This question already has answers here:
What is a concise way to create a 2D slice in Go?
(4 answers)
Closed 2 years ago.
I created a 2D slice using the below code. Say I created [3]3] slice -- [[1 2 3],[4 5 6][7 8 9]]
But if I update the slice say s[1][1]=99 all changes --> [1 99 3], [4 99 6], [7 99 9]]
However, the second slice I have initialized below with variable cost does behave correctly. Not sure what is wrong:
func CreateSparseM() *SparseM{
var m,n,nz int
fmt.Println("Enter the row count of matrix ")
fmt.Scan(&m)
fmt.Println("Enter the column count of matrix ")
fmt.Scan(&n)
fmt.Println("Enter the count of Non Zero elements in the matrix ")
fmt.Scan(&nz)
r:=make([][]int,m)
c:=make([]int,n)
for i:=0;i<m;i++{
r[i] = c
}
fmt.Println(" r ", r)
r[1][1] = 99
fmt.Println(r[1][1])
fmt.Println(r[0][1])
//enter the non-zero elements
var row,col,elem int
for i:=0;i<nz;i++{
fmt.Println("Enter row ")
fmt.Scan(&row)
fmt.Println("Enter col ")
fmt.Scan(&col)
fmt.Println("Enter element ")
fmt.Scan(&elem)
r[row][col] = elem
}
fmt.Println(r)
cost:= [][]int{ {1,1,2,2,3,4,4,5,5},
{2,6,3,7,4,5,7,6,7},
{25,5,12,10,8,16,14,20,18}}
fmt.Println(cost)
cost[1][2]= 777
fmt.Println(cost)
sparseM := &SparseM{m,n,nz,r}
return sparseM
}
A slice contains a reference to an array, the capacity, and the length of the slice. So the following code:
r:=make([][]int,m)
c:=make([]int,n)
for i:=0;i<m;i++{
r[i] = c
}
sets all of r[i] to the same slice c. That is, all r[i] share the same backing array. So if you set r[i][j]=x, you set j'th element of all slices r[i] to x.
The slice you initialized using a literal has three distinct slices, so it does not behave like this.
If you do:
for i:=0;i<m;i++{
r[i] = make([]int,n)
}
then you'll have distinct slices for the first case as well.
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Closed 1 year ago.
The goal is to compute all possible slices of length k that can be formed from a set of n strings.
I tried to do it using a simple recursion. Just printing the result works fine. Putting the results in a map yields some unexpected results for odd k's greater than 2.
What causes differences between the map's keys and their corresponding values?
https://play.golang.org/p/_SGrPRFjJ5g
package main
import (
"fmt"
"strings"
)
func main() {
QQM := make(map[string][]string)
states := []string{
"a",
"b",
}
var perm func(Q []string, k int)
perm = func(Q []string, k int) {
if k == 0 {
QQM[strings.Join(Q, "")] = Q
fmt.Println(QQM)
return
}
for i := 0; i < len(states); i++ {
perm(append(Q, states[i]), k-1)
}
}
perm([]string{}, 4)
}
map[aaaa:[a a a a]]
map[aaaa:[a a a b] aaab:[a a a b]]
map[aaaa:[a a a b] aaab:[a a a b] aaba:[a a b a]]
...
A slice is a view of an underlying array. When you pass slices around, you are not passing the values they contain, but references to those values. So when you put a slice to a map then add elements to that slice, if the slice has the capacity, you would be adding elements to the slice in the map as well.
Copy the slice before putting into the map:
newQ:=make([]string,len(Q))
copy(newQ,Q)
QQM[strings.Join(Q, "")] = newQ
My code:
package main
import (
"fmt"
)
func main() {
a := [10]int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
b := a[1:4]
fmt.Println("a:", a)
fmt.Println("b:", b)
// Works fine even though c is indexing past the end of b.
c := b[4:7]
fmt.Println("c:", c)
// This fails with panic: runtime error: index out of range [4] with length 3
// d := b[4]
}
Output:
a: [0 1 2 3 4 5 6 7 8 9]
b: [1 2 3]
c: [5 6 7]
If I uncomment the line that contains d := b[4], it leads to this this error:
panic: runtime error: index out of range [4] with length 3
My question:
Why is it okay to access b[4:7] even though the index 4 is out of range for b which has a length 3 but it is not okay to access b[4]? What Go language rules explain this behavior?
Relevant rules: Spec: Index expressions and Spec: Slice expressions.
In short: when indexing, index must be less than the length. When slicing, upper index must be less than or equal to the capacity.
