The .txt file has many lines which each contain a single word. So I open the file and pass it to the reader:
file, err := os.Open("file.txt")
check(err)
reader := bufio.NewReader(file)
Now I want to store each line in a slice of strings. I believe I need to use ReadBytes, ReadString, ReadLine, or on of the Scan functions. Any advice on how to implement this would be appreciated. Thanks.
You can use ioutil.ReadFile() to read all lines into a byte slice and then call split on the result:
package main
import (
"fmt"
"io/ioutil"
"log"
"strings"
)
func main() {
data, err := ioutil.ReadFile("/etc/passwd")
if err != nil {
log.Fatal(err)
}
lines := strings.Split(string(data), "\n")
for _, line := range lines {
fmt.Println("line:", string(line))
}
}
Having r as an instance of *bufio.Reader, and myList as a slice of strings, than one could just loop and read lines till EOL.
for {
line, err := r.ReadBytes('\n')
if err != nil {
break
}
myList = append(myList, string(line))
}
Related
This question already has an answer here:
Golang, a proper way to rewind file pointer
(1 answer)
Closed 1 year ago.
I have a program which generates a random number between 1 and the number of lines in a file - call that number n. It then reads the file until iterator i == n and then prints that line from the file.
I'm seeing very strange behaviour though which I'm struggling to explain. For some reason, when I allow my code to seed and generate a random number, the bufio scanner fails and does not print the name from the file. When I comment the number generation part out, the exact same code works and a name is printed as expected.
In the below code, commenting and un-commenting the code between the hashtags inexplicably changes the behaviour of the bufio scanner code - and by that I mean it either prints or doesn't print a name from the file.
The file I refer to is just a list of names e.g.
name1
name2
name3
NOTE:
go version == 1.16.3 darwin/amd64
Code:
package main
import (
"bufio"
"bytes"
"flag"
"fmt"
"io"
"log"
"os"
"path/filepath"
"strings"
)
func main() {
f, err := os.Open(nameFile)
if err != nil {
log.Fatalln(err)
}
defer f.Close()
// ############## RANDOM NUMBER GENERATION ##############
// min := 1
// max, err := lineCounter(f)
// if err != nil {
// log.Fatalln(err)
// }
// rand.Seed(time.Now().UnixNano())
// v := rand.Intn(max-min) + min
// fmt.Println(v)
// ############## RANDOM NUMBER GENERATION ##############
scanner := bufio.NewScanner(f)
i := 0
for scanner.Scan() {
i += 1
if i == 60 {
fmt.Println(scanner.Text())
}
}
if err := scanner.Err(); err != nil {
log.Fatalln(err)
}
}
func lineCounter(r io.Reader) (int, error) {
buf := make([]byte, 32*1024)
count := 0
lineSep := []byte{'\n'}
for {
c, err := r.Read(buf)
count += bytes.Count(buf[:c], lineSep)
switch {
case err == io.EOF:
return count, nil
case err != nil:
return count, err
}
}
}
Your problem can be simplified way, way down:
package main
import (
"bufio"
"io"
"strings"
)
func main() {
f := strings.NewReader(strings.Repeat("north\n", 9))
io.ReadAll(f)
s := bufio.NewScanner(f)
for s.Scan() {
println(s.Text())
}
}
So as you can see, it's nothing to do with the random numbers, and it's not even anything to do with a file. When you are calling that "bad function", you're reading up all the data in the reader, so nothing is left for the Scanner to use.
I'm new to Golang, starting out with some examples. Currently, what I'm trying to do is reading a file line by line and replace it with another string in case it meets a certain condition.
The file is use for testing purposes contains four lines:
one
two
three
four
The code working on that file looks like this:
func main() {
file, err := os.OpenFile("test.txt", os.O_RDWR, 0666)
if err != nil {
panic(err)
}
reader := bufio.NewReader(file)
for {
fmt.Print("Try to read ...\n")
pos,_ := file.Seek(0, 1)
log.Printf("Position in file is: %d", pos)
bytes, _, _ := reader.ReadLine()
if (len(bytes) == 0) {
break
}
lineString := string(bytes)
if(lineString == "two") {
file.Seek(int64(-(len(lineString))), 1)
file.WriteString("This is a test.")
