how to implement the python `zip` function in golang?

Question

Sometimes, it's convenient to combine two lists into a tuple using zip built-in function in Python. How to make this similarly in Go?

For example:

>>> zip ([1,2],[3,4])
[(1,3), (2,4)]

Answer 1

You could do something like this, where you give the tuple type a name:

package main

import "fmt"

type intTuple struct {
    a, b int
}

func zip(a, b []int) ([]intTuple, error) {

    if len(a) != len(b) {
        return nil, fmt.Errorf("zip: arguments must be of same length")
    }

    r := make([]intTuple, len(a), len(a))

    for i, e := range a {
        r[i] = intTuple{e, b[i]}
    }

    return r, nil
}

func main() {
    a := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 0}
    b := []int{0, 9, 8, 7, 6, 5, 4, 3, 2, 1}
    fmt.Println(zip(a, b))
}

Or alternatively use an unnamed type for the tuple, like this:

package main

import "fmt"

func zip(a, b []int) ([][3]int, error) {

    if len(a) != len(b) {
        return nil, fmt.Errorf("zip: arguments must be of same length")
    }

    r := make([][4]int, len(a), len(a))

    for i, e := range a {
        r[i] = [2]int{e, b[i]}
    }

    return r, nil
}

func main() {
    a := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 0}
    b := []int{0, 9, 8, 7, 6, 5, 4, 3, 2, 1}
    fmt.Println(zip(a, b))
}

And finally here's a soft-generic way of doing it:

package main

import (
    "fmt"
    "reflect"
)

func zip(a, b, c interface{}) error {

    ta, tb, tc := reflect.TypeOf(a), reflect.TypeOf(b), reflect.TypeOf(c)

    if ta.Kind() != reflect.Slice || tb.Kind() != reflect.Slice || ta != tb {
        return fmt.Errorf("zip: first two arguments must be slices of the same type")
    }

    if tc.Kind() != reflect.Ptr {
        return fmt.Errorf("zip: third argument must be pointer to slice")
    }

    for tc.Kind() == reflect.Ptr {
        tc = tc.Elem()
    }

    if tc.Kind() != reflect.Slice {
        return fmt.Errorf("zip: third argument must be pointer to slice")
    }

    eta, _, etc := ta.Elem(), tb.Elem(), tc.Elem()

    if etc.Kind() != reflect.Array || etc.Len() != 2 {
        return fmt.Errorf("zip: third argument's elements must be an array of length 2")
    }

    if etc.Elem() != eta {
        return fmt.Errorf("zip: third argument's elements must be an array of elements of the same type that the first two arguments are slices of")
    }

    va, vb, vc := reflect.ValueOf(a), reflect.ValueOf(b), reflect.ValueOf(c)

    for vc.Kind() == reflect.Ptr {
        vc = vc.Elem()
    }

    if va.Len() != vb.Len() {
        return fmt.Errorf("zip: first two arguments must have same length")
    }

    for i := 0; i < va.Len(); i++ {
        ea, eb := va.Index(i), vb.Index(i)
        tt := reflect.New(etc).Elem()
        tt.Index(0).Set(ea)
        tt.Index(1).Set(eb)
        vc.Set(reflect.Append(vc, tt))
    }

    return nil
}

func main() {

    a := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 0}
    b := []int{0, 9, 8, 7, 6, 5, 4, 3, 2, 1}
    c := [][2]int{}

    e := zip(a, b, &c)

    if e != nil {
        fmt.Println(e)
        return
    }

    fmt.Println(c)
}

Answer 2

To zip some number of slice []int lists,

package main

import "fmt"

func zip(lists ...[]int) func() []int {
    zip := make([]int, len(lists))
    i := 0
    return func() []int {
        for j := range lists {
            if i >= len(lists[j]) {
                return nil
            }
            zip[j] = lists[j][i]
        }
        i++
        return zip
    }
}

func main() {
    a := []int{1, 2, 3}
    b := []int{4, 5, 6}
    c := []int{7, 8, 9, 0}
    iter := zip(a, b, c)
    for tuple := iter(); tuple != nil; tuple = iter() {
        fmt.Println("tuple:", tuple)
    }
}

Output:

tuple: [1 4 7]
tuple: [2 5 8]
tuple: [3 6 9]