Golang Read Write and Process data in CSV

package main
 
import (
    "encoding/csv"
    "log"
    "os"
    "strconv"
)
 
func main() {
    rows := readSample()
    appendSum(rows)
    writeChanges(rows)
}
 
func readSample() [][]string {
    f, err := os.Open("sample.csv")
    if err != nil {
        log.Fatal(err)
    }
    rows, err := csv.NewReader(f).ReadAll()
    f.Close()
    if err != nil {
        log.Fatal(err)
    }
    return rows
}
 
func appendSum(rows [][]string) {
    rows[0] = append(rows[0], "SUM")
    for i := 1; i < len(rows); i++ {
        rows[i] = append(rows[i], sum(rows[i]))
    }
}
 
func sum(row []string) string {
    sum := 0
    for _, s := range row {
        x, err := strconv.Atoi(s)
        if err != nil {
            return "NA"
        }
        sum += x
    }
    return strconv.Itoa(sum)
}
 
func writeChanges(rows [][]string) {
    f, err := os.Create("output.csv")
    if err != nil {
        log.Fatal(err)
    }
    err = csv.NewWriter(f).WriteAll(rows)
    f.Close()
    if err != nil {
        log.Fatal(err)
    }
}