Efficient read and write CSV in Go

You're loading the file in memory first then processing it, that can be slow with a big file.

You need to loop and call .Read and process one line at a time.

func processCSV(rc io.Reader) (ch chan []string) {
    ch = make(chan []string, 10)
    go func() {
        r := csv.NewReader(rc)
        if _, err := r.Read(); err != nil { //read header
            log.Fatal(err)
        }
        defer close(ch)
        for {
            rec, err := r.Read()
            if err != nil {
                if err == io.EOF {
                    break
                }
                log.Fatal(err)

            }
            ch <- rec
        }
    }()
    return
}

playground

//note it's roughly based on DaveC's comment.

This is essentially Dave C's answer from the comments sections:

package main

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

func main() {
  // setup reader
  csvIn, err := os.Open("./path/to/datafile.csv")
  if err != nil {
    log.Fatal(err)
  }
  r := csv.NewReader(csvIn)

  // setup writer
  csvOut, err := os.Create("./where/to/write/resultsfile.csv"))
  if err != nil {
    log.Fatal("Unable to open output")
  }
  w := csv.NewWriter(csvOut)
  defer csvOut.Close()

  // handle header
  rec, err := r.Read()
  if err != nil {
    log.Fatal(err)
  }
  rec = append(rec, "score")
  if err = w.Write(rec); err != nil {
    log.Fatal(err)
  }

  for {
    rec, err = r.Read()
    if err != nil {
      if err == io.EOF {
        break
      }
      log.Fatal(err)
    }

    // get float value
    value := rec[1]
    floatValue, err := strconv.ParseFloat(value, 64)
    if err != nil {
      log.Fatal("Record, error: %v, %v", value, err)
    }

    // calculate scores; THIS EXTERNAL METHOD CANNOT BE CHANGED
    score := calculateStuff(floatValue)

    scoreString := strconv.FormatFloat(score, 'f', 8, 64)
    rec = append(rec, scoreString)

    if err = w.Write(rec); err != nil {
      log.Fatal(err)
    }
  w.Flush()
  }
}

Note of course the logic is all jammed into main(), better would be to split it into several functions, but that's beyond the scope of this question.

encoding/csv is indeed very slow on big files, as it performs a lot of allocations. Since your format is so simple I recommend using strings.Split instead which is much faster.

If even that is not fast enough you can consider implementing the parsing yourself using strings.IndexByte which is implemented in assembly: http://golang.org/src/strings/strings_decl.go?s=274:310#L1

Having said that, you should also reconsider using ReadAll if the file is larger than your memory.