Related
Consider the following map
mymap := make(map[string]string)
mymap["a"] = "one"
mymap["b"] = "two"
mymap["c"] = "one"
How to determine if the values are unique?
One strategy is to iterate through the map, create a slice of the values. Then iterate through the slice to find duplicates. Is there a better way?
If you just need true/false of whether there are dupes, without needing to know which values are dupes or how many dupes there are, the most efficient structure to use to track existing values is a map with empty struct values.
See here (pasted below for convenience):
package main
import (
"fmt"
)
func hasDupes(m map[string]string) bool {
x := make(map[string]struct{})
for _, v := range m {
if _, has := x[v]; has {
return true
}
x[v] = struct{}{}
}
return false
}
func main() {
mapWithDupes := make(map[string]string)
mapWithDupes["a"] = "one"
mapWithDupes["b"] = "two"
mapWithDupes["c"] = "one"
fmt.Println(hasDupes(mapWithDupes)) // prints true
mapWithoutDupes := make(map[string]string)
mapWithoutDupes["a"] = "one"
mapWithoutDupes["b"] = "two"
mapWithoutDupes["c"] = "three"
fmt.Println(hasDupes(mapWithoutDupes)) // prints false
}
func main() {
m := map[int]int{
1: 100,
2: 200,
3: 100,
4: 400,
6: 200,
7: 700,
}
mNew := make(map[int]int)
for k, v := range m {
if val, has := mNew[v]; !has {
mNew[v] = k
} else {
fmt.Println(k, m[k], ",", val, m[val])
}
}
swap the map key & value with a new map
Second map wont insert duplicate key, so you can find the values
I'm trying to play around with recursive structs, where when I have multiple I can add them together, creating a new struct with those embedded. However, I'm not sure what the proper way to approach this is.
I've included a code snippet below to further illustrate what I mean.
package main
import "fmt"
type Container struct {
F int
Collection []SubContainer
}
type SubContainer struct {
Key string
Value int
}
func main() {
commits := map[string]int{
"a": 1,
"b": 2,
"c": 3,
"d": 4,
}
sc := []SubContainer{}
c := []Container{}
count := 0
for k, v := range commits {
sc = append(sc, SubContainer{Key: k, Value: v})
count++
if len(sc) == 2 {
c = append(c, Container{Collection: sc, F: count})
sc = nil
}
}
for _, r := range c {
fmt.Println(r)
}
}
Result:
{2 [{a 1} {b 2}]}
{4 [{c 3} {d 4}]}
Desired result:
{6 {2 [{a 1} {b 2}]} {4 [{c 3} {d 4}]}}
Playground link: https://play.golang.org/p/j6rbhgcOoT
One caveat I'm having trouble wrapping my head around is that the commits length may change (I was initially thinking I could just create a different parent struct). Any suggestions would be appreciated... Is doing this somehow with recursive structs the right approach to accomplish this? Thanks!
I attempted to get close to the desired output without being sure about the exact goal, you will find below a modified version of the snippet you provided.
You can use the String() string method on a collection to customize the format.
package main
import "fmt"
type ContainerCollection struct {
Count int
List []Container
}
func (cc ContainerCollection) String() string {
total := 0
for _, c := range cc.List {
total += c.F
}
return fmt.Sprintf("{%d %v}", total, cc.List)
}
type Container struct {
F int
Collection []SubContainer
}
type SubContainer struct {
Key string
Value int
}
func main() {
commits := map[string]int{
"a": 1,
"b": 2,
"c": 3,
"d": 4,
}
c := ContainerCollection{Count: 0}
sc := []SubContainer{}
for k, v := range commits {
sc = append(sc, SubContainer{Key: k, Value: v})
c.Count++
if len(sc) == 2 {
c.List = append(c.List, Container{Collection: sc, F: c.Count})
sc = []SubContainer{}
}
}
// Should also cover odd number of commits
if len(sc) != 0 {
c.Count++
c.List = append(c.List, Container{Collection: sc, F: c.Count})
}
// for _, r := range c.List { fmt.Println(r) }
fmt.Println(c)
}
Result:
{6 [{2 [{a 1} {b 2}]} {4 [{c 3} {d 4}]}]}
Playground
Here's something with minimal modification to your code (just added a 'super' container, which is basically a summary struct). One probably need this only if this is being passed to another library/package/over the net etc., otherwise just maintaining the totalCount may be enough.