}
fmt.Printf(lineString + "\n")
}
file.Close()
}
As you can see in the code snippet, I want to replace the string "two" with "This is a test" as soon as this string is read from the file.
In order to get the current position within the file I use Go's Seek method.
However, what happens is that always the last line gets replaced by This is a test, making the file looking like this:
one
two
three
This is a test
Examining the output of the print statement which writes the current file position to the terminal, I get that kind of output after the first line has been read:
2016/12/28 21:10:31 Try to read ...
2016/12/28 21:10:31 Position in file is: 19
So after the first read, the position cursor already points to the end of my file, which explains why the new string gets appended to the end. Does anyone understand what is happening here or rather what is causing that behavior?
The Reader is not controller by the file.Seek. You have declared the reader as: reader := bufio.NewReader(file) and then you read one line at a time bytes, _, _ := reader.ReadLine() however the file.Seek does not change the position that the reader is reading.
Suggest you read about the ReadSeeker in the docs and switch over to using that. Also there is an example using the SectionReader.
Aside from the incorrect seek usage, the difficulty is that the line you're replacing isn't the same length as the replacement. The standard approach is to create a new (temporary) file with the modifications. Assuming that is successful, replace the original file with the new one.
package main
import (
"bufio"
"io"
"io/ioutil"
"log"
"os"
)
func main() {
// file we're modifying
name := "text.txt"
// open original file
f, err := os.Open(name)
if err != nil {
log.Fatal(err)
}
defer f.Close()
// create temp file
tmp, err := ioutil.TempFile("", "replace-*")
if err != nil {
log.Fatal(err)
}
defer tmp.Close()
// replace while copying from f to tmp
if err := replace(f, tmp); err != nil {
log.Fatal(err)
}
// make sure the tmp file was successfully written to
if err := tmp.Close(); err != nil {
log.Fatal(err)
}
// close the file we're reading from
if err := f.Close(); err != nil {
log.Fatal(err)
}
// overwrite the original file with the temp file
if err := os.Rename(tmp.Name(), name); err != nil {
log.Fatal(err)
}
}
func replace(r io.Reader, w io.Writer) error {
// use scanner to read line by line
sc := bufio.NewScanner(r)
for sc.Scan() {
line := sc.Text()
if line == "two" {
line = "This is a test."
}
if _, err := io.WriteString(w, line+"\n"); err != nil {
return err
}
}
return sc.Err()
}
For more complex replacements, I've implemented a package which can replace regular expression matches. https://github.com/icholy/replace
import (
"io"
"regexp"
"github.com/icholy/replace"
"golang.org/x/text/transform"
)
func replace2(r io.Reader, w io.Writer) error {
// compile multi-line regular expression
re := regexp.MustCompile(`(?m)^two$`)
// create replace transformer
tr := replace.RegexpString(re, "This is a test.")
// copy while transforming
_, err := io.Copy(w, transform.NewReader(r, tr))
return err
}
OS package has Expand function which I believe can be used to solve similar problem.
Explanation:
file.txt
one
two
${num}
four
main.go
package main
import (
"fmt"
"os"
)
var FILENAME = "file.txt"
func main() {
file, err := os.ReadFile(FILENAME)
if err != nil {
panic(err)
}
mapper := func(placeholderName string) string {
switch placeholderName {
case "num":
return "three"
}
return ""
}
fmt.Println(os.Expand(string(file), mapper))
}
output
one
two
three
four
Additionally, you may create a config (yml or json) and
populate that data in the map that can be used as a lookup table to store placeholders as well as their replacement strings and modify mapper part to use this table to lookup placeholders from input file.
e.g map will look like this,
table := map[string]string {
"num": "three"
}
mapper := func(placeholderName string) string {
if val, ok := table[placeholderName]; ok {
return val
}
return ""
}
References:
os.Expand documentation: https://pkg.go.dev/os#Expand
Playground
I start learning Go and I am a bit puzzled by the fact it includes the EOF when using the ioutil.ReadFile function. I want, for example, to read a file and parse all its lines on a field separator.
