How do I read input from the user?

← Getting Started · Ref: Q4

Use the built-in Stdin type:

  stdin <- Stdin()

Read a single line with a guard (only executes if input is available):

  if line <- stdin.next()
    stdout.println(`You entered: ${line}`)

Iterate over all lines until EOF:

  while stdin?
    stdout.println(stdin.next())

In a stream pipeline, stdin is a source:

  cat stdin | filter by notEmpty > stdout

Stdin implements the StringInput trait with these methods:

  hasNext()    check if more input is available
  next()       get the next line as String
  ?            isSet operator, true when input available

Stdin works with both interactive input and piped data:

  echo "Hello" | ek9 myProgram.ek9
  cat data.txt | ./myProgram.ek9
  ls -la | ek9 myProgram.ek9

The same program handles all three cases without any changes.

When piped input ends or the user sends EOF (Ctrl+D on Unix, Ctrl+Z on Windows), hasNext() returns false.

For the full Stdin API, run: ek9 -h Stdin

See Q3 for printing output. See Q37 for string operations on input lines.

Example

defines module qa.getting.started.read.input

  defines program
    ReadInput()
      stdin <- Stdin()
      stdout <- Stdout()

      stdout.println("Type something and press Enter:")
      if line <- stdin.next()
        stdout.println(`You said: ${line}`)

Common mistakes

E50060 — EK9 Stdin uses next() to read the next line, not readLine(). AI often uses Java-style method names. Triggers E50060 — method not resolved. See ek9 -h Stdin for the full API.

Incorrect:

if line <- stdin.readLine()

Correct:

if line <- stdin.next()

E08020 — Declaring stdin without initialising it (as Stdin?) then calling stdin.next() uses an uninitialised variable as a method receiver. Triggers E08020 — might be used before being initialised. Always initialise with <- Stdin(). See ek9 -h E08020 for details.

Incorrect:

stdin as Stdin?

Correct:

stdin <- Stdin()

Coming from another language?

Python: input(), Java: Scanner(System.in), Rust: stdin().read_line(), Go: fmt.Scanln() or bufio.NewReader. EK9 uses a guard pattern with Stdin — the compiler ensures you only process input when it is available.

Keywords: beginner, console, keyboard, first, user, pipe, start, interactive, read, stdin, readline, intro, input