I can not take input from the user in Golang by use fmt.scan().
package main
import "fmt"
func main() {
fmt.Print("Enter text: ")
var input string
e, _ := fmt.Scanln(&input)
fmt.Println(input)
fmt.Println(e)
}
image of code
After stopping the debugger:
image of code
The err added to code, but nothing happened.
func main() {
fmt.Print("Enter text: ")
var input string
e, err := fmt.Scanln(&input)
if err != nil {
fmt.Fprintln(os.Stderr, err)
return
}
fmt.Println(input)
fmt.Println(e)
}
Image after add err in my Code. What is "not available" in Next line (after my Input value: "51213")
You code has no problem. If you build your code with go build and run the binary directly in a terminal, you will see your code runs.
The problem you hit is because of the Delve and vscode debug console. The vscode debug console doesn't support read from stdin. You can check this issue: Cannot debug programs which read from STDIN for details.
No need for 'e'. Replace it with an underscore and remove the print statement.
import (
"fmt"
"os"
)
func main() {
fmt.Print("Enter text: \n")
var input string
_, err := fmt.Scanln(&input)
if err != nil {
fmt.Fprintln(os.Stderr, err)
return
}
fmt.Println(input)
}
For the record. If you want to input a numeric value, fmt.Scan stores the value in a variable as a string, if you would like perform any mathematical operation with it, you need to convert it either to int or float. A quick example:
func main() {
fmt.Println("Type your age: ")
var input string
_, err := fmt.Scanln(&input)
if err != nil {
fmt.Fprintln(os.Stderr, err)
return
}
fmt.Printf("%T\n", input) // outputs string
inputInt,_ := strconv.Atoi(input)
fmt.Printf("%T\n", inputInt) // outputs int
fmt.Printf("You were born in %d\n", 2021-inputInt)
}
Took me a while to figure it out, hope it helps!
Related
I need to automate the input of a code segment like bellow where the inputs of the ReadString are distinct.
package main
import (
"bufio"
"fmt"
"os"
)
func main() {
buf := bufio.NewReader(os.Stdin)
value, err := buf.ReadString('\n')
if err != nil {
fmt.Println(err)
} else {
fmt.Println(value)
}
buf = bufio.NewReader(os.Stdin)
value, err = buf.ReadString('\n')
if err != nil {
fmt.Println(err)
} else {
fmt.Println(value)
}
}
I have tried several formats like the bellow one following answers of this question, unfortunately, none worked.
>> echo "data1
data2" | go run main.go
output: data1
EOF
Here data1 and data2 and input of the separate ReadString methods. I don't have control over the source code. So, I can only try changing the bash input. How to resolve this issue?
This is happening because the second string does not end with a newline. Looking at the documentation for ReadString:
If ReadString encounters an error before finding a delimiter, it returns the data read before the error and the error itself (often io.EOF).
So, even though error is non-nil, you have the data. The following change should work for this specific case:
buf = bufio.NewReader(os.Stdin)
value, err = buf.ReadString('\n')
if value!="" {
fmt.Println(value)
}
if err != nil {
fmt.Println(err)
}
In general, even if you get an error from ReadString, you may still have nonempty data returned from the function.
So I have a Go program that reads from STDIN as such below. I want the username and password to be entered from keyboard or device but the string slice can be passed using pipe. If I run the command as below:
echo "Hello World" | go run main.go
os.Stdin will be set to read from pipes and never the keyboard. Is there a way that I can change os.Stdin FileMode as such that it will be reading from device, i.e. keyboard for username and password?
I tried using os.Stdin.Chmod(FileMode) but received this error:
chmod /dev/stdin: invalid argument
func main() {
var n = []string{}
scanner := bufio.NewScanner(os.Stdin)
fmt.Println("Please type anything with Newline Separated, empty line signals termination")
for scanner.Scan() {
h := scanner.Text()
if h == "" {
break
}
n = append(n, h)
}
if err := scanner.Err(); err != nil {
fmt.Printf("Error in reading from STDIN: %v\n", err)
}
reader := bufio.NewReader(os.Stdin)
os.Stdout.WriteString("Username: ")
username, err := reader.ReadString('\n')
if err != nil {
fmt.Printf("Unable to read username: %v\n", err)
}
username = strings.TrimSpace(username)
os.Stdout.WriteString("Password: ")
bytePassword, _ := terminal.ReadPassword(int(os.Stdin.Fd()))
password := string(bytePassword)
os.Stdout.WriteString("\n")
}
Probably scanf could help, check this example:
https://play.golang.org/p/tteQNl0trJp
package main
import (
"fmt"
)
func main() {
fmt.Println("Enter your name")
var name string
fmt.Scanf("%s", &name)
fmt.Printf("name = %s\n", name)
}
Something a little more elaborated to check if there is something to read from stdin and if not prompt the user:
https://play.golang.org/p/7qeAQ5UNhdQ
package main
import (
"bufio"
"fmt"
"log"
"os"
)
func main() {
// check if there is somethinig to read on STDIN
stat, _ := os.Stdin.Stat()
if (stat.Mode() & os.ModeCharDevice) == 0 {
var stdin []byte
scanner := bufio.NewScanner(os.Stdin)
for scanner.Scan() {
stdin = append(stdin, scanner.Bytes()...)