package main
import "fmt"
type SuperContainer struct {
TotalCount int
Containers []Container
}
type Container struct {
F int
Collection []SubContainer
}
type SubContainer struct {
Key string
Value int
}
func main() {
var totalCount int
commits := map[string]int{
"a": 1,
"b": 2,
"c": 3,
"d": 4,
}
sc := []SubContainer{}
c := []Container{}
count := 0
for k, v := range commits {
sc = append(sc, SubContainer{Key: k, Value: v})
count++
if len(sc) == 2 {
totalCount += count
c = append(c, Container{Collection: sc, F: count})
sc = nil
}
}
for _, r := range c {
fmt.Println(r)
}
supC := SuperContainer{TotalCount: totalCount, Containers: c}
fmt.Println(supC)
}
Playground: https://play.golang.org/p/yN3N3gHaCX
I'm trying to figure out the best way to recursively go through a [string]int map in Go. I'm building a game in which multiple countries are involved, and grouped together by teams of two in the end.
The goal is to match the first two countries with the lowest 'score' into its own group of two, and add it back to the collection giving the new map a total value of the scores of those countries.
Then recursively doing that to all the groups, ending up with one group and one total value in the end.
For example, if you had:
score := map[string]int{
"Canada": 7,
"US": 2,
"Germany": 3,
"Korea": 4,
}
group1 = {[US:2] [Germany:3]} with a total of 5
group1 would now be put back into the initial collection with a 'score' of 5 since it takes the two lowest scores. We would now have:
score := map[string]int{
"Canada": 7,
"Korea": 4,
group1: `US:2 Germany:3` with a total of 5
}
If this was now the lowest score in the collection, the next iteration would be:
group2 = {[Korea:4] [group1:5]}
score := map[string]int{
"Canada": 7,
group2: `Korea:4 group1:5` with a total of 9
}
And so on until you're left with one.. I think the basic structure should be something like this. However, I'm unsure of the proper way to do this since the data structure is now encompassing a [string]int map, as well as this new map.
I realize this is not such a generic question, but could an interface be used for this? I'm very new to Go, so advice would be helpful.
Here is an example to further illustrate what I mean:
https://play.golang.org/p/cnkTc0HBY4
Your problem can "easy" be solved using a heap data structure.
package main
import (
"container/heap"
"fmt"
)
// Something that has a score
type Scoreable interface {
fmt.Stringer
Score() int
}
// A country has a name and a score
type Country struct {
name string
score int
}
// Country implements Scoreable
func (c Country) Score() int {
return c.score
}
// ... and fmt.Stringer
func (c Country) String() string {
return fmt.Sprintf("%s [%d]", c.name, c.score)
}
// A team consists of two Scoreable's and has itself a score
type Team struct {
team1, team2 Scoreable
score int
}
// Team implements Scoreable
func (t Team) Score() int {
return t.score
}
// ... and fmt.Stringer
func (t Team) String() string {
return fmt.Sprintf("(%s + %s)", t.team1.String(), t.team2.String())
}
// The heap will be implemented using a slice of Scoreables
type TeamHeap []Scoreable
// TeamHeap implements heap.Interface
func (th TeamHeap) Len() int {
return len(th)
}
func (th TeamHeap) Less(i, j int) bool {
return th[i].Score() < th[j].Score()
}
func (th TeamHeap) Swap(i, j int) {
th[i], th[j] = th[j], th[i]
}
func (th *TeamHeap) Push(t interface{}) {
*th = append(*th, t.(Scoreable))
}
func (th *TeamHeap) Pop() interface{} {
old := *th
n := len(old)
t := old[n-1]
*th = old[0 : n-1]
return t
}
// The main function
func main() {
// Create a heap and initialize it
teams := &TeamHeap{}
heap.Init(teams)
// Push the countries (NB: heap.Push(), not teams.Push())
heap.Push(teams, Country{"Canada", 7})
heap.Push(teams, Country{"US", 2})
heap.Push(teams, Country{"Germany", 3})
heap.Push(teams, Country{"Korea", 4})
// Take the two teams with lowest score and make a new team of them
// Repeat this until there's only one team left
for teams.Len() > 1 {
t1 := heap.Pop(teams).(Scoreable)
t2 := heap.Pop(teams).(Scoreable)
heap.Push(teams, Team{t1, t2, t1.Score() + t2.Score()})
}
// Print the teams that we now have in the heap
for teams.Len() > 0 {
t := heap.Pop(teams).(Team)
fmt.Println(t)
}
}
You can find runnable code on the Go Playground.
