Java noob: How do i ask a true or false question?

phoenixfire · 07-30-2011, 12:02 AM

I need to write a program that will ask the user a true of false question eg. "Is today a weekend?"

For what i've looked up i think it has something to do with a Boolean but i'm not really understanding how it works.

And help at all would be greatly appreciated.

Nylex · 07-30-2011, 12:08 AM

What don't you understand? Yes, Java has a data type called boolean that can store either true or false. You can simply ask the user the question and set your boolean variable to true or false depending on their answer.

Tell us specifically what you don't understand and post any attempt you've made at writing the code.

phoenixfire · 07-30-2011, 12:24 AM

The first thing i don't understand is how to enter the question. I am assuming it's something like:

Quote:

h1 = keyboard.nextInt(boolvar);
boolean boolvar;
if (h1 == true) {System.out.println ("It is a weekned");}
else if (h1 == false) {System.out.println ("It is not a weekend");}

Am i close?

Nylex · 07-30-2011, 12:43 AM

So the problem is you don't know how to read input? That's what you mean, right?

I'm not sure what class your keyboard object is, but the simple way to get keyboard input is like this.

The rest after getting the input is OK. Remember, though, the braces on the if and else are optional when you have only one statement.

Out of interest, does your class/book not show you how to do basic things like reading input?

Nylex · 07-30-2011, 12:51 AM

For future reference, you might want to look at the Java Tutorials, specifically "Trails Covering the Basics".

paulsm4 · 07-30-2011, 01:52 AM

Hi, phoenixfire -

Perhaps this example might help:

Code:

import java.io.*;

class Tmp
{
  public static void main (String[] args) throws IOException
  {
    // Prompt user to enter something
    System.out.print ("Please enter a weekday (EX: \"monday\"): ");

    // Read their input
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String input = br.readLine ();

    // See if it's a valid weekday
    if (input.compareTo ("monday") == 0)
      System.out.println ("You entered Monday.");
    else if (input.compareTo ("tuesday") == 0)
      System.out.println ("You entered Tuesday.");
    // ... etc etc
    else if (input.compareTo ("sunday") == 0)
      System.out.println ("You entered Sunday.");
    else
      System.out.println ("Sorry: I don't recognize the day \"" + input + "\"");
  }
}

Sample output:

Quote:

C:\temp>javac Tmp.java

C:\temp>java Tmp
Please enter a weekday (EX: "monday"): tuesday
You entered Tuesday.

C:\temp>java Tmp
Please enter a weekday (EX: "monday"): Moose
Sorry: I don't recognize the day "Moose"

Proud · 07-31-2011, 05:20 AM

Quote:

if (input.compareTo ("monday") == 0)

Why not
if( string1.equals( string2 ) )
?

Also, Java 7 can do Switch/Case statements on Strings now.

And for a GUI example, http://download.oracle.com/javase/tu...og.html#create

Code:

final JOptionPane optionPane = new JOptionPane(
    "The only way to close this dialog is by\n"
    + "pressing one of the following buttons.\n"
    + "Do you understand?",
    JOptionPane.QUESTION_MESSAGE,
    JOptionPane.YES_NO_OPTION);

http://download.oracle.com/javase/tu...nPaneMetal.png
Or

Code:

int n = JOptionPane.showConfirmDialog(
    frame,
    "Would you like green eggs and ham?",
    "An Inane Question",
    JOptionPane.YES_NO_OPTION);

http://download.oracle.com/javase/tu...nTextMetal.png

phoenixfire · 07-31-2011, 10:38 PM

Nylex thanks you very much for your useful link you've been a great help so far in both this and my other thread.

Thank you Paulsm4 for your well commented example, very helpful.

And thank you to Proud for providing a nice alternative.

I'll go over everything you've all given me and try to understand it completely and i'll let you know if my program works. Thanks again!

Edit: Nylex, the book we're using is called "Java: An Introduction to Problem Solving and Programming (6th Edition)" but unfortunately i haven't received it yet from amazon.com. Screwed if i'm paying the price they're asking for it at the Uni bookshop.

phoenixfire · 07-31-2011, 11:41 PM

Alright, now i'm getting

Quote:

Error: The operator && is undefined for the argument type(s) int, boolean

When trying to compile. As i think i'm getting really close i'll just post the entire question and my program.

