I read go slice usage and internals and Slice and Effective go#slice but there is nothing about slicing a slice with 3 number like this : slice[a:b:c]
For example this code :
package main
import "fmt"
func main() {
var s = []string{"a", "b", "c", "d", "e", "f", "g"}
fmt.Println(s[1:2:6], len(s[1:2:6]), cap(s[1:2:6]))
fmt.Println(s[1:2:5], len(s[1:2:5]), cap(s[1:2:5]))
fmt.Println(s[1:2], len(s[1:2]), cap(s[1:2]))
}
go playground result is this :
[b] 1 5
[b] 1 4
[b] 1 6
I can understand that the third one is something about capacity, but what is the exact meaning of this?
Do I miss something in documents?
The syntax has been introduced in Go 1.2, as I mentioned in "Re-slicing slices in Golang".
It is documented in Full slice expressions:
a[low : high : max]
constructs a slice of the same type, and with the same length and elements as the simple slice expression a[low : high].
Additionally, it controls the resulting slice's capacity by setting it to max - low.
Only the first index may be omitted; it defaults to 0.
After slicing the array a:
a := [5]int{1, 2, 3, 4, 5}
t := a[1:3:5]
the slice t has type []int, length 2, capacity 4, and elements
t[0] == 2
t[1] == 3
The design document for that feature had the following justification:
It would occasionally be useful, for example in custom []byte allocation managers, to be able to hand a slice to a caller and know that the caller cannot edit values beyond a given subrange of the true array.
The addition of append to the language made this somewhat more important, because append lets programmers overwrite entries between len and cap without realizing it or even mentioning cap.
2022: svitanok adds for Go 1.19+:
while the capacity of a "derivative" slice doesn't exceed the one specified by the third index during its creation the slice is still "from" the same spot in the memory as its original ("true") slice, so the changes applied to it will affect the original slice.
And if then, for example, you append to this derivative slice the amount of elements that would cause its capacity to be increased, this new slice will occupy a different place in the memory, and so the changes made to it will not affect the slice it originated from.
In a slice expression slice[a:b:c] or aSlice[1:3:5]
a:b or 1:3 -> gives length
a:c or 1:5 -> gives capacity
We can extract both length and capacity from a slice expression with 3 numbers/indices, without looking at the source slice/array.
expression| aSlice[low:high:max] or aSlice[a:b:c] or aSlice[1:3:7]
------------------------------------------------------------------------
Length | len(aSlice[low:high]) or len(aSlice[a:b]) or len(aSlice[1:3])
Capacity | len(aSlice[low:max]) or len(aSlice[a:c]) or len(aSlice[1:7])
------------------------------------------------------------------------
Read more here at Slice Expressions
Playground
Actually Go slice have a pointer and pointing to the array and it holds length and capacity of the array and we can show it like will be
pointer:length:capacity
and append is used for adding same new length.
sl1 := make([]int, 6)
fmt.Println(sl1)
sl2 := append(sl1, 1)
fmt.Println(sl2)
[0 0 0 0 0 0]
[0 0 0 0 0 0 1]
Related
I have the following multivariate array:
x := [2][3]int{
{3, 2, 1},
{3, 2, 1},
}
Both rows and columns are fixed-size.
I'm trying to check that the rows are sorted, and I undertand the sort function requires arrays without a known size. How can I ask the go to treat the item with a fixed-, known size as if it had an unknown size?
var allTrue bool = true
for i := range x {
t := sort.Ints(x[i]) == []int{1, 2, 3}
allTrue = allTrue && t
}
I get:
./main.go:xx:yy: sort.Ints(x[i]) used as value
./main.go:xx:yy: cannot use x[i] (type [3]int) as type []int in argument to sort.Ints
Am I reading this error message correctly?
Notwithstanding this other answer, which provides guidance regarding appropriate slicing to use the sort package, I have added this answer to provide more description on some further issues with the code you have posted. I hope this aids your understanding of the Go language.
Introduction to Slices
I undertand the sort function requires arrays without a known size [sic]
As others have said, this is not the terminology to describe this concept in Go. All Go arrays are of fixed size, as defined by the language spec. As you know, arrays are of type [N]T for an array which contains some non-negative number N of elements of type T. This is fixed at compile time and never changes during runtime of your program.
"Arrays without a known size" most closely maps to slices. Slices are distinct types in Go which allow for representation of sequences of data of a particular type, where their length is managed dynamically by the Go runtime. They are of type []T for elements of type T. In particular, their size is not part of their type definition and can change at runtime. For some slice variable x []T, the implementation provides:
an internal backing array of similar elemental type, where the implementation manages the allocation of memory and expansion of the array as the slice length increases
its length len(x) – denoting the number of elements the slice currently contains
its capacity cap(x) – the total length of the slice plus the additional extent of the backing array, which may extend beyond the length due to slicing operations restricting the view on the array or a larger array being allocated by the runtime to allow for appending more items to the slice.
