In C/C++ (and many languages of that family), a common idiom to declare and initialize a variable depending on a condition uses the ternary conditional operator :
int index = val > 0 ? val : -val
Go doesn't have the conditional operator. What is the most idiomatic way to implement the same piece of code as above ? I came to the following solution, but it seems quite verbose
var index int
if val > 0 {
index = val
} else {
index = -val
}
Is there something better ?
As pointed out (and hopefully unsurprisingly), using if+else is indeed the idiomatic way to do conditionals in Go.
In addition to the full blown var+if+else block of code, though, this spelling is also used often:
index := val
if val <= 0 {
index = -val
}
and if you have a block of code that is repetitive enough, such as the equivalent of int value = a <= b ? a : b, you can create a function to hold it:
func min(a, b int) int {
if a <= b {
return a
}
return b
}
...
value := min(a, b)
The compiler will inline such simple functions, so it's fast, more clear, and shorter.
No Go doesn't have a ternary operator. Using if/else syntax is the idiomatic way.
Why does Go not have the ?: operator?
There is no ternary testing operation in Go. You may use the following to achieve the same result:
if expr {
n = trueVal
} else {
n = falseVal
}
The reason ?: is absent from Go is that the language's designers had seen the operation used too often to create impenetrably complex expressions. The if-else form, although longer, is unquestionably clearer. A language needs only one conditional control flow construct.
— Frequently Asked Questions (FAQ) - The Go Programming Language
Suppose you have the following ternary expression (in C):
int a = test ? 1 : 2;
The idiomatic approach in Go would be to simply use an if block:
var a int
if test {
a = 1
} else {
a = 2
}
However, that might not fit your requirements. In my case, I needed an inline expression for a code generation template.
I used an immediately evaluated anonymous function:
a := func() int { if test { return 1 } else { return 2 } }()
This ensures that both branches are not evaluated as well.
The map ternary is easy to read without parentheses:
c := map[bool]int{true: 1, false: 0} [5 > 4]
Foreword: Without arguing that if else is the way to go, we can still play with and find pleasure in language-enabled constructs.
Go 1.18 generics update: Go 1.18 adds generics support. It is now possible to create a generic If() function like this. Note: This is available in github.com/icza/gog, as gog.If() (disclosure: I'm the author).
func If[T any](cond bool, vtrue, vfalse T) T {
if cond {
return vtrue
}
return vfalse
}
Which you can use like this:
min := If(i > 0, i, 0)
The pre-1.18 answer follows:
The following If construct is available in my github.com/icza/gox library with lots of other methods, being the gox.If type.
Go allows to attach methods to any user-defined types, including primitive types such as bool. We can create a custom type having bool as its underlying type, and then with a simple type conversion on the condition, we have access to its methods. Methods that receive and select from the operands.
Something like this:
type If bool
func (c If) Int(a, b int) int {
if c {
return a
}
return b
}
How can we use it?
i := If(condition).Int(val1, val2) // Short variable declaration, i is of type int
|-----------| \
type conversion \---method call
For example a ternary doing max():
i := If(a > b).Int(a, b)
A ternary doing abs():
i := If(a >= 0).Int(a, -a)
This looks cool, it's simple, elegant, and efficient (it's also eligible for inlining).
One downside compared to a "real" ternary operator: it always evaluates all operands.
To achieve deferred and only-if-needed evaluation, the only option is to use functions (either declared functions or methods, or function literals), which are only called when / if needed:
func (c If) Fint(fa, fb func() int) int {
if c {
return fa()
}
return fb()
}
Using it: Let's assume we have these functions to calculate a and b:
func calca() int { return 3 }
func calcb() int { return 4 }
Then:
i := If(someCondition).Fint(calca, calcb)
For example, the condition being current year > 2020:
i := If(time.Now().Year() > 2020).Fint(calca, calcb)
If we want to use function literals:
i := If(time.Now().Year() > 2020).Fint(
func() int { return 3 },
func() int { return 4 },
)
Final note: if you would have functions with different signatures, you could not use them here. In that case you may use a function literal with matching signature to make them still applicable.