When indexing: a[x]
the index x is in range if 0 <= x < len(a), otherwise it is out of range
When slicing: a[low: high]
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.
When you do this:
b := a[1:4]
b will be a slice sharing the backing array with a, b's length will be 3 and its capacity will be 9. So later it is perfectly valid to slice b even beyond its length, up to its capacity which is 9. But when indexing, you can always index only the part covered by the slice's length.
We use indexing to access current elements of a slice or array, and we use slicing if we want to create a fragment of an array or slice, or if we want to extend it. Extending it means we want a bigger portion (but what is still covered by the backing array).
I'm dealing with a programming problem
Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
and with input n = 5, k = 4, the output should be [[1,2,3,4],[1,2,3,5],[1,2,4,5],[1,3,4,5],[2,3,4,5]], the following is my golang solution
func combine(n int, k int) [][]int {
result := [][]int{}
comb := []int{}
subcom(0, k, n, &comb, &result)
return result
}
func subcom(s, k, n int, comb *[]int, result *[][]int) {
if k > 0 {
for i := s + 1; i <= n-k+1; i++ {
c := append(*comb, i)
subcom(i, k-1, n, &c, result)
}
} else {
*result = append(*result, *comb)
}
}
I think my solution is right, but it return [[1 2 3 5] [1 2 3 5] [1 2 4 5] [1 3 4 5] [2 3 4 5]].
After debugging, I found [1 2 3 4] was added to the result slice at the beginning, but later changed to [1 2 3 5], resulting in the repetition of two [1 2 3 5]s. But I can't figure out what's wrong here.
This is a common mistake when using append.
When your code runs c:=append(*comb,i), it tries to first use the allocated memory in the underlying array to add a new item and only create a new slice when it failed to do so. This is what changes the [1 2 3 4] to [1 2 3 5] - because they share the same underlying memory.
To fix this, copy when you want to append into result:
now := make([]int,len(*comb))
copy(now,*comb)
*result = append(*result,now)
Or use a shortcut of copying:
*result = append(*result, append([]int{},*comb...))
Update:
To understand what I mean by underlying memory, one should understandd the internal model of Go's slice.
In Go, a slice has a data structure called SliceHeader which is accessible through reflect package and is what being referred to when you use unsafe.Sizeof and taking address.
The SliceHeader taking cares of three elements: Len,Cap and a Ptr. The fisrt two is trivail: they are what len() and cap() is for. The last one is a uintptr that points to the memory of the data the slice is containing.
When you shallow-copy a slice, a new SliceHeader is created but with the same content, including Ptr. So the underlying memory is not copied, but shared.
I'm trying to make a very simple program that modifies arrays, but ran across some interesting behavior if I converted them to types. https://play.golang.org/p/KC7mqmHuLw It appears that if I have an array go passes by reference, but if I have a type then go passes by value. Is this correct?
I have two variables b and c, both are arrays of 3 integers, but c is of type cT, in every other respect they should be identical. I can assign values as b[0]=-1 and c[0]=-1, but if I pass those arrays as parameters to a function they act very differently.
The output of the program is:
before b: [1 2 3]
before c: [1 2 3]
*after b: [-1 2 0]
*after c: [-1 2 3]
*what? c: [-1 2 0]
My initial assumption is that the lines "after b" and "after c" should have been the same. Am I doing something incorrectly or am I correct about types passing to functions by value (ie. creating copy of the variable before passing to the function)?
package main
import "fmt"
type cT [3]int
func main() {
b := []int{1, 2, 3}
c := cT{1, 2, 3}
fmt.Println("before b:", b)
fmt.Println("before c:", c)
b[0] = -1
c[0] = -1
mangleB(b) // ignore return value
mangleC(c) // ignore return value
fmt.Println("*after b:", b)
fmt.Println("*after c:", c)
c = mangleC(c)
fmt.Println("*what? c:", c)
}
func mangleB(row []int) []int {
row[2] = 0
return row
}
func mangleC(row cT) cT{
row[2] = 0
return row
}
The Go Programming Language Specification
Array types
An array is a numbered sequence of elements of a single type, called
the element type.
Slice types
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.
Calls
In a function call, the function value and arguments are evaluated in
the usual order. After they are evaluated, the parameters of the call
are passed by value to the function and the called function begins
execution. The return parameters of the function are passed by value
back to the calling function when the function returns.
type cT [3]int
b := []int{1, 2, 3}
c := cT{1, 2, 3}
I have two variables, b and c, both are arrays of 3 integers
No, you don't!
b is a slice of int with length (len(b)) 3 and capacity (cap(b)) 3, c is an array of (len(c)) 3 int.