See the Tour of Go and the language spec on slices for more details.
Resolving the problem with the code
As noted above, slices are of distinct type to arrays, so you cannot use something of type [N]T for some N and T where something of type []T is required.
sort.Ints sorts a slice of integers in-place – it has type signature func Ints(a []int). Your call sort.Ints(x[i]) indexes the array x at index i, which will return an array of type [3]int. This is incompatible with the sort function and leads to the compile-time error you observe.
To obtain a slice from an array, you use a slice expression. Such expressions allow arrays, slices and some other types to be used to construct new slices.
Slice expressions are given in the form a[low : high] where low and high are optional integers providing indices into the backing array or slice which specify the range to return in the new slice. The language spec link above has more details which I recommend you read; suffice to say the simplest slice expression a[:] for some array or slice a is syntactic sugar to mean a[0:len(a)-1], i.e. transform the array/slice into a slice of the same length.
Using this trick, obtain a slice of type []int from your multi-dimensional array by slicing: x[i][:]:
x[i] returns an array of type [3]int, as before
slicing the returned array returns a slice of type []int, which is compatible with sort.Ints.
sort.Ints does not return a value, and slices are not comparable
Even if you fix these issues with your code, there remain two issues with the following line:
t := sort.Ints(x[i]) == []int{1, 2, 3}
sort.Ints sorts in-place; it does not return a value, so the equality test is meaningless.
sort.Ints operates on slices, which are not comparable. It is not possible to call A == B where either A or B is a slice, unless either A or B is the special identifier nil. This is a subtle point which is covered in the language spec. (Aside: read that page, as you will note arrays are comparable.)
As you cannot compare slices directly using the == equality operator, verifying element-wise equality of slices requires:
Slices to be of the same length (dissimilar lengths implies one slice has more elements than the other)
Elements at each index of one slice are identical to other slices.
(I am ignoring the fact that one slice may have a dissimilar capacity to another, as we only care about the element-wise equality.)
This can be verified by looping through one of the slices and verifying elements at each index correspond at the same index in the other slice. This example code provides an example of that (playground link):
package main
import (
"fmt"
)
func CheckEquality(a, b []int) bool {
// Slices of dissimilar length are not equal
if len(a) != len(b) {
return false
}
for i, el := range a {
if b[i] != el {
return false
}
}
return true
}
func main() {
var mySlice = []int{1, 2, 3, 4, 5}
var mySlice2 = []int{1, 2, 3, 4, 5} // same as mySlice
var otherSlice = []int{5, 6, 7, 8, 9} // dissimilar slice
var longSlice = []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
fmt.Println(CheckEquality(mySlice, mySlice2)) // Expect true
fmt.Println(CheckEquality(mySlice, otherSlice)) // Expect false
fmt.Println(CheckEquality(mySlice, longSlice)) // Expect false
}
You have to slice the array with the [:] operator before you can use it with the sort package. See: A Tour of Go: Slices and the "sort" package godoc.
Also, you can check whether the slice is sorted more efficiently than sorting it, using sort.IntsAreSorted([]int), like this.
var allTrue bool = true
for i := range x {
if !sort.IntsAreSorted(x[i][:]) {
allTrue = false
break
}
}
You can get a slice from an array by using the [:] operator, e.g.:
arr := [3]int{1, 2, 3}
slc := arr[:] // []int{1, 2, 3}, backed by arr
As such, you could use the sort.Ints(...) function:
sort.Ints(x[0][:])
sort.Ints(x[1][:])
// Now both elements of "x" are sorted.
I'm taking the tour on Golang site, and I'm trying to digest one of the examples. It is unclear how it works:
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
printSlice(s)
// Slice the slice to give it zero length.
s = s[:0]
printSlice(s)
// Extend its length.
s = s[:4]
printSlice(s)
// Drop its first two values.
s = s[2:]
printSlice(s)
}
func printSlice(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}
The output is:
len=6 cap=6 [2 3 5 7 11 13]
len=0 cap=6 []
len=4 cap=6 [2 3 5 7]
len=2 cap=4 [5 7]
After the first slice, s = s[:0] the slice length is 0. Then there is another slicing of s = s[:4]. Although the length is 0, this seems to work. But how this happens? Shouldn't the underlaying array be in accessible from s?
What confuses me more is, the next time we slice it, s = s[2:] we slice the old value of s (which is 4 elements) and not the original array.
Can someone shed some lights what is the difference between the two cases?
