Today, I stumbled on a pretty idiom that surprised me. If you want to filter a slice in place without allocating, use two slices with the same backing array:
s := []T{
// the input
}
s2 := s
s = s[:0]
for _, v := range s2 {
if shouldKeep(v) {
s = append(s, v)
}
}
Here's a specific example of removing duplicate strings:
s := []string{"a", "a", "b", "c", "c"}
s2 := s
s = s[:0]
var last string
for _, v := range s2 {
if len(s) == 0 || v != last {
last = v
s = append(s, v)
}
}
If you need to keep both slices, simply replace s = s[:0] with s = nil or s = make([]T, 0, len(s)), depending on whether you want append() to allocate for you.
There are a couple of nice ways to filter a slice without allocations or new dependencies. Found in the Go wiki on Github:
Filter (in place)
n := 0
for _, x := range a {
if keep(x) {
a[n] = x
n++
}
}
a = a[:n]
And another, more readable, way:
Filtering without allocating
This trick uses the fact that a slice shares the same backing array
and capacity as the original, so the storage is reused for the
filtered slice. Of course, the original contents are modified.
b := a[:0]
for _, x := range a {
if f(x) {
b = append(b, x)
}
}
For elements which must be garbage collected, the following code can
be included afterwards:
for i := len(b); i < len(a); i++ {
a[i] = nil // or the zero value of T
}
One thing I'm not sure about is whether the first method needs clearing (setting to nil) the items in slice a after index n, like they do in the second method.
EDIT: the second way is basically what MicahStetson described in his answer. In my code I use a function similar to the following, which is probably as good as it gets in terms on performance and readability:
func filterSlice(slice []*T, keep func(*T) bool) []*T {
newSlice := slice[:0]
for _, item := range slice {
if keep(item) {
newSlice = append(newSlice, item)
}
}
// make sure discarded items can be garbage collected
for i := len(newSlice); i < len(slice); i++ {
slice[i] = nil
}
return newSlice
}
Note that if items in your slice are not pointers and don't contain pointers you can skip the second for loop.
There isn't an idiomatic way you can achieve the same expected result in Go in one single line as in Ruby, but with a helper function you can obtain the same expressiveness as in Ruby.
You can call this helper function as:
Filter(strs, func(v string) bool {
return strings.HasPrefix(v, "foo_") // return foo_testfor
}))
Here is the whole code:
package main
import "strings"
import "fmt"
// Returns a new slice containing all strings in the
// slice that satisfy the predicate `f`.
func Filter(vs []string, f func(string) bool) []string {
vsf := make([]string, 0)
for _, v := range vs {
if f(v) && len(v) > 7 {
vsf = append(vsf, v)
}
}
return vsf
}
func main() {
var strs = []string{"foo1", "foo2", "foo3", "foo3", "foo_testfor", "_foo"}
fmt.Println(Filter(strs, func(v string) bool {
return strings.HasPrefix(v, "foo_") // return foo_testfor
}))
}
And the running example: Playground
you can use the loop as you did and wrap it to a utils function for reuse.
For multi-datatype support, copy-paste will be a choice. Another choice is writing a generating tool.
And final option if you want to use lib, you can take a look on https://github.com/ledongthuc/goterators#filter that I created to reuse aggregate & transform functions.
It requires the Go 1.18 to use that support generic + dynamic type you want to use with.
filteredItems, err := Filter(list, func(item int) bool {
return item % 2 == 0
})
filteredItems, err := Filter(list, func(item string) bool {
return item.Contains("ValidWord")
})
filteredItems, err := Filter(list, func(item MyStruct) bool {
return item.Valid()
})
It also supports Reduce in case you want to optimize the way you select.
Hope it's useful with you!
"Select Elements from Array" is also commonly called a filter function. There's no such thing in go. There are also no other "Collection Functions" such as map or reduce. For the most idiomatic way to get the desired result, I find https://gobyexample.com/collection-functions a good reference:
[...] in Go it’s common to provide collection functions if and when they are specifically needed for your program and data types.
They provide an implementation example of the filter function for strings:
func Filter(vs []string, f func(string) bool) []string {
vsf := make([]string, 0)
for _, v := range vs {
if f(v) {
vsf = append(vsf, v)
}
}
return vsf
}
However, they also say, that it's often ok to just inline the function:
Note that in some cases it may be clearest to just inline the
collection-manipulating code directly, instead of creating and calling
a helper function.
In general, golang tries to only introduce orthogonal concepts, meaning that when you can solve a problem one way, there shouldn't be too many more ways to solve it. This adds simplicity to the language by only having a few core concepts, such that not every developer uses a different subset of the language.
Take a look at this library: github.com/thoas/go-funk
It provides an implementation of a lot of life-saving idioms in Go (including filtering of elements in array for instance).
r := funk.Filter([]int{1, 2, 3, 4}, func(x int) bool {
return x%2 == 0
}
Here is an elegant example of both Fold and Filter that uses recursion to accomplish filtering. FoldRight is also generally useful. It is not stack safe but could be made so with trampolining. Once Golang has generics it can be entirely generalized for any 2 types:
func FoldRightStrings(as, z []string, f func(string, []string) []string) []string {
if len(as) > 1 {//Slice has a head and a tail.
h, t := as[0], as[1:len(as)]
return f(h, FoldRightStrings(t, z, f))
} else if len(as) == 1 {//Slice has a head and an empty tail.