For example if calca() and calcb() would have parameters too (besides the return value):
func calca2(x int) int { return 3 }
func calcb2(x int) int { return 4 }
This is how you could use them:
i := If(time.Now().Year() > 2020).Fint(
func() int { return calca2(0) },
func() int { return calcb2(0) },
)
Try these examples on the Go Playground.
func Ternary(statement bool, a, b interface{}) interface{} {
if statement {
return a
}
return b
}
func Abs(n int) int {
return Ternary(n >= 0, n, -n).(int)
}
This will not outperform if/else and requires cast but works. FYI:
BenchmarkAbsTernary-8 100000000 18.8 ns/op
BenchmarkAbsIfElse-8 2000000000 0.27 ns/op
If all your branches make side-effects or are computationally expensive the following would a semantically-preserving refactoring:
index := func() int {
if val > 0 {
return printPositiveAndReturn(val)
} else {
return slowlyReturn(-val) // or slowlyNegate(val)
}
}(); # exactly one branch will be evaluated
with normally no overhead (inlined) and, most importantly, without cluttering your namespace with a helper functions that are only used once (which hampers readability and maintenance). Live Example
Note if you were to naively apply Gustavo's approach:
index := printPositiveAndReturn(val);
if val <= 0 {
index = slowlyReturn(-val); // or slowlyNegate(val)
}
you'd get a program with a different behavior; in case val <= 0 program would print a non-positive value while it should not! (Analogously, if you reversed the branches, you would introduce overhead by calling a slow function unnecessarily.)
As others have noted, golang does not have a ternary operator or any equivalent. This is a deliberate decision thought to improve readability.
This recently lead me to a scenario where constructing a bit-mask in a very efficient manner became hard to read when written idiomatically, or very inefficient when encapsulated as a function, or both, as the code produces branches:
package lib
func maskIfTrue(mask uint64, predicate bool) uint64 {
if predicate {
return mask
}
return 0
}
producing:
text "".maskIfTrue(SB), NOSPLIT|ABIInternal, $0-24
funcdata $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
funcdata $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
movblzx "".predicate+16(SP), AX
testb AL, AL
jeq maskIfTrue_pc20
movq "".mask+8(SP), AX
movq AX, "".~r2+24(SP)
ret
maskIfTrue_pc20:
movq $0, "".~r2+24(SP)
ret
What I learned from this was to leverage a little more Go; using a named result in the function (result int) saves me a line declaring it in the function (and you can do the same with captures), but the compiler also recognizes this idiom (only assign a value IF) and replaces it - if possible - with a conditional instruction.
func zeroOrOne(predicate bool) (result int) {
if predicate {
result = 1
}
return
}
producing a branch-free result:
movblzx "".predicate+8(SP), AX
movq AX, "".result+16(SP)
ret
which go then freely inlines.
package lib
func zeroOrOne(predicate bool) (result int) {
if predicate {
result = 1
}
return
}
type Vendor1 struct {
Property1 int
Property2 float32
Property3 bool
}
// Vendor2 bit positions.
const (
Property1Bit = 2
Property2Bit = 3
Property3Bit = 5
)
func Convert1To2(v1 Vendor1) (result int) {
result |= zeroOrOne(v1.Property1 == 1) << Property1Bit
result |= zeroOrOne(v1.Property2 < 0.0) << Property2Bit
result |= zeroOrOne(v1.Property3) << Property3Bit
return
}
produces https://go.godbolt.org/z/eKbK17
movq "".v1+8(SP), AX
cmpq AX, $1
seteq AL
xorps X0, X0
movss "".v1+16(SP), X1
ucomiss X1, X0
sethi CL
movblzx AL, AX
shlq $2, AX
movblzx CL, CX
shlq $3, CX
orq CX, AX
movblzx "".v1+20(SP), CX
shlq $5, CX
orq AX, CX
movq CX, "".result+24(SP)
ret
eold's answer is interesting and creative, perhaps even clever.
