Variadic Functions

Variadic Functions

Beginner·2 min read

Fast recall

Quick revision

Core answer

func sum(nums ...int) — the function receives nums as a SLICE ([]int). Call with any count: sum(1,2,3), sum(), sum(slice...) (spread a slice). The ... is the variadic marker, on the parameter and on the spread.

The variadic parameter collects arguments into a slice:

func sum(nums ...int) int {
    total := 0
    for _, n := range nums { total += n }
    return total
}

sum(1, 2, 3)   // 6 — three separate args, collected into []int{1,2,3}
sum()           // 0 — zero args, empty slice
sum([]int{4, 5}...)  // 9 — spread an existing slice

Executed. Inside the function, nums IS a []int — the same loop, indexing, len, slicing as any slice.

The rules: exactly ONE variadic parameter, and it must be the LAST. func f(a string, nums ...int) is legal (fixed args first); func f(nums ...int, a string) is a compile error.

The spread operator (...) — the same symbol, different position: in the declaration it means 'collect'; at the call site after a slice it means 'expand.' The pair composes: variadic collection + spread = passing slices through.

Interview-ready guide

Interview answer

The variadic parameter collects arguments into a slice:

func sum(nums ...int) int {
    total := 0
    for _, n := range nums { total += n }
    return total
}

sum(1, 2, 3)   // 6 — three separate args, collected into []int{1,2,3}
sum()           // 0 — zero args, empty slice
sum([]int{4, 5}...)  // 9 — spread an existing slice

Executed. Inside the function, nums IS a []int — the same loop, indexing, len, slicing as any slice.

The rules: exactly ONE variadic parameter, and it must be the LAST. func f(a string, nums ...int) is legal (fixed args first); func f(nums ...int, a string) is a compile error.

03 · Deep dive

About 2 min with examples

Direct Answer

Deep Dive & Architecture

What it's for: printf-shaped APIs (fmt.Println takes ...interface{} — any count of anything), option patterns, builders. What it's NOT: the only way to pass slices — the spread form means a slice-based call is always available.

The interface{} variadic: func log(args ...interface{}) — any count of any type. fmt's whole API is built on this — and so is every print-shaped helper you've written.

Common Mistakes & Pitfalls

Giving a definition without explaining a practical trade-off.

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The variadic parameter collects arguments into a slice:

func sum(nums ...int) int {
    total := 0
    for _, n := range nums { total += n }
    return total
}

sum(1, 2, 3)   // 6 — three separate args, collected into []int{1,2,3}
sum()           // 0 — zero args, empty slice
sum([]int{4, 5}...)  // 9 — spread an existing slice

Executed. Inside the function, nums IS a []int — the same loop, indexing, len, slicing as any slice.

The rules: exactly ONE variadic parameter, and it must be the LAST. func f(a string, nums ...int) is legal (fixed args first); func f(nums ...int, a string) is a compile error.