h := as[0]
return f(h, FoldRightStrings([]string{}, z, f))
}
return z
}
func FilterStrings(as []string, p func(string) bool) []string {
var g = func(h string, accum []string) []string {
if p(h) {
return append(accum, h)
} else {
return accum
}
}
return FoldRightStrings(as, []string{}, g)
}
Here is an example of its usage to filter out all the strings with length < 8
var p = func(s string) bool {
if len(s) < 8 {
return true
} else {
return false
}
}
FilterStrings([]string{"asd","asdfas","asdfasfsa","asdfasdfsadfsadfad"}, p)
I`m developing this library: https://github.com/jose78/go-collection. PLease try this example to filter elements:
package main
import (
"fmt"
col "github.com/jose78/go-collection/collections"
)
type user struct {
name string
age int
id int
}
func main() {
newMap := generateMapTest()
if resultMap, err := newMap.FilterAll(filterEmptyName); err != nil {
fmt.Printf("error")
} else {
fmt.Printf("Result: %v\n", resultMap)
result := resultMap.ListValues()
fmt.Printf("Result: %v\n", result)
fmt.Printf("Result: %v\n", result.Reverse())
fmt.Printf("Result: %v\n", result.JoinAsString(" <---> "))
fmt.Printf("Result: %v\n", result.Reverse().JoinAsString(" <---> "))
result.Foreach(simpleLoop)
err := result.Foreach(simpleLoopWithError)
if err != nil {
fmt.Println(err)
}
}
}
func filterEmptyName(key interface{}, value interface{}) bool {
user := value.(user)
return user.name != "empty"
}
func generateMapTest() (container col.MapType) {
container = col.MapType{}
container[1] = user{"Alvaro", 6, 1}
container[2] = user{"Sofia", 3, 2}
container[3] = user{"empty", 0, -1}
return container
}
var simpleLoop col.FnForeachList = func(mapper interface{}, index int) {
fmt.Printf("%d.- item:%v\n", index, mapper)
}
var simpleLoopWithError col.FnForeachList = func(mapper interface{}, index int) {
if index > 0 {
panic(fmt.Sprintf("Error produced with index == %d\n", index))
}
fmt.Printf("%d.- item:%v\n", index, mapper)
}
Result of execution:
Result: map[1:{Alvaro 6 1} 2:{Sofia 3 2}]
Result: [{Sofia 3 2} {Alvaro 6 1}]
Result: [{Alvaro 6 1} {Sofia 3 2}]
Result: {Sofia 3 2} <---> {Alvaro 6 1}
Result: {Alvaro 6 1} <---> {Sofia 3 2}
0.- item:{Sofia 3 2}
1.- item:{Alvaro 6 1}
0.- item:{Sofia 3 2}
Recovered in f Error produced with index == 1
ERROR: Error produced with index == 1
Error produced with index == 1
The DOC currently are located in wiki section of the project. You can try it in this link. I hope you like it...
REgaRDS...
I am sort of new to golang, and also kind of new to programming. And go has been very hard for me. This is one thing that always confuses me: data types. If you run this(not on the playground) then it will spit out:
./main.go:40: cannot use recorded (type string) as type SVC in append
and if I reverse the values in the append call, it will spit out:
./main.go:40: first argument to append must be slice; have string
What I am trying to do is grab all of the stuff in the home directory, append all of the values with the modifications to an array, then put the array into a file using ioutil. All I want(as of now) is to append the values to the slice in func record. Can anybody help?
package main
import "os"
import "io/ioutil"
import "fmt"
type SVC struct {
key string
val string
}
func main() {
os.Chdir("../../../../../../..")
var data, err = ioutil.ReadDir("home")
checkerr(err)
for _, data := range data {
fmt.Println(data.Name())
}
os.Chdir("home/beanboybunny/repos/bux/go/src/bux")
}
func checkerr(err1 error) {
if err1 != nil {
fmt.Println("error")
}
}
func writer(dataname string) {
f := "var "
uname := dataname
q := " = VVC{\n"
w := " bux: 1,\n"
e := " op: true,\n"
c := "}"
b2 := f + uname + q + w + e + c
record(b2)
}
func record(recorded string) {
cache := []SVC{}
record SVC := recorded
appenda := append(cache, recorded)
}
Your type SVC struct has two private string fields. If you just want an array of strings you don't need the SVC type.
If you're just trying to build an array of strings with your specially formatted transform of the items in the home directory, here is a start showing the use of append and format strings: https://play.golang.org/p/eUKTKRxwfp
package main
import (
"fmt"
"io/ioutil"
"os"
"strings"
)
func main() {
os.Chdir("../../../../../../..")
var data, err = ioutil.ReadDir("home")
checkerr(err)
lines := []string{}
for _, data := range data {
fmt.Println(data.Name())
lines = append(lines, buildLine(data.Name()))
}
fmt.Println(strings.Join(lines, "\n"))
os.Chdir("home/beanboybunny/repos/bux/go/src/bux")
}
func checkerr(err1 error) {
if err1 != nil {
fmt.Printf("error: %v", err1)
}
}
func buildLine(dataname string) string {
return fmt.Sprintf("var %s = VVC{\n bux: 1,\n op: true,\n}", dataname)
}
You can not add meters to liters. So you can't append string type to a list of SVC records.
If you want []SVC as output (list of SVC) you need to implement a parser that transform a string to an SVC object and append it.
If you want []string as output you need to implement a serializer that transform an SVC to a string and append it.
How to do it is a separate question that is out of scope for this question.