Returning the lenght of a vector idiomatically

Question

I'm writing a function that returns a sequence of numbers of variable length:

func fib(n int) ??? {
    retval := ???
    a, b := 0, 1
    for ; n > 0; n-- {
        ??? // append a onto retval here
        c := a + b
        a = b
        b = c
    }
}

It can be observed that the final length of the returned sequence will be n. How and what should fib return to achieve idiomatic Go? If the length was not known in advance, how would the return value, and usage differ? How do I insert values into retval?

Answer 1

Here, we know how many numbers; we want n Fibonacci numbers.

package main

import "fmt"

func fib(n int) (f []int) {
    if n < 0 {
        n = 0
    }
    f = make([]int, n)
    a, b := 0, 1
    for i := 0; i < len(f); i++ {
        f[i] = a
        a, b = b, a+b
    }
    return
}

func main() {
    f := fib(7)
    fmt.Println(len(f), f)
}

Output: 7 [0 1 1 2 3 5 8]

---

Here, we don't know how many numbers; we want all the Fibonacci numbers less than or equal to n.

package main

import "fmt"

func fibMax(n int) (f []int) {
    a, b := 0, 1
    for a <= n {
        f = append(f, a)
        a, b = b, a+b
    }
    return
}

func main() {
    f := fibMax(42)
    fmt.Println(len(f), f)
}

Output: 10 [0 1 1 2 3 5 8 13 21 34]

---

You could also use IntVector from the Go vector package. Note that type IntVector []int.

Answer 2

Don't use Vectors, use slices. Here are some mapping of various vector operations to idiomatic slice operations.