However, it would be recommended to instead do:
var index int
if val > 0 {
index = printPositiveAndReturn(val)
} else {
index = slowlyReturn(-val) // or slowlyNegate(val)
}
Yes, they both compile down to essentially the same assembly, however this code is much more legible than calling an anonymous function just to return a value that could have been written to the variable in the first place.
Basically, simple and clear code is better than creative code.
Additionally, any code using a map literal is not a good idea, because maps are not lightweight at all in Go. Since Go 1.3, random iteration order for small maps is guaranteed, and to enforce this, it's gotten quite a bit less efficient memory-wise for small maps.
As a result, making and removing numerous small maps is both space-consuming and time-consuming. I had a piece of code that used a small map (two or three keys, are likely, but common use case was only one entry) But the code was dog slow. We're talking at least 3 orders of magnitude slower than the same code rewritten to use a dual slice key[index]=>data[index] map. And likely was more. As some operations that were previously taking a couple of minutes to run, started completing in milliseconds.\
One-liners, though shunned by the creators, have their place.
This one solves the lazy evaluation problem by letting you, optionally, pass functions to be evaluated if necessary:
func FullTernary(e bool, a, b interface{}) interface{} {
if e {
if reflect.TypeOf(a).Kind() == reflect.Func {
return a.(func() interface{})()
}
return a
}
if reflect.TypeOf(b).Kind() == reflect.Func {
return b.(func() interface{})()
}
return b
}
func demo() {
a := "hello"
b := func() interface{} { return a + " world" }
c := func() interface{} { return func() string { return "bye" } }
fmt.Println(FullTernary(true, a, b).(string)) // cast shown, but not required
fmt.Println(FullTernary(false, a, b))
fmt.Println(FullTernary(true, b, a))
fmt.Println(FullTernary(false, b, a))
fmt.Println(FullTernary(true, c, nil).(func() string)())
}
Output
hello
hello world
hello world
hello
bye
Functions passed in must return an interface{} to satisfy the internal cast operation.
Depending on the context, you might choose to cast the output to a specific type.
If you wanted to return a function from this, you would need to wrap it as shown with c.
The standalone solution here is also nice, but could be less clear for some uses.
Now with the release of go1.18 generics, it's very easy to do it with a generic function like this, and it is reusable through your whole app
package main
import (
"fmt"
)
func Ternary[T any](condition bool, If, Else T) T {
if condition {
return If
}
return Else
}
func main() {
fmt.Println(Ternary(1 < 2, "yes", "no")) // yes
fmt.Println(Ternary(1 < 2, 1, 0)) // 1
fmt.Println(Ternary[bool](1 < 2, true, false)) // true
}
be aware if you use it in this case it will crash.
in this case, just use an if statement,
(because you passing into the function a nil pointer VS an if statement is not calling that section if it is false)
var a *string
fmt.Println(Ternary(a != nil, *a, "some thing else"))
the solution call it with a function, so it will not be excuted if it's false
func TernaryPointer[T any](condition bool, If, Else func() T) T {
if condition {
return If()
}
return Else()
}
var pString *string
fmt.Println(TernaryPointer(
pString != nil, // condition
func() string { return *pString }, // true
func() string { return "new data" }, // false
))
but in this case, I think a regular if statement is cleaner (except if go adds arrow functions in the future)
playground
give credit for this answer he already answered it
I have compiled some items and compared the speed.
