Using Golang to read csv, reorder columns then write result to a new csv with Concurrency

Here's my starting point.

It is a Golang script to read in a csv with 3 columns, re-order the columns and write the result to a new csv file.

package main
import (
"fmt"
"encoding/csv"
"io"
"os"
"math/rand"
"time"
)
func main(){
start_time := time.Now()
// Loading csv file
rFile, err := os.Open("data/small.csv") //3 columns
if err != nil {
fmt.Println("Error:", err)
return
}
defer rFile.Close()
// Creating csv reader
reader := csv.NewReader(rFile)
lines, err := reader.ReadAll()
if err == io.EOF {
fmt.Println("Error:", err)
return
}
// Creating csv writer
wFile, err := os.Create("data/result.csv")
if err != nil {
fmt.Println("Error:",err)
return
}
defer wFile.Close()
writer := csv.NewWriter(wFile)
// Read data, randomize columns and write new lines to results.csv
rand.Seed(int64(time.Now().Nanosecond()))
var col_index []int
for i,line :=range lines{
if i == 0 {
//randomize column index based on the number of columns recorded in the 1st line
col_index = rand.Perm(len(line))
}
writer.Write([]string{line[col_index[0]], line[col_index[1]], line[col_index[2]]}) //3 columns
writer.Flush()
}
//print report
fmt.Println("No. of lines: ",len(lines))
fmt.Println("Time taken: ", time.Since(start_time))
}

Question:

1. Is my code idiomatic for Golang?

2. How can I add concurrency to this code?

Your code is OK. There are no much case for concurrency. But you can at least reduce memory consumption reordering on the fly. Just use Read() instead of ReadAll() to avoid allocating slice for hole input file.

for line, err := reader.Read(); err == nil; line, err = reader.Read(){
if err = writer.Write([]string{line[col_index[0]], line[col_index[1]], line[col_index[2]]}); err != nil {
fmt.Println("Error:", err)
break
}
writer.Flush()
}

Move the col_index initialisation outside the write loop:

if len(lines) > 0 {
//randomize column index based on the number of columns recorded in the 1st line
col_index := rand.Perm(len(lines[0]))
newLine := make([]string, len(col_index))
for _, line :=range lines[1:] {
for from, to := range col_index {
newLine[to] = line[from]
}
writer.Write(newLine)
writer.Flush()
}
}

To use concurrency, you must not use reader.ReadAll. Instead make a goroutine that calls reader.Read and write the output on a channel that would replace the lines array. The main goroutine would read the channel and do the shuffle and the write.