In Go, all parameters are passed by value. b is passed as a slice descriptor, c is passed as an array. A slice descriptor is a struct with a slice length and capacity, and a pointer to the underlying array.
See comments:
func main() {
b := []int{1, 2, 3} // slice
c := cT{1, 2, 3} // array
fmt.Println("before b:", b)
fmt.Println("before c:", c)
b[0] = -1
c[0] = -1
// passing in a slice which you can think of as ref to array
// pass by value, and it is copy of ref to array
mangleB(b) // ignore return value
// passing in copy of array (pass by value)
// yes full shallow copy of array
mangleC(c) // ignore return value
// if you ignore return modifications are lost
fmt.Println("*after b:", b)
fmt.Println("*after c:", c)
// return value is modified array
c = mangleC(c)
// c now copy of array from line 24
fmt.Println("*what? c:", c)
}
when I'm calling slice a ref I'm simplifying details here https://blog.golang.org/go-slices-usage-and-internals
https://play.golang.org/p/OAaCMhc-Ug
What is the difference between cap and len of a slice in golang?
According to definition:
A slice has both a length and a capacity.
The length of a slice is the number of elements it contains.
The capacity of a slice is the number of elements in the underlying array, counting from the first element in the slice.
x := make([]int, 0, 5) // len(b)=0, cap(b)=5
Does the len mean non null values only?
A slice is an abstraction that uses an array under the covers.
cap tells you the capacity of the underlying array. len tells you how many items are in the array.
The slice abstraction in Go is very nice since it will resize the underlying array for you, plus in Go arrays cannot be resized so slices are almost always used instead.
Example:
s := make([]int, 0, 3)
for i := 0; i < 5; i++ {
s = append(s, i)
fmt.Printf("cap %v, len %v, %p\n", cap(s), len(s), s)
}
Will output something like this:
cap 3, len 1, 0x1040e130
cap 3, len 2, 0x1040e130
cap 3, len 3, 0x1040e130
cap 6, len 4, 0x10432220
cap 6, len 5, 0x10432220
As you can see once the capacity is met, append will return a new slice with a larger capacity. On the 4th iteration you will notice a larger capacity and a new pointer address.
Play example
I realize you did not ask about arrays and append but they are pretty foundational in understanding the slice and the reason for the builtins.
From the source code:
// The len built-in function returns the length of v, according to its type:
// Array: the number of elements in v.
// Pointer to array: the number of elements in *v (even if v is nil).
// Slice, or map: the number of elements in v; if v is nil, len(v) is zero.
// String: the number of bytes in v.
// Channel: the number of elements queued (unread) in the channel buffer;
// if v is nil, len(v) is zero.
func len(v Type) int
// The cap built-in function returns the capacity of v, according to its type:
// Array: the number of elements in v (same as len(v)).
// Pointer to array: the number of elements in *v (same as len(v)).
// Slice: the maximum length the slice can reach when resliced;
// if v is nil, cap(v) is zero.
// Channel: the channel buffer capacity, in units of elements;
// if v is nil, cap(v) is zero.
func cap(v Type) int
Simple explanation
Slice are self growing form of array so there are two main properties.
Length is total no of elements() the slice is having and can be used for looping through the elements we stored in slice. Also when we print the slice all elements till length gets printed.
Capacity is total no elements in underlying array, when you append more elements the length increases till capacity. After that any further append to slice causes the capacity to increase automatically(apprx double) and length by no of elements appended.
The real magic happens when you slice out sub slices from a slice where all the actual read/write happens on the underlaying array. So any change in sub slice will also change data both in original slice and underlying array. Where as any sub slices can have their own length and capacity.
Go through the below program carefully. Its modified version of golang tour example
package main
import "fmt"
func main() {
sorig := []int{2, 3, 5, 7, 11, 13}
printSlice(sorig)
// Slice the slice to give it zero length.
s := sorig[:0]
printSlice(s)
// Extend its length.
s = s[:4]
s[2] = 555
printSlice(s)
// Drop its first two values.
s = s[2:]
printSlice(s)
printSlice(sorig)
}
func printSlice(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
//Output
//len=6 cap=6 [2 3 5 7 11 13]
//len=0 cap=6 []
//len=4 cap=6 [2 3 555 7]
//len=2 cap=4 [555 7]
//len=6 cap=6 [2 3 555 7 11 13]