}
if err := scanner.Err(); err != nil {
log.Fatal(err)
}
fmt.Printf("stdin = %s\n", stdin)
} else {
fmt.Println("Enter your name")
var name string
fmt.Scanf("%s", &name)
fmt.Printf("name = %s\n", name)
}
}
You can instead read from /dev/tty as this is always the terminal (if the program runs on a terminal). This is portable only to Unix-like systems (Linux, BSD, macOS, etc) and won't work on Windows.
// +build !windows
tty, err := os.Open("/dev/tty")
if err != nil {
log.Fatalf("can't open /dev/tty: %s", err)
}
scanner := bufio.NewScanner(tty)
// as you were ...
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
Here's an issue that's bedeviling me at the moment. When getting input from the user, I want to employ a loop to ask the user to retry until they enter valid input:
// user_input.go
package main
import (
"fmt"
)
func main() {
fmt.Println("Please enter an integer: ")
var userI int
for {
_, err := fmt.Scanf("%d", &userI)
if err == nil {
break
}
fmt.Println("Sorry, invalid input. Please enter an integer: ")
}
fmt.Println(userI)
}
Running the above, if the user enters valid input, no problem:
Please enter an integer:
3
3
exit code 0, process exited normally.
But try inputting a string instead?
Please enter an integer:
what?
Sorry, invalid input. Please enter an integer:
Sorry, invalid input. Please enter an integer:
Sorry...
Etc, and it keeps looping character by character until the string is exhausted.
Even inputting a single character loops twice, I assume as it parses the newline.
Anyways, there must be a way to flush Stdin in Go?
P.S. In the absence of such a feature, how would you work around it to provide equivalent functionality? I've failed even at that...
I would fix this by reading until the end of the line after each failure. This clears the rest of the text.
package main
import (
"bufio"
"fmt"
"os"
)
func main() {
stdin := bufio.NewReader(os.Stdin)
fmt.Println("Please enter an integer: ")
var userI int
for {
_, err := fmt.Fscan(stdin, &userI)
if err == nil {
break
}
stdin.ReadString('\n')
fmt.Println("Sorry, invalid input. Please enter an integer: ")
}
fmt.Println(userI)
}
Is it bad to wake up an old question?
I prefer to use fmt.Scanln because A) it doesn't require importing another library (e.g. reader) and B) it doesn't involve an explicit for loop.
func someFunc() {
fmt.Printf("Please enter an integer: ")
// Read in an integer
var i int
_, err := fmt.Scanln(&i)
if err != nil {
fmt.Printf("Error: %s", err.Error())
// If int read fails, read as string and forget
var discard string
fmt.Scanln(&discard)
return
}
fmt.Printf("Input contained %d", i)
}
However, it seems like there ought to be a more elegant solution. Particularly in the case of fmt.Scanln it seems odd that the read stops after the first non-number byte rather than "scanning the line".
I ran into a similar problem for getting user input but solved it in a slightly different way. Adding to the thread in case someone else finds this useful:
package main
import (
"bufio"
"fmt"
"os"
"strings"
)
// Get first word from stdin
func getFirstWord() (string) {
input := bufio.NewScanner(os.Stdin)
input.Scan()
ans := strings.Fields(input.Text())
if len(ans) == 0 {
return ""
} else {
return ans[0]
}
}
func main() {
fmt.Printf("Would you like to play a game?\n> ")
ans := getFirstWord()
fmt.Printf("Your answer: %s\n", ans)
}
I know this has already been answered but this was my implementation:
func flush (reader *bufio.Reader) {
var i int
for i = 0; i < reader.Buffered(); i++ {
reader.ReadByte()
}
}
This should work in every situation, including ones where "stdin.ReadString('\n')" cannot be used.
