Sorting array of structs (missing Len method)

I have two structs defined as below:

// DuplicatedAd format
type DuplicatedAd struct {
	// Ad Identifier
	AdID int `form:"ad_id" json:"ad_id,omitempty" xml:"ad_id"`
	// Parameters of an Ad, can be empty
	AdParams string `form:"ad_params" json:"ad_params,omitempty" xml:"ad_params"`
	// Body of the Ad
	Body string `form:"body" json:"body,omitempty" xml:"body"`
	// Body Duplicate Percentage
	BodyScore float64 `form:"body_score" json:"body_score,omitempty" xml:"body_score"`
	// Category of the Ad
	Category int `form:"category" json:"category,omitempty" xml:"category"`
	// The ad uploaded main image
	Image string `form:"image,omitempty" json:"image,omitempty" xml:"image,omitempty"`
	// Ad Listing ID
	ListID int `form:"list_id" json:"list_id,omitempty" xml:"list_id"`
	// Listing time for the ad
	ListTime string `form:"list_time" json:"list_time,omitempty" xml:"list_time"`
	// the ad Phone number
	Phone string `form:"phone,omitempty" json:"phone,omitempty" xml:"phone,omitempty"`
	// Region of the Ad
	Region int `form:"region" json:"region,omitempty" xml:"region"`
	// Ad Status
	Status string `form:"status" json:"status,omitempty" xml:"status"`
	// Subject of the Ad
	Subject string `form:"subject" json:"subject,omitempty" xml:"subject"`
	// Subject Duplicate Percentage
	SubjectScore float64 `form:"subject_score" json:"subject_score,omitempty" xml:"subject_score"`
	// Type of the Ad
	Type string `form:"type" json:"type,omitempty" xml:"type"`
	// Total Score
	TotalScore float64 `form:"total_score" json:"total_score,omitempty" xml:"total_score"`
}

// CpResponse format
type CpResponse struct {
	// Number of doublets
	Count int `json:"count"`
	// The Doublet Ads to show
	Duplicated []DuplicatedAd `json:"duplicated"`
}

i'm defining a variable var DuplicatedAds = CpResponse{}and in a loop i'm appending some results to it as follow:

DuplicatedAds.Duplicated = append(DuplicatedAds.Duplicated, t)
DuplicatedAds.Count = len(DuplicatedAds.Duplicated)

i wish that the DuplicatedAds.Duplicated be sorted by TotalScore.

i'm new to golang, so after a little research i found out that i must implement theses three functions before i can sort

func (slice DuplicatedAd) Len() int {
	return len(slice)
}

func (slice DuplicatedAd) Less(i, j int) bool {
	return slice[i].TotalScore < slice[j].TotalScore
}

func (slice DuplicatedAd) Swap(i, j int) {
	slice[i], slice[j] = slice[j], slice[i]
}

but when i try to compile i get this errors:

./duplicates.go:165: cannot use DuplicatedAds.Duplicated (type []DuplicatedAd) as type sort.Interface in argument to sort.Sort:
	[]DuplicatedAd does not implement sort.Interface (missing Len method)
./types.go:68: invalid argument slice (type DuplicatedAd) for len
./types.go:72: invalid operation: slice[i] (type DuplicatedAd does not support indexing)
./types.go:72: invalid operation: slice[j] (type DuplicatedAd does not support indexing)
./types.go:76: invalid operation: slice[i] (type DuplicatedAd does not support indexing)
./types.go:76: invalid operation: slice[j] (type DuplicatedAd does not support indexing)
./types.go:76: invalid operation: slice[i] (type DuplicatedAd does not support indexing)

How could i have an CpResponse object which his Duplicated array is sorted by DuplicatedAd.TotalScore ?

Starting from Go 1.8, you can use the following function to sort your slice:

sort.Slice(DuplicatedAds.Duplicated, func(i int, j int) bool {
    return DuplicatedAds.Duplicated[i].TotalScore < DuplicatedAds.Duplicated[j].TotalScore
})

You don't need to define those three functions up there anymore (Len, Less and Swap). Notice how we used the same implementation of Len as a parameter of sort.Slice.

cd1 offers a good solution. As for the reason your methods aren't working, it's because they're operating on a single struct and not a slice of structs.

You would want something more like this where each method works on the DuplicatedAdSlice type rather than the DuplicatedAd type.

type DuplicatedAdSlice []DuplicatedAd // Type alias

func (slice DuplicatedAdSlice) Len() int {
    return len(slice)
}

And update the CpResponse struct to use the type alias

// The Doublet Ads to show
Duplicated DuplicatedAdSlice `json:"duplicated"`