/*
go test ternary_op_test.go -v -bench="^BenchmarkTernaryOperator" -run=none -benchmem
*/
package _test
import (
"testing"
)
func BenchmarkTernaryOperatorIfElse(b *testing.B) {
for i := 0; i < b.N; i++ {
if i%2 == 0 {
_ = i
} else {
_ = -i
}
}
}
// https://stackoverflow.com/a/45886594/9935654
func Ternary(statement bool, a, b interface{}) interface{} {
if statement {
return a
}
return b
}
func BenchmarkTernaryOperatorTernaryFunc(b *testing.B) {
for i := 0; i < b.N; i++ {
_ = Ternary(i%2 == 0, i, -i).(int)
}
}
// https://stackoverflow.com/a/34636594/9935654
func BenchmarkTernaryOperatorWithFunc(b *testing.B) {
for i := 0; i < b.N; i++ {
_ = func() int {
if i%2 == 0 {
return i
} else {
return -i
}
}
}
}
// https://stackoverflow.com/a/31483763/9935654
func BenchmarkTernaryOperatorMap(b *testing.B) {
for i := 0; i < b.N; i++ {
_ = map[bool]int{true: i, false: -i}[i%2 == 0]
}
}
output
goos: windows
goarch: amd64
cpu: Intel(R) Core(TM) i7-8565U CPU # 1.80GHz
BenchmarkTernaryOperatorIfElse
BenchmarkTernaryOperatorIfElse-8 1000000000 0.4460 ns/op 0 B/op 0 allocs/op
BenchmarkTernaryOperatorTernaryFunc
BenchmarkTernaryOperatorTernaryFunc-8 1000000000 0.3602 ns/op 0 B/op 0 allocs/op
BenchmarkTernaryOperatorWithFunc
BenchmarkTernaryOperatorWithFunc-8 659517496 1.642 ns/op 0 B/op 0 allocs/op
BenchmarkTernaryOperatorMap
BenchmarkTernaryOperatorMap-8 13429532 82.48 ns/op 0 B/op 0 allocs/op
PASS
ok command-line-arguments 4.365s
One more suggestion for the idiomatic approach in Go of ternary operator:
package main
import (
"fmt"
)
func main() {
val := -5
index := func (test bool, n, d int) int {
if test {
return n
}
return d
}(val > 0, val, -val)
fmt.Println(index)
}
Go Playground
I was playing with a solution that doesn't use the three arguments function.
Don't take me wrong, the three arguments solution works great but personally i like to name things explicitly.
What i'd love is an explicit interface like that:
When(<condition>).Then(<true value>).Else(<false value>)
I implemented this like that:
type Else[T any] interface {
ElseDo(fn func() T) T
Else(value T) T
}
type Then[T any] interface {
ThenDo(fn func() T) Else[T]
Then(value T) Else[T]
}
type Condition[T any] struct {
condition bool
thenValue T
thenFn func() T
}
func When[T any](condition bool) Then[T] {
return &Condition[T]{condition: condition}
}
func (c *Condition[T]) ThenDo(fn func() T) Else[T] {
c.thenFn = fn
return c
}
func (c *Condition[T]) Then(value T) Else[T] {
c.thenValue = value
return c
}
func (c *Condition[T]) ElseDo(fn func() T) T {
if c.condition {
return c.then()
}
return fn()
}
func (c *Condition[T]) Else(value T) T {
if c.condition {
return c.then()
}
return value
}
func (c *Condition[T]) then() T {
if c.thenFn != nil {
return c.thenFn()
}
return c.thenValue
}
Usage:
When[int](something == "expectedValue").Then(0).Else(1)
When[int](value > 0).Then(value).Else(1)
When[int](value > 0).ThenDo(func()int {return value * 4}).Else(1)
When[string](boolean == true).Then("it is true").Else("it is false")
Unfortunately i didn't find a way to get rid of the explicit type when calling the When function. The type is not automatically inferred by the return types of Then/Else 🤷♂️
Related
I'm making a program in Golang using the flag package, and I'm trying to check if more than one flag in a specific list is true. Right now, this is my solution:
List := 0
if *md5flag {
List++
}
if *sha1flag {
List++
}
if *sha256flag {
List++
}
if *sha512flag {
List++
}
if List > 1 {
// Do stuff
os.Exit(1)
}
Would there be an easier way to do this, and by using a single if statement?
If all you need is to check that at most one flag is set and exit immediately in that case, you can use a loop with short circuiting, though I'm not sure if it's more readable for just 4 values:
flags := []*bool{md5flag, sha1flag, sha256flag, sha512flag}
seenSetFlag := false
for _, f := range flags {
if *f {
if seenSetFlag {
os.Exit(1)
}
seenSetFlag = true
}
}
Go doesn't have any conversion from bool to int, so you need to use an if-statement in some way.