Sorry for digging this back up, but I ran into this today and wanted to improve on the existing answers by using new standard library functionality.
import (
"bufio"
"fmt"
"os"
)
func discardBuffer(r *bufio.Reader) {
r.Discard(r.Buffered())
}
stdin := bufio.NewReader(os.Stdin)
var i int
for true {
if _, err := fmt.Fscanln(stdin, &i); err != nil {
discardBuffer(stdin)
// Handle error, display message, etc.
continue
}
// Do your other value checks and validations
break
}
The basic idea is to always buffer your reads from stdin. When you encounter an error while scanning, just discard the buffer contents. That way you start with an empty buffer for your next scan.
Alternatively, you can discard the buffer before you scan, so any stray inputs by the user before then won't get picked up.
func fscanln(r *bufio.Reader, a ...interface{}) error {
r.Discard(r.Buffered())
_, err := fmt.Fscanln(r, a...)
return err
}
stdin := bufio.NewReader(os.Stdin)
var i int
if err := fscanln(stdin, &i); err != nil {
// Handle error
}
I use this snippet to filter unnecessary leading space/new line
in := bufio.NewReader(os.Stdin)
result, err = in.ReadString('\n')
for len(strings.TrimSpace(result)) == 0 {
result, err = in.ReadString('\n')
}
I usually use bufio.Scanner since the fmt.Scan funcs always split on whitespace.
func promptYN(msg string) bool {
s := bufio.NewScanner(os.Stdin)
for {
fmt.Printf("%s [y/n]: ", msg)
s.Scan()
input := strings.ToLower(s.Text())
if input == "y" || input == "n" {
return input == "y"
}
fmt.Println("Error: expected Y or N.")
}
}
func promptInt(msg string) int {
s := bufio.NewScanner(os.Stdin)
for {
fmt.Printf("%s [int]: ", msg)
s.Scan()
output, err := strconv.Atoi(s.Text())
if err == nil {
return output
}
fmt.Println("Error: expected an integer.")
}
}
Or you could make something more universal:
func prompt(msg string, check func(string) bool) {
s := bufio.NewScanner(os.Stdin)
for {
fmt.Printf("%s: ", msg)
s.Scan()
if check(s.Text()) {
return
}
}
}
Example:
var f float64
prompt("Enter a float", func(s string) bool {
f, err = strconv.ParseFloat(s, 64)
return err == nil
})
I use this, to collect a input and display the input then, but when I do it like this the "something" text will be displayed in a new line, but I want the text to be displayed in the same line, any ideas?
func main() {
fmt.Println("Example")
print("example: ")
in := bufio.NewReader(os.Stdin);
input, err := in.ReadString('\n');
if err != nil {
fmt.Println("Error: ", err)
}
fmt.Println(input, "something")
}
Trim the newline from input. For example,
package main
import (
"bufio"
"fmt"
"os"
"strings"
)
func main() {
fmt.Println("Example")
print("example: ")
in := bufio.NewReader(os.Stdin)
input, err := in.ReadString('\n')
if err != nil {
fmt.Println("Error: ", err)
return
}
fmt.Println(strings.TrimRight(input, "\n"), "something")
}
Input/Output:
Example
example: some input
some input something
fmt.Println() automatically appends a newline to the end of it's output.
You could try using Printf, which takes a "format string" and a list of inputs.
An example would be:
func main() {
fmt.Println("Example")
print("example: ")
in := bufio.NewReader(os.Stdin);
input, err := in.ReadString('\n');
if err != nil {
fmt.Println("Error: ", err)
}
fmt.Printf("something : %s", input)
}
In this case, %s is a placeholder for a string type.
All the placeholders can be found in the fmt godoc: http://golang.org/pkg/fmt/
Also if the input itself has a newline at the end, you can use Trim from the package strings to pull off the \n character.
On windows (not sure if that is relevant) none of the previous answers worked for me. So here's the resolution I found on: https://groups.google.com/d/msg/golang-nuts/hWoESdeZ398/qDbTogJhj88J which links to https://play.golang.org/p/_9N_RwmBvd
The basic idea is that rather than using strings.TrimRight(input, "\n") you would include a \r character, like strings.TrimRight(input, "\r\n")
This fixes the OPs issue.
OP code with fixed input stripping!
func main() {
fmt.Println("Example")
print("example: ")
in := bufio.NewReader(os.Stdin);
input, err := in.ReadString('\n');
if err != nil {
fmt.Println("Error: ", err)
}
fmt.Println(strings.TrimRight(input, "\r\n"), "something")
}
PS: Sorry for the necro I'm just hoping to save someone from wasting 20 minutes googling!