dongmu5596 2013-03-19 07:54
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Golang对类型数组中的切片字段的引用

I'm a Python programmer enjoying Go. The thing that trips me up must often is the use of references; I have mastered it (mostly) but then on occasion something baffles me, such as this one.

I have a simple type ('Fixture'):

type Fixture struct {
    Probabilities      *[]float64
}

If I populate the Probabilities field for a single instance of this type, everything is fine:

c := appengine.NewContext(r)
f := Fixture{}
p := []float64{}
p = append(p, 0.5)
p = append(p, 0.2)
p = append(p, 0.3)
f.Probabilities = &p
c.Infof("%v", *f.Probabilities)

2013/03/19 07:37:36 INFO: [0.5 0.2 0.3]

However if I try and populate this field for an array of these types, the code compiles but the field value is always nil:

c := appengine.NewContext(r)
fixtures := []Fixture{}
f := Fixture{}
fixtures = append(fixtures, f)
for _, f := range fixtures {
    p := []float64{}
    p = append(p, 0.5)
    p = append(p, 0.2)
    p = append(p, 0.3)
    f.Probabilities = &p
}
for _, f := range fixtures {
    // c.Infof("%v", *f.Probabilities) // causes error
    c.Infof("%v", f.Probabilities)
}

2013/03/19 07:37:41 INFO: <nil>

I guess I'm not understanding how arrays/slices work, particularly with respect to references. Can anyone point out where I'm going wrong ?

Thanks!

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2条回答 默认 最新

  • doujiao1538 2013-03-19 08:25
    关注

    In the range statement

    for _, f := range fixtures { ... }
    

    f is a newly declared local variable of type Fixture. It's not a reference to anything. So after setting up its values, it has to be put into to the fixtures slice.

    package main
    
    import "fmt"
    
    type Fixture struct {
            Probabilities *[]float64
    }
    
    func main() {
            fixtures := []Fixture{}
            f := Fixture{}
            fixtures = append(fixtures, f)
            for i, f := range fixtures {
                    p := []float64{}
                    p = append(p, 0.5)
                    p = append(p, 0.2)
                    p = append(p, 0.3)
                    f.Probabilities = &p
                    fixtures[i] = f
            }
            for _, f := range fixtures {
                    fmt.Printf("%v", f.Probabilities)
            }
    }
    

    (Also here)


    Output

    &[0.5 0.2 0.3]
    
    本回答被题主选为最佳回答 , 对您是否有帮助呢?
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