I would do something like this
package main
import (
"fmt"
)
func boolsToInt(flags... bool) int {
value := 0
for _, flag := range flags {
if flag {
value++
}
}
return value
}
func main() {
b1, b2, b3, b4 := false, true, true, false
fmt.Println(boolsToInt(b1, b2, b3, b4))
}
You can use a bitmask. But the bitmask approach is more effective if you set up your flags to be a bitmask to begin with, i.e. the flags should be integers that can be OR'ed together. bool in Go doesn't support | operator.
With a bitmask, to check if more than one bit is set you can use the trick x & (x - 1) != 0. This checks if x is a power of two, when it is not, more than one bit is set. (source)
type alg int
const (
md5flag alg = 1 << iota
sha1flag
sha256flag
sha512flag
)
func main() {
// clients can initialize the value with bitwise-OR
supportedAlgos := md5flag | sha256flag
// check if more than one is set
if supportedAlgos & (supportedAlgos - 1) != 0 {
// do stuff
}
}
If you can't refactor your code to use a bitmask you may still construct it from the individual bool pointers. But then the approach isn't much different than the one you have right now.
func toBitmask(bs ...*bool) int {
bitmask := 0
for i, b := range bs {
if b != nil && *b {
bitmask |= 1 << i
}
}
return bitmask
}
func main() {
list := toBitmask(md5flag, sha1flag, sha256flag, sha512flag)
if list & (list - 1) != 0 {
// do stuff
}
}
Playground: https://play.golang.org/p/PXK_1sS5ZxI
Is there any way to pass the custom function in the sort.Strings() to do the case-insensitive sorting on the list of strings?
data := []string{"A", "b", "D", "c"}
The output should be: A, b, c, D
The equivalent of the above requirement in Python is like :
li = sorted(data, key=lambda s: s.lower())
Do we have something like that in golang?
The translation of the Python code to Go is:
sort.Slice(data, func(i, j int) bool { return strings.ToLower(data[i]) < strings.ToLower(data[j]) })
Run it on the Go Playground.
This approach, like the Python code in the question, can allocate two strings for each comparison. The allocations are probably OK for the example in the question, but can be a problem in other scenarios.
To avoid allocations, compare the strings rune by rune:
func lessLower(sa, sb string) bool {
for {
rb, nb := utf8.DecodeRuneInString(sb)
if nb == 0 {
// The number of runes in sa is greater than or
// equal to the number of runes in sb. It follows
// that sa is not less than sb.
return false
}
ra, na := utf8.DecodeRuneInString(sa)
if na == 0 {
// The number of runes in sa is less than the
// number of runes in sb. It follows that sa
// is less than sb.
return true
}
rb = unicode.ToLower(rb)
ra = unicode.ToLower(ra)
if ra != rb {
return ra < rb
}
// Trim rune from the beginning of each string.
sa = sa[na:]
sb = sb[nb:]
}
}
⋮
sort.Slice(data, func(i, j int) bool { return lessLower(data[i], data[j]) })
Run it on the Go Playground.
Take a look at the collate package if you need to sort by language or culture specific sort orders.
The solution below is more verbose and more performant. The main difference is that in the other answers, using strings.ToLower at each comparison allocates some memory, and the code below takes care of comparing runes without creating any new string.
// lessCaseInsensitive compares s, t without allocating
func lessCaseInsensitive(s, t string) bool {
for {
if len(t) == 0 {
return false
}
if len(s) == 0 {
return true
}
c, sizec := utf8.DecodeRuneInString(s)
d, sized := utf8.DecodeRuneInString(t)
lowerc := unicode.ToLower(c)
lowerd := unicode.ToLower(d)
if lowerc < lowerd {
return true
}
if lowerc > lowerd {
return false
}
s = s[sizec:]
t = t[sized:]
}
}
sort.Slice(data, func(i, j int) bool { return lessCaseInsensitive(data[i], data[j]) })
You can see in this benchmark for example that avoiding allocs makes the case-insensitive sorting 5x faster.