A slice is basically a pointer to memory with some additional information:
1) the number of elements currently used and
2) the capacity, i.e. the remaining length it can occupy.
At the start we create a slice with 6 integers, this makes go create the underlying int array with a total size of 6 as well.
here is your memory locations with addresses (content does not matter here)
* * * * * *
[0][1][2][3][4][5]
^
s points to the start of the memory
len(s) = 6
cap(s) = 6
Next we say: make this slice's len be 0, this is the s = s[:0] which takes a sub-slice of s at position 0 with length 0. Note that s[0:0] is the same, you can omit the first 0.
[0][1][2][3][4][5]
^
s still points to the start of the memory
len(s) = 0
cap(s) = 6
Since the capacity is still the same, we might as well make the length 4 by saying s = s[:4].
* * * *
[0][1][2][3][4][5]
^
s still points to the start of the memory
len(s) = 4
cap(s) = 6
Then we take a sub-slice that does not start at the beginning of the memory by doing s = s[2:].
* *
[0][1][2][3][4][5]
^
s now points to the original address plus two!
len(s) = 2
cap(s) = 4
Leon addressed me to the Go's blog post, where they address exactly my question.
This is the snippet which helped me better understanding this concept:
A slice is a descriptor of an array segment. It consists of a pointer to the array, the length of the segment, and its capacity (the maximum length of the segment).
A slice cannot be grown beyond its capacity. Attempting to do so will cause a runtime panic, just as when indexing outside the bounds of a slice or array. Similarly, slices cannot be re-sliced below zero to access earlier elements in the array.
Slices can be extended if the array has more elements in it, but it can not access elements below 0 of the slice. It's a window to the underlaying array. The blog post explains it in more depth.
I can't seem to wrap my head around the notation of indices when working with Go slices.
Given a slice s.
s := []int{1, 2, 3, 4, 5}
I now want to create a new slice s2 = [2 3].
s2 := s[1:3] // s2 = [2 3]
Now, what is the thought process that I should go through when accessing this value? Am I reading values starting from index 1 up to and including the third element of the slice? Or am I reading values from index 1 up to and excluding index 3?
I am not starting at index 1 and going up to index 3 and neither am I starting at position 1 and going up to position 3 as both of these would result in s2 having 3 elements.
What is the idea behind this notation?
Relevant section from the spec: Slice expressions.
For a string, array, pointer to array, or slice a, the primary expression
a[low : high]
constructs a substring or slice. The indices low and high select which elements of operand a appear in the result. The result has indices starting at 0 and length equal to high - low.
So s2 := s[1:3] creates a new slice with length 3 - 1 = 2, so it will contain 2 elements: s[1] and s[2].
When slicing a slice, low should be the index of the first element you want to include (inclusive), and high should be the index of the last element that will not be incuded (high is exclusive).
So if you want the result to include the elements [2, 3], you need to provide slicing indices 1 and 3:
s2 := s[1:3] // will be [2, 3]
What might be confusing is that the elements in your slice start with 1, but the index starts with 0.
For reasoning behind the inclusive-exclusive indices, see related question: In a Go slice, why does s[lo:hi] end at element hi-1?
The capacity parameter in making a slice in Go does not make much sense to me. For example,
aSlice := make([]int, 2, 2) //a new slice with length and cap both set to 2
aSlice = append(aSlice, 1, 2, 3, 4, 5) //append integers 1 through 5
fmt.Println("aSlice is: ", aSlice) //output [0, 0, 1, 2, 3, 4, 5]
If the slice allows inserting more elements than the capacity allows, why do we need to set it in the make() function?
The builtin append() function uses the specified slice to append elements to if it has a big enough capacity to accomodate the specified elements.
But if the passed slice is not big enough, it allocates a new, big enough slice, copies the elements from the passed slice to the new slice and append the elements to that new slice. And returns this new slice. Quoting from the append() documentation:
The append built-in function appends elements to the end of a slice. If it has sufficient capacity, the destination is resliced to accommodate the new elements. If it does not, a new underlying array will be allocated. Append returns the updated slice. It is therefore necessary to store the result of append, often in the variable holding the slice itself:
When making a slice with make if the length and capacity are the same, the capacity can be omitted, in which case it is defaulted to the specified length:
// These 2 declarations are equivalent:
s := make([]int, 2, 2)
s := make([]int, 2)
Also note that append() appends elements after the last element of the slice. And the above slices already have len(s) == 2 right after declaration so if you append even just 1 element to it, it will cause a reallocation as seen in this example:
s := make([]int, 2, 2)
fmt.Println(s, len(s), cap(s))
s = append(s, 1)
fmt.Println(s, len(s), cap(s))
Output:
[0 0] 2 2
[0 0 1] 3 4
So in your example what you should do is something like this:
s := make([]int, 0, 10) // Create a slice with length=0 and capacity=10
fmt.Println(s, len(s), cap(s))
s = append(s, 1)
fmt.Println(s, len(s), cap(s))
Output:
[] 0 10
[1] 1 10
I recommend the following blog articles if you want to understand slices in more details:
Go Slices: usage and internals
Arrays, slices (and strings): The mechanics of 'append'
It is mainly an optimization, and it is not unique to go, similar structures in other languages have this as well.