Sample input File:
CZG;KCZG;some text
EKY;KEKY;some text
A50;KA50;some text
UKY;UCFL;some text
MIC;KMIC;some text
K2M;K23M;some text
This is what I do to read and parse that file:
import(
"fmt"
"log"
"io/ioutil"
"strings"
)
func main() {
/* Read file */
airportsFile := "/path/to/file/ad_iata"
content, err := ioutil.ReadFile(airportsFile)
if err != nil {
log.Fatal(err)
}
/* split content on EOL */
lines := strings.Split(string(content), "\n")
/* split line on field separator ; */
for _, line := range lines {
lineSplit := strings.Split(line, ";")
fmt.Println(lineSplit)
}
}
The string.Split function adds a empty element at the end of the lineSplit slice when it sees the EOF (nothing to parse). Therefore, if I want to access the second index of that slice (lineSplit[1]) I run into a panic: runtime error: index out of range. I have to restrict the range by doing this
/* split line on field separator ; */
lenLines := len(lines) -1
for _, line := range lines[:lenLines] {
lineSplit := strings.Split(line, ";")
fmt.Println(lineSplit[1])
}
Is there a better way if I want to keep using ReadFile for its terseness ?
The same problem occurs when using ioutil.ReadAll
There is no such thing as an "EOF byte" or "EOF character". What you are seeing is probably caused by a line break character ('\n') at the very end of the file.
To read a file line by line, it's more idiomatic to use bufio.Scanner instead:
file, err := os.Open(airportsFile)
if err != nil {
log.Fatal(err)
}
defer file.Close()
scanner := bufio.NewScanner(file)
for scanner.Scan() {
line := scanner.Text()
// ... use line as you please ...
}
if err := scanner.Err(); err != nil {
log.Fatal(err)
}
And this actually addresses your problem, because Scanner will read the final newline without starting a new line, as evidenced by this playground example.
Your input File seeems to be CSV file, so you can use encoding/csv
airportsFile := "/path/to/file/ad_iata"
content, err := os.Open(airportsFile)
if err != nil {
log.Fatal(err)
}
r := csv.NewReader(content)
r.Comma = ';'
records, err := r.ReadAll() /* split line on field separator ; */
if err != nil {
log.Fatal(err)
}
fmt.Println(records)
which looks terse enough for me and provide correct output
[[CZG KCZG some text] [EKY KEKY some text] [A50 KA50 some text] [UKY UCFL some text] [MIC KMIC some text] [K2M K23M some text]]
You may use scanner.Err() to check for errors on file read.
// Err returns the first non-EOF error that was encountered by the Scanner.
func (s *Scanner) Err() error {
if s.err == io.EOF {
return nil
}
return s.err
}
In general in go the idiomatic way to read and parse a file is to use bufio.NewScanner which accept as an input parameter the file to read and returns a new Scanner.
Considering the above remarks here is a way you can read and parse a file:
package main
import (
"bufio"
"fmt"
"os"
)
func main() {
input, err := os.Open("example.txt")
if err != nil {
panic("Error happend during opening the file. Please check if file exists!")
os.Exit(1)
}
defer input.Close()
scanner := bufio.NewScanner(input)
for scanner.Scan() {
line := scanner.Text()
fmt.Printf("%v\n", line)
}
if err := scanner.Err(); err != nil {
fmt.Fprintln(os.Stderr, "reading input:", err)
}
}
I want to do this:
Read a line from a text file.
Process the line.
Delete the line.
My first thought was to read the entire file into memory with ioutil.Readfile(),
but I'm not sure how to update the text file after the line has been processed,
and what happens if extra lines is added to the text file after it has been read into memory?
I normally write shell scripts and would do something like this:
while read -r line; do
echo "${line}"
sed -i 1d "${myList}"
done < "${myList}"
What is the best way to do this in Golang?
Use the bufio package.
Here's the basic syntax for opening a text file and looping through each line.
package main
import (
"bufio"
"fmt"
"os"
)
func main() {
// Open the file.
f, _ := os.Open("C:\\programs\\file.txt")
// Create a new Scanner for the file.
scanner := bufio.NewScanner(f)
// Loop over all lines in the file and print them.
for scanner.Scan() {
line := scanner.Text()
fmt.Println(line)
}
}
you have some options:
1- read file, process it, then write it back (you need to lock that file).
