I have to basically set the length of an 2 dimension array depending of how many elements I found on a Json File, not sure how to do it. I already have a method who is going to read my Json File, however Im not sure how to set the length of my 2 dimension array after I finished to read it.
json.Unmarshal will allocate slices as needed. Go slices are what you probably mean by variable-sized arrays. Here's a simple example:
b := []byte(`[[1, 2, 3], [4, 5, 6]]`)
var slice2d [][]int
if err := json.Unmarshal(b, &slice2d); err != nil {
log.Fatal(err)
}
fmt.Println("Unmarshaled", slice2d)
If you're reading into something else and not directly into a 2d slice, then perhaps you're just looking for make?
This will allocate a 2D slice, NxM:
twoD := make([][]int, N)
for i := 0; i < N; i++ {
twoD[i] = make([]int, M)
}
// Now you can write into twoD[i][j], given i<N, j<M
// and no extra allocation will be incurred.
Related
I want to copy an array but the copied array always makes changes to the original array as well. Why is this the case?
Below is a code example of the issue:
package main
import (
"fmt"
)
func main() {
array := make([]int, 2)
for i := range array {
array[i] = 0
}
oldArray := array
array[0] = 3 // why does this also change oldArray?
oldArray[1] = 2 // why does this also change the original array?
fmt.Println(oldArray, array)
// I expected [0,2], [3,0]
// but it returns [3,2], [3,2]
}
I tried to initiate the variable oldArray before array but the result is the same.
In your example code, you're working with slices, not arrays.
From the slice documentation:
A slice is a descriptor for a contiguous segment of an underlying array and provides access to a numbered sequence of elements from that array.
When you assign a slice to a variable, you're creating a copy of that descriptor and therefore dealing with the same underlying array. When you're actually working with arrays, it has the behavior you're expecting.
Another snippet from the slice documentation (emphasis mine):
A slice, once initialized, is always associated with an underlying array that holds its elements. A slice therefore shares storage with its array and with other slices of the same array; by contrast, distinct arrays always represent distinct storage.
Here's a code sample (for the slices, the memory address of the first element is in parentheses, to clearly point out when two slices are using the same underlying array):
package main
import (
"fmt"
)
func main() {
// Arrays
var array [2]int
newArray := array
array[0] = 3
newArray[1] = 2
fmt.Printf("Arrays:\narray: %v\nnewArray: %v\n\n", array, newArray)
// Slices (using copy())
slice := make([]int, 2)
newSlice := make([]int, len(slice))
copy(newSlice, slice)
slice[0] = 3
newSlice[1] = 2
fmt.Printf("Slices (different arrays):\nslice (%p): %v \nnewSlice (%p): %v\n\n", slice, slice, newSlice, newSlice)
// Slices (same underlying array)
slice2 := make([]int, 2)
newSlice2 := slice2
slice2[0] = 3
newSlice2[1] = 2
fmt.Printf("Slices (same array):\nslice2 (%p): %v \nnewSlice2 (%p): %v\n\n", slice2, slice2, newSlice2, newSlice2)
}
Output:
Arrays:
array: [3 0]
newArray: [0 2]
Slices (different arrays):
slice (0xc000100040): [3 0]
newSlice (0xc000100050): [0 2]
Slices (same array):
slice2 (0xc000100080): [3 2]
newSlice2 (0xc000100080): [3 2]
Go Playground
use copy function.
oldArray := make([]int, len(array))
copy(oldArray, array)
https://play.golang.org/p/DsLJ2PDIy_N
A basic question that I'm struggling to find an answer for as there are a lot of answers about how to join two slices using the append function and the spread operator which erroneously use the word 'array'.
I am new to Go and have made the assumption that using sized arrays is good practice where the size is known. However I am struggling to work with arrays as I can't figure out how to do simple operations such as concatenation. Here is some code.
var seven [7]int
five := [5]int{1,2,3,4,5}
two := [2]int{6,7}
//this doesn't work as both the inputs and assignment are the wrong type
seven = append(five,two)
//this doesn't work as the assignment is still the wrong type
seven = append(five[:],two[:])
//this works but I'm not using arrays anymore so may as well use slices everywhere and forget sizing
seven2 := append(five[:],two[:])
As far as I can see I can either just give up on arrays and use slices exclusively or I could write a loop to explicitly construct the new array. Is there a third option?
append() can only be used to append elements to a slice. If you have an array, you can't pass that directly to append().
What you may do is slice the array, so you get a slice (which will use the array as its backing store), and you can use that slice as the target and source of elements.
For example:
s := seven[:0]
s = append(s, five[:]...)
s = append(s, two[:]...)
fmt.Println(seven)
This will print (try it on the Go Playground):
[1 2 3 4 5 6 7]
Also note that since append() returns the resulting slice, it's possible to write all this in one line:
_ = append(append(seven[:0], five[:]...), two[:]...)
