What is the idiomatic Go equivalent of C's ternary operator?

Question

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 ?

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

The map ternary is easy to read without parentheses:

c := map[bool]int{true: 1, false: 0} [5 > 4]

Answer 5

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

Answer 6

Foreword: Without arguing that if else is the way to go, we can still play with and find pleasure in language-enabled constructs.

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.

Answer 7

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.)

Answer 8

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.\

Answer 9

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.

Answer 10

As others have noted, golang does not have a ternary operator or any equivalent. This is a deliberate decision thought to intend readability.

This recently lead me to a scenario constructing a bit-mask in a very efficient manner became hard to read when written idiomatically because it took up a lot of lines of screen, 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