package main
import (
"container/heap"
"fmt"
)
//Recursive data structure may looks something like
type Group struct {
Score int
Left *Group
Right *Group
Country string
}
//You can use slice to hold them organized in tree
type GrHeap []Group
//To implement your logic you can use stdlib/container/heap Heap interface
//you must implement Heap interface for your slice
func (h GrHeap) Len() int { return len(h) }
func (h GrHeap) Less(i, j int) bool { return h[i].Score < h[j].Score }
func (h GrHeap) Swap(i, j int) { h[i], h[j] = h[j], h[i] }
func (h *GrHeap) Push(x interface{}) {
// Push and Pop use pointer receivers because they modify the slice's length,
// not just its contents.
*h = append(*h, x.(Group))
}
func (h *GrHeap) Pop() interface{} {
old := *h
n := len(old)
x := old[n-1]
*h = old[0 : n-1]
return x
}
func main() {
//you most likely already have a map
//anyway it will be handy to keep it for convenient access to individual country
score := map[string]int{
"Canada": 7,
"US": 2,
"Germany": 3,
"Korea": 4,
}
//here we allocate heap
gr := make(GrHeap, 0)
//populate it from map
for k, v := range score {
g := Group{v, nil, nil, k}
gr = append(gr, g)
}
//and initialize
heap.Init(&gr)
//and here we use heap magic to implement your logic
for len(gr) > 2 {
l := heap.Pop(&gr).(Group)
r := heap.Pop(&gr).(Group)
ng := Group{l.Score + r.Score, &l, &r, ""}
heap.Push(&gr, ng)
}
fmt.Println(gr)
fmt.Println(gr[1].Left)
fmt.Println(gr[1].Right.Left)
//and you can see it works https://play.golang.org/p/gugJxJb7rr
}
You can try map[string]interface{} with Type assertion。
Here is the demo
package main
import "fmt"
const total = "total"
func GetValue(i interface{}) int {
value, ok := i.(int)
if ok {
return value
}
return i.(map[string]interface{})[total].(int)
}
func main() {
score := map[string]interface{}{
"Canada": 7,
"US": 2,
"Germany": 3,
"Korea": 4,
}
groupCount := 0
for len(score) > 2 {
var (
firstMin = math.MaxInt32
secondMin = math.MaxInt32
firstKey = ""
secondKey = ""
)
for k, v := range score {
iv := GetValue(v)
if iv < firstMin {
secondMin = firstMin
secondKey = firstKey
firstMin = iv
firstKey = k
continue
}
if iv < secondMin {
secondMin = iv
secondKey = k
continue
}
}
groupCount++
score[fmt.Sprintf("Group%d", groupCount)] = map[string]interface{}{
firstKey: score[firstKey],
secondKey: score[secondKey],
total: GetValue(score[firstKey])+ GetValue(score[secondKey]),
}
delete(score, firstKey)
delete(score, secondKey)
}
fmt.Println(score)
}
Here is the link https://play.golang.org/p/qq5qwAsh1m
Is there a foreach construct in the Go language?
Can I iterate over a slice or array using a for?
From For statements with range clause:
A "for" statement with a "range" clause iterates through all entries
of an array, slice, string or map, or values received on a channel.
For each entry it assigns iteration values to corresponding iteration
variables and then executes the block.
As an example:
for index, element := range someSlice {
// index is the index where we are
// element is the element from someSlice for where we are
}
If you don't care about the index, you can use _:
for _, element := range someSlice {
// element is the element from someSlice for where we are
}
The underscore, _, is the blank identifier, an anonymous placeholder.
Go has a foreach-like syntax. It supports arrays/slices, maps and channels.
Iterate over an array or a slice:
// index and value
for i, v := range slice {}
// index only
for i := range slice {}
// value only
for _, v := range slice {}
Iterate over a map:
// key and value
for key, value := range theMap {}
// key only
for key := range theMap {}
// value only
for _, value := range theMap {}
Iterate over a channel:
for v := range theChan {}
Iterating over a channel is equivalent to receiving from a channel until it is closed:
for {
v, ok := <-theChan
if !ok {
break
}
}
Following is the example code for how to use foreach in Go:
package main
import (
"fmt"
)
func main() {
arrayOne := [3]string{"Apple", "Mango", "Banana"}
for index,element := range arrayOne{
fmt.Println(index)
fmt.Println(element)
}
}
This is a running example https://play.golang.org/p/LXptmH4X_0
Yes, range:
The range form of the for loop iterates over a slice or map.