(Storing the result is not needed here because we have and want to use only the backing array, but in general that is not the case.)
This outputs the same, try it on the Go Playground. Although this isn't very readable, so it's not worth compacting it into a single line.
Although when you have the target array, "appending" arrays is nothing more than copying them to the target, to the proper position. For that, you may use the builtin copy() function too. Note that the copy() function also accepts only slices, so you have to slice the arrays here too.
copy(seven[:], five[:])
copy(seven[len(five):], two[:])
fmt.Println(seven)
This will output the same. Try this one on the Go Playground.
You can use copy
copy(seven[:], five[:])
copy(seven[5:], two[:])
fmt.Printf("%v\n", seven)
> [1 2 3 4 5 6 7]
You can concatenate two arrays in go using copy function
package main
import "fmt"
func main() {
five := [5]int{1, 2, 3, 4, 5}
two := [2]int{6, 7}
var n [len(five) + len(two)]int
copy(n[:], five[:])
copy(n[len(five):], two[:])
fmt.Println(n)
}
https://blog.golang.org/go-slices-usage-and-internals
Golang runtime used to check whether current index exceeds the maximum possible.
On the side of array, it look ups its type (which contain its len and reference to the element type), because that's type, that can be registered only at compile time.
// each array mention with unique size creates new type
array := [5]byte{1,2,3,4,5}
On the side of slice, it look ups their header which looks like:
type slice {
data *byte
len int
cap int // capacity, the maximum possible index
}
As you can see, any slice is a single structure with data and len, cap fields, meanwhile array is just single pointer to data (*byte).
When you trying to convert array to slice, it just creates slice header and fills fields with:
slice := array[:]
==
slice := Slice{}
slice.data = array
slice.len = type_of(array).len
slice.cap = type_of(array).len
you can do that simply by converting array into slice:
arr1 := [...]int {1,2,3,}
arr2 := [...]int {4,5,6, }
//arr3 = arr1 + arr2 // not allowed
// converting arrays into slice
slc_arr1, slc_arr2 := arr1[:], arr2[:]
slc_arr3 := make([]int, 0)
slc_arr3 = append(slc_arr1, slc_arr2...)
fmt.Println(slc_arr3) // [1 2 3 4 5 6]
There is a more general way of appending an array of any type(once Golang has generics, but for now this solution is specific to strings. Just change the type as appropriate). The notion of Fold comes from Functional Programming. Note I have also included a filter function which also uses Fold. The solution is not stack safe but in many cases that does not matter. It can be made stack safe with trampolining. At the end is an example of its usage.
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)
}
func AppendStrings(as1, as2 []string) []string {
var g = func(h string, accum []string) []string {
return append(accum, h)
}
return FoldRightStrings(as1, as2, g)
}
func TestAppendStringArrays(t *testing.T) {
strings := []string{"a","b","c"}
bigarray := AppendStrings(AppendStrings(strings, strings),AppendStrings(strings, strings))
if diff := deep.Equal(bigarray, []string{"a","b","c","c","b","a","a","b","c","c","b","a"}); diff != nil {
t.Error(diff)
}
}
I have an array of T wrapped in an interface. I know the size of the array beforehand. How do I write a generic function that gets back an array (or a slice) for any array length? E.g. for size 3 I want something like
var values interface{} = [3]byte{1, 2, 3}
var size = 3 // I know the size
var _ = values.([size]byte) // wrong, array bound must be a const expression
I can't really do a type switch because [1]byte is a different type from [2]byte etc so I'd have to explicitly enumerate all possible sizes.
Reflect is your friend here:
package main
import (
"fmt"
"reflect"
)
func main() {
var in interface{} = [3]byte{1, 2, 3} // an element from your []interface{}
var size = 3 // you got this
out := make([]byte, size) // slice output
for i := 0; i < size; i++ {
idxval := reflect.ValueOf(in).Index(i) // magic here
uidxval := uint8(idxval.Uint()) // you may mess around with the types here
out[i] = uidxval // and dump in output
}
fmt.Printf("%v\n", out)
}
Slices are the better choice output here, since you indicate that you have an undefined length.
What Magic here does is indexing the value of your input interface through reflect. This is not quick, but it does the trick.
I just started playing with Go. I started creating a function that accepts an array of integers and returning the chunks of that array. To see what I mean, here is that program:
package main
import (
"fmt"
"math"
)
func main() {
a:= []int{1,2,3,4,5,6,2, 231, 521,21, 51}
c:=chunks(a[:], 3)
fmt.Println(c) // [[1 2 3] [4 5 6] [2 231 521] [51]]
}
func chunks(a []int, size int) [][]int{
var f float64 = float64(len(a)) / float64(size)
size_of_wrapper := int(math.Ceil(f))
i := 0
j := 0
twoD := make([][]int, size_of_wrapper )
for i < len(a) {
if i + size < len(a) {
twoD[j] = make([]int, size)
twoD[j] = append(a[i:i+size])
i = i + size
j++
} else {
if i + size == len(a){
i++
} else {
twoD[j] = make([]int, 1)
twoD[j] = append(a[len(a)-1:])
i++
}
}
}
return twoD
}
Now, I have a question. Can I turn this function to be able to receive an array that has strings in it or any other types? Also, can I set to return that same type at the end? In this case, I return an array of arrays that contain only integer values.
