使用Golang将包含结构的映射作为值导出到csv

Question

I have a map which has a struct as value, as below:

type Record struct {
    ID   int
    Type string
    Year string
}

m := make(map[int]Record)

Once I populate this map with some records, I'm trying to export them as a csv with:

file, err := os.Create("export.csv")
checkError("Error:", err)
defer file.Close()
writer := csv.NewWriter(file)
defer writer.Flush()

for key, value := range m {
    r := make([]string, 0, 1+len(value))
    r = append(r, key)
    r = append(r, value)
}
writer.Flush()

I'm getting the error invalid argument value (type Record) for len. How should I handle the struct here to? Do I have to convert it to string somehow?

Answer 1

There are a few issues here:

1. writer.Write takes an argument of type []string so trying to append anything other than a string to r (which I assume is a shorthand representation of record) will not work.
2. You cannot get the length of a struct. See the builtin documentation for more info about the types you can pass in to len.

I've made a few modifications to your code:

• provide headers for the csv for ease of readability
• update r to accept capacity of len(headers), equals 4- I wasn't sure what you wanted the capacity to be but you can easily update this to be (probably 3, not 4).
• convert ID field to string in order to add to []string for writer.Write

Test here in Go Playground.

Example:

type Record struct {
    ID   int
    Type string
    Year string
}

m := make(map[int]Record)

// populate this map with some records

file, err := os.Create("export.csv")
checkError("Error:", err)
defer file.Close()
writer := csv.NewWriter(file)
defer writer.Flush()

// define column headers
headers := []string{
    "id",
    "type",
    "year",
}

// write column headers
writer.Write(headers)

var idString string

for key := range m {

    r := make([]string, 0, 1+len(headers)) // capacity of 4, 1 + the number of properties your struct has & the number of column headers you are passing

    // convert the Record.ID to a string in order to pass into []string
    idString = strconv.Itoa(m[key].ID)

    r = append(
        r,
        idString,
        m[key].Type,
        m[key].Year,
    )

    writer.Write(r)
}