Convert csvs with rows of fields to rows of objects

convert.go

package main

import (
	"encoding/csv"
	"fmt"
	"os"
	"strconv"
)

type Record struct {
	key   string
	value string
}

func main() {
	if len(os.Args) < 2 {
		fmt.Println("Usage: go run convert.go example.csv")
		return
	}
	input := os.Args[1]
	f, err := os.Open(input)
	if err != nil {
		fmt.Println("Error opening input file", err)
		return
	}
	r := csv.NewReader(f)

	// Fields maps object ids to sets of defined key,value pairs
	fields := make(map[int][]Record)
	uniqueKeys := make(map[string]struct{})

	for record, err := r.Read(); err == nil; record, err = r.Read() {
		if len(record) < 3 {
			fmt.Println("Unexpected short record set", record)
			return
		}
		id, err := strconv.Atoi(record[0])
		if err != nil {
			fmt.Println("Expected integer ID for record", record)
			return
		}
		fields[id] = append(fields[id], Record{record[1], record[2]})
		uniqueKeys[record[1]] = struct{}{}
	}

	i := 1
	keyIndices := make(map[string]int)
	for k := range uniqueKeys {
		keyIndices[k] = i
		i++
	}

	outFile, err := os.Create("converted" + input)
	if err != nil {
		fmt.Println("Error opening output file", err)
		return
	}

	w := csv.NewWriter(outFile)

	headers := make([]string, len(keyIndices)+1)
	headers[0] = "id"
	for k, i := range keyIndices {
		headers[i] = k
	}
	w.Write(headers)

	for id, records := range fields {
		toWrite := make([]string, len(keyIndices)+1)
		toWrite[0] = strconv.Itoa(id)
		for _, record := range records {
			toWrite[keyIndices[record.key]] = record.value
		}

		w.Write(toWrite)
	}
	w.Flush()
}