You need a type that implements sort.Interface.
https://play.golang.org/p/JTm0AjuxCRV
I have a recursive function and I want one execute some statement only for the outermost call to the function. How do I achieve this functionality?
func fact(n int) int {
if n == 0 {
return 1
}
fact := n * fact(n-1)
if outer_most{
fmt.Printf(strconv.Itoa(n))
}
return fact
}
func main() {
fact(4)
}
This should print only 4
To answer the question itself: if for some reason you really want to run something that is only for outermost func call and don't want to change api, Golang has a runtime lib for that.
You may do it as:
package main
import (
"fmt"
"runtime"
"strconv"
)
func outer_most() bool {
pc:=make([]uintptr,2)
runtime.Callers(2,pc) //skip: 1 - runtime.Caller, 2 - outer_most itself
return runtime.FuncForPC(pc[0])!=runtime.FuncForPC(pc[1]) // test if the caller of the caller is the same func, otherwise it is the outermost
}
func fact(n int) int {
if n == 0 {
return 1
}
fact := n * fact(n-1)
if outer_most() {
fmt.Printf(strconv.Itoa(n))
}
return fact
}
func main() {
fact(4)
}
playground: https://play.golang.org/p/ro1ZOn6yIR7
It is no good practice, but solve the question most straightfowardly.
Note:
Use a global variable is very likely to cause glitches. You need to set it every time you call the func, and if there is concurency, data race would be involved.
And if you are unconfortable with the fact that the extra bool argument get allocated every recursive call (which could be countless time), you may look at #Adrian 's answer or wrap it as a method of a bool.
You could pass something like a depth that gets incremented at each call. Eg:
func fact(depth int, n int) int {
if n == 0 {
return 1
}
fact := n * fact(depth + 1, n-1)
if depth == 0 {
fmt.Println(fact) // I assume you meant to print fact here.
}
return fact
}
func main() {
fact(0, 4)
}
If this is really your use case, you're much better off doing:
func fact(n int) int {
if n == 0 {
return 1
}
return n * fact(n-1)
}
func main() {
fmt.Println(fact(4))
}
Answer to edited question:
You can use he second pattern below again:
func fact(n int, outerMost bool) int {
if n == 0 {
return 1
}
fact := n * fact(n-1, false)
if outerMost {
fmt.Printf(strconv.Itoa(n))
}
return fact
}
func main() {
fact(4, true)
}
Again, you can use closures or helper functions to clean this up.
Original answer:
Try using a global variable:
var outerMost bool = true
func fact(n int) int {
if outerMost {
fmt.Printf(strconv.Itoa(n))
outerMost = false
}
if n == 0 {
return 1
}
return n * fact(n-1)
}
func main() {
fact(4)
}
There are other ways to achieve this. For example, you can use similar technique with closures as well. Or add another parameter to fact:
func fact(n int, outerMost bool) int {
if outerMost {
fmt.Printf(strconv.Itoa(n))
outerMost = false
}
if n == 0 {
return 1
}
return n * fact(n-1, outerMost)
}
func main() {
fact(4, true)
}
A simple anonymous function is probably the cleanest, as it does not add a parameter, complicating the API for external callers just to implement internal logic.
func fact(n int) int {
var facto func(n int) int
facto = func(n int) int {
if n == 0 {
return 1
}
fact := n * facto(n-1)
return fact
}
n = facto(n)
fmt.Printf("%d", n)
return n
}
This achieves the same functionality, but the caller doesn't have to know to pass an additional bool or int value that's irrelevant to it. Full example here: https://play.golang.org/p/7vHwPDN2_FL
I imported the math library in my program, and I was trying to find the minimum of three numbers in the following way:
v1[j+1] = math.Min(v1[j]+1, math.Min(v0[j+1]+1, v0[j]+cost))
where v1 is declared as:
t := "stackoverflow"
v1 := make([]int, len(t)+1)
However, when I run my program I get the following error:
./levenshtein_distance.go:36: cannot use int(v0[j + 1] + 1) (type int) as type float64 in argument to math.Min
I thought it was weird because I have another program where I write
fmt.Println(math.Min(2,3))
and that program outputs 2 without complaining.