When ranging over a slice, two values are returned for each iteration. The first is the index, and the second is a copy of the element at that index.
Example:
package main
import "fmt"
var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}
func main() {
for i, v := range pow {
fmt.Printf("2**%d = %d\n", i, v)
}
for i := range pow {
pow[i] = 1 << uint(i) // == 2**i
}
for _, value := range pow {
fmt.Printf("%d\n", value)
}
}
You can skip the index or value by assigning to _.
If you only want the index, drop the , value entirely.
The following example shows how to use the range operator in a for loop to implement a foreach loop.
func PrintXml (out io.Writer, value interface{}) error {
var data []byte
var err error
for _, action := range []func() {
func () { data, err = xml.MarshalIndent(value, "", " ") },
func () { _, err = out.Write([]byte(xml.Header)) },
func () { _, err = out.Write(data) },
func () { _, err = out.Write([]byte("\n")) }} {
action();
if err != nil {
return err
}
}
return nil;
}
The example iterates over an array of functions to unify the error handling for the functions. A complete example is at Google´s playground.
PS: it shows also that hanging braces are a bad idea for the readability of code. Hint: the for condition ends just before the action() call. Obvious, isn't it?
You can in fact use range without referencing its return values by using for range against your type:
arr := make([]uint8, 5)
i,j := 0,0
for range arr {
fmt.Println("Array Loop", i)
i++
}
for range "bytes" {
fmt.Println("String Loop", j)
j++
}
https://play.golang.org/p/XHrHLbJMEd
This may be obvious, but you can inline the array like so:
package main
import (
"fmt"
)
func main() {
for _, element := range [3]string{"a", "b", "c"} {
fmt.Print(element)
}
}
outputs:
abc
https://play.golang.org/p/gkKgF3y5nmt
I'm seeing a lot of examples using range. Just a heads up that range creates a copy of whatever you're iterating over. If you make changes to the contents in a foreach range you will not be changing the values in the original container, in that case you'll need a traditional for loop with an index you increment and deference indexed reference. E.g.:
for i := 0; i < len(arr); i++ {
element := &arr[i]
element.Val = newVal
}
I have just implemented this library: https://github.com/jose78/go-collection.
This is an example of how to use the Foreach loop:
package main
import (
"fmt"
col "github.com/jose78/go-collection/collections"
)
type user struct {
name string
age int
id int
}
func main() {
newList := col.ListType{user{"Alvaro", 6, 1}, user{"Sofia", 3, 2}}
newList = append(newList, user{"Mon", 0, 3})
newList.Foreach(simpleLoop)
if err := newList.Foreach(simpleLoopWithError); err != nil{
fmt.Printf("This error >>> %v <<< was produced", err )
}
}
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 > 1{
panic(fmt.Sprintf("Error produced with index == %d\n", index))
}
fmt.Printf("%d.- item:%v\n", index, mapper)
}
The result of this execution should be:
0.- item:{Alvaro 6 1}
1.- item:{Sofia 3 2}
2.- item:{Mon 0 3}
0.- item:{Alvaro 6 1}
1.- item:{Sofia 3 2}
Recovered in f Error produced with index == 2
ERROR: Error produced with index == 2
This error >>> Error produced with index == 2
<<< was produced
Try this code in playGrounD.
How would you implement the deleteRecords function in the code below:
Example:
type Record struct {
id int
name string
}
type RecordList []*Record
func deleteRecords( l *RecordList, ids []int ) {
// Assume the RecordList can contain several 100 entries.
// and the number of the of the records to be removed is about 10.
// What is the fastest and cleanest ways to remove the records that match
// the id specified in the records list.
}
I did some micro-benchmarking on my machine, trying out most of the approaches given in the replies here, and this code comes out fastest when you've got up to about 40 elements in the ids list:
func deleteRecords(data []*Record, ids []int) []*Record {
w := 0 // write index
loop:
for _, x := range data {
for _, id := range ids {
if id == x.id {
continue loop
}
}
data[w] = x
w++
}
return data[:w]
}
You didn't say whether it's important to preserve the order of records in the list. If you don't then this function is faster than the above and still fairly clean.
func reorder(data []*Record, ids []int) []*Record {
n := len(data)
i := 0
loop:
for i < n {
r := data[i]
for _, id := range ids {
if id == r.id {
data[i] = data[n-1]
n--
continue loop
}
}
i++
}
return data[0:n]
}
As the number of ids rises, so does the cost of the linear search. At around 50 elements, using a map or doing a binary search to look up the id becomes more efficient, as long as you can avoid rebuilding the map (or resorting the list) every time. At several hundred ids, it becomes more efficient to use a map or a binary search even if you have to rebuild it every time.
