How can I use something like (input == '\n') to determine when to stop accepting user input?

Question

I have to convert an infix operation to a postfix one, however the infix operation must be inputted as one character per line. So instead of inputting something like this: 3-2, you would need to input something like this:

3
-
2

I had an idea of using =='\\n' to determine whether the inputted character is a next line function so that would determine the end of the equation, but it doesn't work. I tried replacing it with a different character such as =='e', and that works perfectly. What can I do to fix this?

   String string = "";
   Scanner input = new Scanner(System.in);
   boolean flag = true;
   while (flag==true)
   {
       char charIn = input.next().charAt(0);
       string = string + charIn;
       if (charIn=='e') //inputting 'e' gives me my desired result
       {
           flag = false;
       }
   }
   //code that passes string to InfixToPostfix method and prints out the answer. this part works fine

Answer 1

You did not specify that this was a school assignment or that you had certain restrictions, so this answer is admittedly a shot in the dark.

I would recommend using a StringBuilder within a loop and reading nextLine() instead of simply next() . This allows you to determine if the entry was empty (ie: the enter key was pressed without entering a character).

Also, we should allow the user to enter more than one character anyway (what happens when they try to enter 22 as a number). Abandoning the char type allows for this.

public static void main(String[] args) {
    StringBuilder string = new StringBuilder();
    Scanner input = new Scanner(System.in);
    boolean flag = true;
    while (flag) {

        // Capture all characters entered, including numbers with multiple digits
        String in = input.nextLine();

        // If no characters were entered, then the [ENTER] key was pressed
        if (in.isEmpty()) {
            // User is done adding characters; exit the loop
            flag = false;
        } else {

            // Otherwise, get the text entered and add it to our final string
            string.append(in);
        }
    }

    System.out.println("Final String: " + string);
}

Does this meet your needs?

Answer 2

This should do what you want. Reading just the first character has its limitations.

String string = "";
Scanner input = new Scanner(System.in);
boolean flag = true;
while (flag==true)
{
     String nextLine = input.nextLine();
     char charIn;

     if(nextLine.length() > 0) {
       charIn = nextLine.charAt(0); //This is bad idea as you can only operate on single digit numbers
       System.out.println("charIn = " + charIn);;
       string = string + charIn;
     }
     else
          flag = false;
}