so I ended up casting the values as float64, so that math.Min could work:
v1[j+1] = math.Min(float64(v1[j]+1), math.Min(float64(v0[j+1]+1), float64(v0[j]+cost)))
With this approach, I got the following error:
./levenshtein_distance.go:36: cannot use math.Min(int(v1[j] + 1), math.Min(int(v0[j + 1] + 1), int(v0[j] + cost))) (type float64) as type int in assignment
so to get rid of the problem, I just casted the result back to int
I thought this was extremely inefficient and hard to read:
v1[j+1] = int(math.Min(float64(v1[j]+1), math.Min(float64(v0[j+1]+1), float64(v0[j]+cost))))
I also wrote a small minInt function, but I think this should be unnecessary because the other programs that make use of math.Min work just fine when taking integers, so I concluded this has to be a problem of my program and not the library per se.
Is there anything that I'm doing terrible wrong?
Here's a program that you can use to reproduce the issues above, line 36 specifically:
package main
import (
"math"
)
func main() {
LevenshteinDistance("stackoverflow", "stackexchange")
}
func LevenshteinDistance(s string, t string) int {
if s == t {
return 0
}
if len(s) == 0 {
return len(t)
}
if len(t) == 0 {
return len(s)
}
v0 := make([]int, len(t)+1)
v1 := make([]int, len(t)+1)
for i := 0; i < len(v0); i++ {
v0[i] = i
}
for i := 0; i < len(s); i++ {
v1[0] = i + 1
for j := 0; j < len(t); j++ {
cost := 0
if s[i] != t[j] {
cost = 1
}
v1[j+1] = int(math.Min(float64(v1[j]+1), math.Min(float64(v0[j+1]+1), float64(v0[j]+cost))))
}
for j := 0; j < len(v0); j++ {
v0[j] = v1[j]
}
}
return v1[len(t)]
}
Until Go 1.18 a one-off function was the standard way; for example, the stdlib's sort.go does it near the top of the file:
func min(a, b int) int {
if a < b {
return a
}
return b
}
You might still want or need to use this approach so your code works on Go versions below 1.18!
Starting with Go 1.18, you can write a generic min function which is just as efficient at run time as the hand-coded single-type version, but works with any type with < and > operators:
func min[T constraints.Ordered](a, b T) T {
if a < b {
return a
}
return b
}
func main() {
fmt.Println(min(1, 2))
fmt.Println(min(1.5, 2.5))
fmt.Println(min("Hello", "世界"))
}
There's been discussion of updating the stdlib to add generic versions of existing functions, but if that happens it won't be until a later version.
math.Min(2, 3) happened to work because numeric constants in Go are untyped. Beware of treating float64s as a universal number type in general, though, since integers above 2^53 will get rounded if converted to float64.
There is no built-in min or max function for integers, but it’s simple to write your own. Thanks to support for variadic functions we can even compare more integers with just one call:
func MinOf(vars ...int) int {
min := vars[0]
for _, i := range vars {
if min > i {
min = i
}
}
return min
}
Usage:
MinOf(3, 9, 6, 2)
Similarly here is the max function:
func MaxOf(vars ...int) int {
max := vars[0]
for _, i := range vars {
if max < i {
max = i
}
}
return max
}
For example,
package main
import "fmt"
func min(x, y int) int {
if x < y {
return x
}
return y
}
func main() {
t := "stackoverflow"
v0 := make([]int, len(t)+1)
v1 := make([]int, len(t)+1)
cost := 1
j := 0
v1[j+1] = min(v1[j]+1, min(v0[j+1]+1, v0[j]+cost))
fmt.Println(v1[j+1])
}
Output:
1
Though the question is quite old, maybe my package imath can be helpful for someone who does not like reinventing a bicycle. There are few functions, finding minimal of two integers: ix.Min (for int), i8.Min (for int8), ux.Min (for uint) and so on. The package can be obtained with go get, imported in your project by URL and functions referred as typeabbreviation.FuncName, for example:
package main
import (
"fmt"
"<Full URL>/go-imath/ix"
)
func main() {
a, b := 45, -42
fmt.Println(ix.Min(a, b)) // Output: -42
}
As the accepted answer states, with the introduction of generics in go 1.18 it's now possible to write a generic function that provides min/max for different numeric types (there is not one built into the language). And with variadic arguments we can support comparing 2 elements or a longer list of elements.
