Given two arrays or slices for eg:
a := []int{1, 2, 3, 4, 5}
b := []int{3, 4, 5, 6, 7, 8, 9}
The slices may not be sorted and order doesn't matter.
What is the most efficient way to compute values such that you end up with the common elements of both slices, and the remainder of elements present in one but not the other i.e for the two arrays given above the return values would be:
common := []int{3, 4, 5}
inAButNotB := []int{1, 2}
inBButNotA := []int{6, 7, 8, 9}
Its easy to compute the intersection converting one slice into a map and then iterating over the one to see if values exist. Is there a way to compute the other two values within the same loop?
O(len(a) + len(b)) is efficient. For example,
package main
import (
"fmt"
)
func main() {
a := []int{1, 2, 3, 4, 5}
b := []int{3, 4, 5, 6, 7, 8, 9}
fmt.Println(a)
fmt.Println(b)
m := make(map[int]uint8)
for _, k := range a {
m[k] |= (1 << 0)
}
for _, k := range b {
m[k] |= (1 << 1)
}
var inAAndB, inAButNotB, inBButNotA []int
for k, v := range m {
a := v&(1<<0) != 0
b := v&(1<<1) != 0
switch {
case a && b:
inAAndB = append(inAAndB, k)
case a && !b:
inAButNotB = append(inAButNotB, k)
case !a && b:
inBButNotA = append(inBButNotA, k)
}
}
fmt.Println(inAAndB)
fmt.Println(inAButNotB)
fmt.Println(inBButNotA)
}
Playground: https://play.golang.org/p/RvGaC9Wfjiv
Output:
[1 2 3 4 5]
[3 4 5 6 7 8 9]
[3 4 5]
[1 2]
[8 6 7 9]
The Go Programming Language Specification
& bitwise AND integers | bitwise OR integers ^ bitwise XOR integers &^ bit clear (AND NOT) integers << left shift integer << unsigned integer >> right shift integer >> unsigned integer
We have 8 bits for uint8. Bit 0 (1 << 0, 1 shift left 0) is a and bit 1 (1 << 1; 1 shift left 1) is b. For uint8 bits, 00000001 is a, 00000010 is b, 00000011 is a and b, and 00000000 is nether a nor b. The | operator sets a bit, the & operator reads a bit.
The Go Programming Language Specification
A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keys of another type, called the key type.
The comparison operators == and != must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice. If the key type is an interface type, these comparison operators must be defined for the dynamic key values; failure will cause a run-time panic.
The algorithm works for any slice type whose elements can be a map key. The comparison operators == and != must be fully defined for operands of the key type.