When you append more than the capacity, the runtime needs to allocate more memory for the new elements. This is costly and can also cause memory fragmentation.
By specifying the capacity, the runtime allocates what is needed in advance, and avoids reallocations. However if you do not know the estimated capacity in advance or it changes, you do not have to set it, and the runtime reallocates what is needed and grows the capacity itself.
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According to the Tour of Go, in a Go slice s, the expression s[lo:hi] evaluates to a slice of the elements from lo through hi-1, inclusive:
package main
import "fmt"
func main() {
p := []int{0, // slice position 0
10, // slice position 1
20, // slice position 2
30, // slice position 3
40, // slice position 4
50} // slice position 5
fmt.Println(p[0:3]) // => [0 10 20]
}
In my code example above, "p[0:3]" would seem to intuitively "read" as: "the slice from position 0 to position 3", equating to [0, 10, 20, 30]. But of course, it actually equates to [0 10 20].
So my question is: what is the design rationale for the upper value evaluating to hi-1 rather than simply hi? It feels unintuitive, but there must be some reason for it that I'm missing, and I'm curious what that might be.
Thanks in advance.
This is completely a matter of convention, and there are certainly other ways to do it (for example, Matlab uses arrays whose first index is 1). The choice really comes down to what properties you want. As it turns out, using 0-indexed arrays where slicing is inclusive-exclusive (that is, a slice from a to b includes element a and excludes element b) has some really nice properties, and thus it's a very common choice. Here are a few advantages.
Advantages of 0-indexed arrays and inclusive-exclusive slicing
(note that I'm using non-Go terminology, so I'll talk about arrays in the way that C or Java would talk about them. Arrays are what Go calls slices, and slices are sub-arrays (ie, "the slice from index 1 to index 4"))
Pointer arithmetic works. If you're in a language like C, arrays are really just pointers to the first element in the array. Thus, if you use 0-indexed arrays, then you can say that the element at index i is just the element pointed at by the array pointer plus i. For example, if we have the array [3 2 1] with the address of the array being 10 (and assuming that each value takes up one byte of memory), then the address of the first element is 10 + 0 = 10, the address of the second is 10 + 1 = 11, and so on. In short, it makes the math simple.
The length of a slice is also the place to slice it. That is, for an array arr, arr[0:len(arr)] is just arr itself. This comes in handy a lot in practice. For example, if I call n, _ := r.Read(arr) (where n is the number of bytes read into arr), then I can just do arr[:n] to get the slice of arr corresponding to the data that was actually written into arr.
Indices don't overlap. This means that if I have arr[0:i], arr[i:j], arr[j:k], arr[k:len(arr)], these slices fully cover arr itself. You may not often find yourself partitioning an array into sub-slices like this, but it has a number of related advantages. For example, consider the following code to split an array based on non-consecutive integers:
func consecutiveSlices(ints []int) [][]int {
ret := make([][]int, 0)
i, j := 0, 1
for j < len(ints) {
if ints[j] != ints[j-1] + 1 {
ret = append(ret, ints[i:j])
i = j
}
}
ret = append(ret, ints[i:j])
}
(this code obviously doesn't handle some edge cases well, but you get the idea)
If we were to try to write the equivalent function using inclusive-inclusive slicing, it would be significantly more complicated.
If anyone can think of any more, please feel free to edit this answer and add them.
The Go Programming Language Specification
Slice types
Slice expressions
For a string, array, pointer to array, or slice a, the primary
expression
a[low : high]
constructs a substring or slice. The indices low and high select which
elements of operand a appear in the result. The result has indices
starting at 0 and length equal to high - low.
For convenience, any of the indices may be omitted. A missing low
index defaults to zero; a missing high index defaults to the length of
the sliced operand
For arrays or strings, the indices are in range if 0 <= low <= high <=
len(a), otherwise they are out of range. For slices, the upper index
bound is the slice capacity cap(a) rather than the length. A constant
index must be non-negative and representable by a value of type int;
for arrays or constant strings, constant indices must also be in
range. If both indices are constant, they must satisfy low <= high. If
the indices are out of range at run time, a run-time panic occurs.
For q := p[m:n], q is a slice of p starting at index m for a length of n-m elements.