2- use binary file and invent (make use of) special data structure (like linked list) to optimize text processing (with line locking).
3- use ready made databases.
4- use Virtual filesystem inside your file, and treat each line like one file, see: https://github.com/lotrfan/vfs and https://github.com/blang/vfs
using file manager (like database server) solves the file locking dilemma.
and if the purpose of using file is one way communication which sender program just adds new line and receiver program just removes it, it is better to use os pipes (named pipe (FIFO)) or other interop methods.
see for Linux: Unix FIFO in go?
for Windows: https://github.com/natefinch/npipe
sample file writer:
package main
import (
"bufio"
"fmt"
"os"
"time"
)
func main() {
f, err := os.OpenFile("/tmp/file.txt", os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
if err != nil {
panic(err)
}
defer f.Close()
for i := 0; ; i++ {
w := bufio.NewWriter(f)
_, err := fmt.Fprintln(w, i)
if err != nil {
panic(err)
}
w.Flush() // Flush writes any buffered data to the underlying io.Writer.
f.Sync() // commit the current contents of the file to stable storage.
fmt.Println("write", i)
time.Sleep(500 * time.Millisecond)
}
}
sample file reader:
package main
import (
"fmt"
"os"
"time"
)
func main() {
f, err := os.OpenFile("/tmp/file.txt", os.O_RDWR, 0666)
if err != nil {
panic(err)
}
defer f.Close()
i := 0
for {
n, err := fmt.Fscanln(f, &i)
if n == 1 {
fmt.Println(i)
}
if err != nil {
fmt.Println(err)
return
}
time.Sleep(500 * time.Millisecond)
}
}
I currently have this piece of code that will read a file line by line (delimited by a \n)
file, _ := os.Open(filename) //deal with the error later
defer file.Close()
buf := bufio.NewReader(file)
for line, err := buf.ReadString('\n'); err != io.EOF; line, err = buf.ReadString('\n')
{
fmt.Println(strings.TrimRight(line, "\n"))
}
However I don't feel comfortable with writing buf.ReadString("\n") twice in the for loop, does anyone have any suggestions for improvement?
bufio.ReadString reads until the first occurrence of delim in the input,
returning a string containing the data up to and including the
delimiter. If ReadString encounters an error before finding a
delimiter, it returns the data read before the error and the error
itself (often io.EOF). ReadString returns err != nil if and only if
the returned data does not end in delim.
If buf.ReadString('\n') returns an error other than io.EOF, for example bufio.ErrBufferFull, you will be in an infinite loop. Also, if the file doesn't end in a '\n', you silently ignore the data after the last '\n'.
Here's a more robust solution, which executes buf.ReadString('\n') once.
package main
import (
"bufio"
"fmt"
"io"
"os"
"strings"
)
func main() {
filename := "FileName"
file, err := os.Open(filename)
if err != nil {
fmt.Println(err)
return
}
defer file.Close()
buf := bufio.NewReader(file)
for {
line, err := buf.ReadString('\n')
if err != nil {
if err != io.EOF || len(line) > 0 {
fmt.Println(err)
return
}
break
}
fmt.Println(strings.TrimRight(line, "\n"))
}
}
Most code that reads line by line can be improved by not reading line by line. If your goal is to read the file and access the lines, something like the following is almost always better.
package main
import (
"fmt"
"io/ioutil"
"log"
"strings"
)
func main() {
b, err := ioutil.ReadFile("filename")
if err != nil {
log.Fatal(err)
}
s := string(b) // convert []byte to string
s = strings.TrimRight(s, "\n") // strip \n on last line
ss := strings.Split(s, "\n") // split to []string
for _, s := range ss {
fmt.Println(s)
}
}
Any errors come to you at a single point so error handling is simplified. Stripping a newline off the last line allows for files that may or may not have that final newline, as Peter suggested. Most text files are tiny compared to available memory these days, so reading these in one gulp is appropriate.