I seem to really struggle to find the solution to this problem. One of the posts that I have read recommended to use interface type for this kind of job. Is that the only one?
Can I turn this function to be able to receive an array that has strings in it or any other types?
Also, can I set to return that same type at the end? I
Unfortunately the Go programming language does not have generics. AFAIK Go will have generics in the next 2.0 release so right now you mainly have 3 maybe 4 options.
1. Using the []interface{} type
Your function declaration will look something like this.
func chunks(a []interface{}, size int) [][]interface{}
Using this approach will imply some type assertion.
2. Using the reflect package
The definition will look the same but this time in your implementation, instead of using the type assertion technique you will use the reflect package to determine your type and retrieve values. Remember you will pay a reasonable cost using this approach.
3. Using some unsafe.Pointer magic.
You could use this generic pointer type and do some pointer arithmetic in a C spirit way.
func chunks(a unsafe.Pointer, len uintptr, size uintptr) unsafe.Pointer
I know if you search Go does not oficially support doing pointer arithmetic but you can fake it using tricks like.
package main
import "fmt"
import "unsafe"
func main() {
vals := []int{10, 20, 30, 40}
start := unsafe.Pointer(&vals[0])
size := unsafe.Sizeof(int(0))
for i := 0; i < len(vals); i++ {
item := *(*int)(unsafe.Pointer(uintptr(start) + size*uintptr(i)))
fmt.Println(item)
}
}
4. Try using go generate in your building phase
You can find more information on how to generate go code base on what types you provide.
Because Go does not yet have the generics, the options are to use reflection or to duplicate code for each type. Here's how to use the reflect package:
// chunkAny slices a into size chunks and puts result in cp.
// The variable cp must be pointer to slice of a's type.
func chunkAny(a interface{}, size int, cp interface{}) {
av := reflect.ValueOf(a)
cv := reflect.ValueOf(cp).Elem()
cv.SetLen(0) // reset length in case backing array allocated
i, j := 0, size
for j < av.Len() {
cv.Set(reflect.Append(cv, av.Slice(i, j)))
i += size
j += size
}
cv.Set(reflect.Append(cv, av.Slice(i, av.Len())))
}
Call the function like this:
a := []string{"a", "b", "c", "d"}
var b [][]string
chunkAny(a, 3, &b)
Run it on the playground.
Having read the following at http://golang.org/doc/effective_go.html#arrays...
Arrays are values. Assigning one array to another copies all the
elements.
In particular, if you pass an array to a function, it will receive a copy
of the array, not a pointer to it.
... I expect in the following code that arr2 to be distinct from arr, and main()'s arr to be distinct from shuffle()'s arr. Can someone please explain why the following code shuffles arr2? I know Go is still a young language; perhaps the treatment of arrays has changed?
package main
import (
"fmt"
"rand"
"time"
)
func shuffle(arr []int) {
rand.Seed(time.Nanoseconds())
for i := len(arr) - 1; i > 0; i-- {
j := rand.Intn(i)
arr[i], arr[j] = arr[j], arr[i]
}
}
func main() {
arr := []int{1, 2, 3, 4, 5}
arr2 := arr
shuffle(arr)
for _, i := range arr2 {
fmt.Printf("%d ", i)
}
}
I think your problem is that you're confusing arrays and slices.
Arrays are fixed-length lists of values. You're actually not using any arrays in your example. Arrays can be declared a few ways:
arr1 := [3]int{1, 2, 3} // an array of 3 integers, 1-3
arr2 := [...]int{1, 2, 3} // same as the previous line, but we're letting
// the compiler figure out the size of the array
var arr3 [3]int // a zeroed out array of 3 integers
You're using slices. A slice is a reference to an underlying array. There are a few ways to allocate new slices:
slice1 := []int{1, 2, 3} // a slice of length 3 containing the integers 1-3
slice2 := make([]int, 3) // a slice of length 3 containing three zero-value integers
slice3 := make([]int, 3, 5) // a slice of length 3, capacity 5 that's all zeroed out
Any other slice assignments are just duplicating a reference to an array.
Now that we've established that, the line
arr := []int{1, 2, 3, 4, 5}
creates a slice referencing an anonymous underlying array that contains the numbers 1-5.
arr2 := arr
duplicates that reference -- it does not copy the underlying array. So there's one underlying array and two references to it. That's why the contents of arr2 change when you modify the contents of arr. They're referencing the same array.