func Min[T constraints.Ordered](args ...T) T {
min := args[0]
for _, x := range args {
if x < min {
min = x
}
}
return min
}
func Max[T constraints.Ordered](args ...T) T {
max := args[0]
for _, x := range args {
if x > max {
max = x
}
}
return max
}
example calls:
Max(1, 2) // 2
Max(4, 5, 3, 1, 2) // 5
Could use https://github.com/pkg/math:
import (
"fmt"
"github.com/pkg/math"
)
func main() {
a, b := 45, -42
fmt.Println(math.Min(a, b)) // Output: -42
}
Since the issue has already been resolved, I would like to add a few words. Always remember that the math package in Golang operates on float64. You can use type conversion to cast int into a float64. Keep in mind to account for type ranges. For example, you cannot fit a float64 into an int16 if the number exceeds the limit for int16 which is 32767. Last but not least, if you convert a float into an int in Golang, the decimal points get truncated without any rounding.
If you want the minimum of a set of N integers you can use (assuming N > 0):
import "sort"
func min(set []int) int {
sort.Slice(set, func(i, j int) bool {
return set[i] < set[j]
})
return set[0]
}
Where the second argument to min function is your less function, that is, the function that decides when an element i of the passed slice is less than an element j
Check it out here in Go Playground: https://go.dev/play/p/lyQYlkwKrsA
I am trying to represent a hypergraph in memory. Are there any better data structures for this task beside nested matrices? A nested matrix is a matrix which can have elements of both the "native" type (let's say int for the sake of simplicity) and matrices.
This is the beginning of such a matrix. Are there any rough edges in the code, to make it look more idiomatic? How to make it look more idiomatic?
The code:
package main
import "fmt"
type Matricial interface {
Put(interface{}, ...int)
Get(...int) interface{}
}
type Matrix struct {
Matricial
values map[int]interface{}
}
func NewMatrix() *Matrix {
m := &Matrix{}
m.values = make(map[int]interface{})
return m
}
func (m *Matrix) Set(atom interface{}, pos ...int) {
firstdim := pos[0]
if val, ok := m.values[firstdim]; ok {
fmt.Println("map key exists", val)
switch converted := val.(type) {
case int:
m.values[firstdim] = converted
default:
fmt.Println("ERR: unknown type: %T", val)
}
} else {
if len(pos[1:]) > 0 {
newm := NewMatrix()
m.values[firstdim] = newm
newm.Set(atom, pos[1:]...)
} else {
m.values[firstdim] = atom
}
}
}
func (m *Matrix) Get(pos ...int) interface{} {
if len(pos) == 1 {
return m.values[pos[0]]
} else {
switch accessor := m.values[pos[0]].(type) {
case Matricial:
return accessor.Get(pos[1:]...)
default:
return nil
}
}
return nil
}
func main() {
m := NewMatrix()
m.Set(42, 2, 3, 4)
m.Set(43, 0)
fmt.Println(m.Get(2, 3))
fmt.Println(m.Get(2, 3, 4))
fmt.Println(m.Get(0))
}
The data structure must allow connecting hyperedges with other hyperedges (i.e. handling hyperedges as though they were nodes).
A nested matrix (adopting your definition of the term) seems a reasonable representation for hypergraph, not knowing anything more about your application anyway. An example Go implementation is the power set example at Rosetta code.
It is not idiomatic to embed an interface. For example, if you rename the Put method of Matricial to be Set, which is what I think you meant, then you can just delete the Matricial field of Matrix and your program produces the same output.