如何使用Golang计算地图中某个值出现的次数？

Question

I have created a map with the following structure:

m := make(map[int]Record)

The Record is a struct as follows:

type Record struct {
    UID  int
    Type string
    Year string
}

The SumRecord struct is supposed to store information about the number of occurences of every given Type / Year Value in map m.

type SumRecord struct {
    Sum  int
    Type string
    Year string
}

The struct is supposed to hold information about book publication years, ie {1, "Type": "fiction", "Year": 1996}, {2, "Type": "non-fiction", "Year": 1996}

I am unsuccessfully trying to create a second map where I would store the sum of each publication type per year (similar to a SUM / GROUP BY in SQL). How can I achieve that with Go?

Answer 1

Here's an alternative solution to the one provided by @ThunderCat.

This creates a new mapping of a SumRecord to an integer, representing the sum of occurrences for that specific grouping of Type/Year.

See full example here.

type Record struct {
    UID  int
    Type string
    Year string
}

type SumRecord struct {
    Type string
    Year string
}

m := make(map[int]Record)

// e.g. [{"1996","non-fiction"}:4], representing 4 occurrences of {"1996","non-fiction"}
srMap := make(map[SumRecord]int)

// add records

// loop over records
for key := range m {
    sr := SumRecord{
        Type: m[key].Type,
        Year: m[key].Year,
    }
    // creates new counter or increments existing pair counter by 1
    srMap[sr] += 1
}
// print all mappings
fmt.Println(srMap)

// specific example
fmt.Println(srMap[SumRecord{
    Year: "1996",
    Type: "non-fiction",
}])