If you wish to preserve original contents of the slice, something like this is more appropriate:
func deletePreserve(data []*Record, ids []int) []*Record {
wdata := make([]*Record, len(data))
w := 0
loop:
for _, x := range data {
for _, id := range ids {
if id == x.id {
continue loop
}
}
wdata[w] = x
w++
}
return wdata[0:w]
}
For a personal project, I did something like this:
func filter(sl []int, fn func(int) bool) []int {
result := make([]int, 0, len(sl))
last := 0
for i, v := range sl {
if fn(v) {
result = append(result, sl[last:i]...)
last = i + 1
}
}
return append(result, sl[last:]...)
}
It doesn't mutate the original, but should be relatively efficient.
It's probably better to just do:
func filter(sl []int, fn func(int) bool) (result []int) {
for _, v := range sl {
if !fn(v) {
result = append(result, v)
}
}
return
}
Simpler and cleaner.
If you want to do it in-place, you probably want something like:
func filter(sl []int, fn func(int) bool) []int {
outi := 0
res := sl
for _, v := range sl {
if !fn(v) {
res[outi] = v
outi++
}
}
return res[0:outi]
}
You can optimize this to use copy to copy ranges of elements, but that's twice
the code and probably not worth it.
So, in this specific case, I'd probably do something like:
func deleteRecords(l []*Record, ids []int) []*Record {
outi := 0
L:
for _, v := range l {
for _, id := range ids {
if v.id == id {
continue L
}
}
l[outi] = v
outi++
}
return l[0:outi]
}
(Note: untested.)
No allocations, nothing fancy, and assuming the rough size of the list of Records and the list of ids you presented, a simple linear search is likely to do as well as fancier things but without any overhead. I realize that my version mutates the slice and returns a new slice, but that's not un-idiomatic in Go, and it avoids forcing the slice at the callsite to be heap allocated.
For the case you described, where len(ids) is approximately 10 and len(*l) is in the several hundreds, this should be relatively fast, since it minimizes memory allocations by updating in place.
package main
import (
"fmt"
"strconv"
)
type Record struct {
id int
name string
}
type RecordList []*Record
func deleteRecords(l *RecordList, ids []int) {
rl := *l
for i := 0; i < len(rl); i++ {
rid := rl[i].id
for j := 0; j < len(ids); j++ {
if rid == ids[j] {
copy(rl[i:len(*l)-1], rl[i+1:])
rl[len(rl)-1] = nil
rl = rl[:len(rl)-1]
break
}
}
}
*l = rl
}
func main() {
l := make(RecordList, 777)
for i := range l {
l[i] = &Record{int(i), "name #" + strconv.Itoa(i)}
}
ids := []int{0, 1, 2, 4, 8, len(l) - 1, len(l)}
fmt.Println(ids, len(l), cap(l), *l[0], *l[1], *l[len(l)-1])
deleteRecords(&l, ids)
fmt.Println(ids, len(l), cap(l), *l[0], *l[1], *l[len(l)-1])
}
Output:
[0 1 2 4 8 776 777] 777 777 {0 name #0} {1 name #1} {776 name #776}
[0 1 2 4 8 776 777] 772 777 {1 name #1} {3 name #3} {775 name #775}
Instead of repeatedly searching ids, you could use a map. This code preallocates the full size of the map, and then just moves array elements in place. There are no other allocations.
func deleteRecords(l *RecordList, ids []int) {
m := make(map[int]bool, len(ids))
for _, id := range ids {
m[id] = true
}
s, x := *l, 0
for _, r := range s {
if !m[r.id] {
s[x] = r
x++
}
}
*l = s[0:x]
}
Use the vector package's Delete method as a guide, or just use a Vector instead of a slice.
Here is one option but I would hope there are cleaner/faster more functional looking ones:
func deleteRecords( l *RecordList, ids []int ) *RecordList {
var newList RecordList
for _, rec := range l {
toRemove := false
for _, id := range ids {
if rec.id == id {
toRemove = true
}
if !toRemove {
newList = append(newList, rec)
}
}
return newList
}
With large enough l and ids it will be more effective to Sort() both lists first and then do a single loop over them instead of two nested loops