Quote:

////////////////////////////// PROBLEM STATEMENT //////////////////////////////
// Given a day of the week encoded as 0=Sun, 1=Mon, 2=Tue, ...6=Sat, and a //
// boolean indicating if we are on vacation, print a string of the form //
// "7:00" indicating when the alarm clock should ring. Weekdays, the alarm //
// should be "7:00" and on the weekend it should be "10:00". Unless we are //
// on vacation -- then on weekdays it should be "10:00" and weekends it //
// should be "off". //
// 1, false -> "7:00" //
// 1, false -> "7:00" //
// 1, false -> "7:00" //
///////////////////////////////////////////////////////////////////////////////

// >>>>>> Your Java Code Fragment starts here <<<<<<
Scanner keyboard = new Scanner (System.in);
int d1;
System.out.println("What day is it today? (0=sun, 1=mon, 5=fri, 6=sat, ect");
d1 = keyboard.nextInt( );
System.out.println("Is it a holiday today?");
Boolean h1 = keyboard.nextBoolean();
if (d1 >= 1 && d1<=5 && h1 == true) {System.out.println ("10:00");}
else if (d1 >= 1 && d1<=5 && h1==false) {System.out.println ("7:00");}
else if (d1 == 0||6 && h1==true) {System.out.println ("off");}
else if (d1 == 0||6 && h1==false) {System.out.println ("10:00");}

// >>>>>> Your Java Code Fragment ends here <<<<<<

So how can && be undefined? Does it not mean AND?

paulsm4 · 08-01-2011, 12:07 AM

Hi -

Suggestion:

1. Read input:
a) int day // Day
b) boolean isHoliday // Holiday

2. Write a "method" (for example, "alarmTime()") to compute the alarm time:
EXAMPLE:

Code:

class MyClass
{
  ...
  string alarmTime (int day, boolean isHoliday)
  {
    if (isHoliday)
    {
      if ( (day == 0) || (day == 6) )
        return "off"
      else
        return "10:00";
    }
    else
    {
      if ( (day == 0) || (day == 6) )
        return "10:00";
      else
        return "7:00";
    ...

phoenixfire · 08-01-2011, 12:22 AM

Thanks for your reply but i have it working another way now.
Now i'm getting some kind of error.

For testing if we've completed the question or not they have given us a program that enters values into the program we've written and checks them against what the answer should be.

For the end result i've got

Quote:

Scanner keyboard = new Scanner (System.in);
int d1;
boolean h1;
//System.out.println("What day is it today? (0=sun, 1=mon, 5=fri, 6=sat, ect");
d1 = keyboard.nextInt( );
//System.out.println("Is it a holiday today?");
h1 = keyboard.nextBoolean();
if (d1 >= 1 && d1<=5 && h1 == true) {System.out.println ("10:00");}
else if (d1 >= 1 && d1<=5 && h1==false) {System.out.println ("7:00");}
else if (d1 <= 0 && d1 >=6 && h1==true) {System.out.println ("off");}
else if (d1 <= 6 && d1 >=6 && h1==false) {System.out.println ("10:00");}

With the questions commented out so that the program will only read the "10:00" ect.
The strange thing is it will run fine if i don't have them commented out but won't pass since the output also includes my 'please enter' questions but when they are commented out i get

Quote:

java.lang.StringIndexOutOfBoundsException: String index out of range: -1

when i try to run it.

Why would this happen?

devnull10 · 08-01-2011, 07:37 AM

Sounds like the program is reading all the input from your program. One (very ugly) workaround might be to print the questions to stderr instead of stdout.
I'd imagine your error may well be buffer related (ie, not being flushed) but I wouldn't like to say for definite as I've only had a 2 second glance at your code.

paulsm4 · 08-01-2011, 11:34 PM

Quote:

Q: Why would this happen?

That's what debuggers are for

.

Familiarize yourself with JDB (or your IDE's debugger, if you're using an IDE) and single step through it.

I believe that commenting out your prompt is probably "coincidence"; I do *not* believe stdin and stdout are somehow getting co-mingled together.

Another alternative, of course, is to try "BufferedReader" I suggested, or the JSwing approach another poster suggested.

But your best bet is a debugger. You should definitely familiarize yourself with at least one debugger; better sooner than later.

IMHO .. PSM

Proud · 08-02-2011, 04:04 AM

What was originally

Code:

if (d1 == 0||6 && h1==false) {System.out.println ("10:00");}

now seems to be

Code:

if (d1 <= 6 && d1 >=6 && h1==false) {System.out.println ("10:00");}

Firstly I think you're now aware that d1 == 0||6 isn't the correct shorthand for d1 == 0 || d1 == 6, but it also isn't equivalent to the latter logically.
d1 <= 6 && d1 >=6 is equal to d1 == 6, which mean d1 == 0 isn't tested. I think you meant d1 <= 0 && d1 >=6

As to why you're program behaves differently when you're not interleaving the out.printlns with the keyboard.nextTypes, consider that you're using a Scanner class wrapping System.in. Check how it behaves around giving empty tokens, blocking(aka waiting without you writing specific code to make it) differently when calling next() vs hasNext() (maybe you should wait while( !hasNext() ) ?), and that it may determine the end of the input has been reached before you want it to.