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 Posted: Sun, 5th Sep 2010 18:11 Post subject: Java begineer problem |
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So yeah, picked up java 1-2 weeks ago and now i'm stuck on my first problem.
Basically I want to create a program where a window pops up and asks you to write code in the following way. HH:MM:SS. Once you've done that another window should pop up where the info is showed in the following way:
3 hours
4 minutes
22 seconds.
Now the problem i'm having is that the text is shown the following way.
3 hours
:4 minutes
:22 seconds
I've been through the code over and over and i can't see where i went wrong. it seems quite logical to me but there's probably a small detail that is most likely evident for you guys. NOTE: I haven't added the hours, seconds, minutes text yet to the end message.
code:
| Code: |
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import javax.swing.*;
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public class Marathon {
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public static void main (String[] arg) {
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String a;
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a = JOptionPane.showInputDialog("Write time in format: TT:MM:SS");
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int b = a.indexOf(':');
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int c = a.lastIndexOf(':');
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String d;
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d = a.substring(0, b);
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String e;
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e = a.substring(b, c);
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String f;
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f = a.substring(c);
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String g;
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g = d + "\n" + e + "\n" + f;
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JOptionPane.showMessageDialog(null, g);
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System.exit(0);
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}
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}
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Thanks.
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| Posted: Sun, 5th Sep 2010 18:20 Post subject: |
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Correction:
12. e = a.substring(b+1, c);
14. f = a.substring(c+1);
Edit: I forgot to explain, b and c are the indexes of the ':' character but you don't want this character included in your output, so you want to begin with the character that comes AFTER the index of ':'. Got it?
Last edited by me7 on Sun, 5th Sep 2010 18:23; edited 1 time in total
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LeoNatan
Banned
Posts: 73193
Location: Ramat Gan, Israel 🇮🇱
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| Posted: Sun, 5th Sep 2010 18:22 Post subject: |
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Why not just split the string:
| Code: | String[] results = a.split(":");
String output = results[0] + " hours\n" + results[1] + " minutes\n" + results[2] + " seconds";
JOptionPane.showMessageDialog(null, output); |
Anyway, in your method, try
| Code: | e = a.substring(b + 1, c);
...
f = a.substring(c + 1); |
You want to take the string from one place after the index of the ":".
BTW, naming parameters "a" "b" "c" is a really bad habit, one you should avoid very dearly.
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| Posted: Sun, 5th Sep 2010 18:28 Post subject: |
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Ah I see, didn't know that you could use +1 in substring.
Thanks for the help guys.
Also iNatan, how should i name the strings then? I'm new so any tips to avoid future bad habits is welcomed.
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LeoNatan
Banned
Posts: 73193
Location: Ramat Gan, Israel 🇮🇱
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| Posted: Sun, 5th Sep 2010 18:40 Post subject: |
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You should always give them a meaningful name.
For example:
| Code: | String input = JOptionPane.showInputDialog("Write time in format: TT:MM:SS");
int indexHours = input.indexOf(':');
int indexMinutes = input.lastIndexOf(':');
String hours = input.substring(0, indexHours);
String minutes = index.substring(indexHours + 1, indexMinutes);
String seconds = index.substring(indexMinutes + 1);
String output = hours + "\n" + minutes + "\n" + seconds;
JOptionPane.showMessageDialog(null, output); |
That way you never have a variable or parameter which you have no clue what it is. With modern IDEs you don't have to worry about typing too much, so don't worry about length (within reason, of course). It is important to be as descriptive as possible so you can remember what each variable is the instant you see it.
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| Posted: Sun, 5th Sep 2010 18:46 Post subject: |
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| iNatan wrote: | You should always give them a meaningful name.
For example:
| Code: | String input = JOptionPane.showInputDialog("Write time in format: TT:MM:SS");
int indexHours = input.indexOf(':');
int indexMinutes = input.lastIndexOf(':');
String hours = input.substring(0, indexHours);
String minutes = index.substring(indexHours + 1, indexMinutes);
String seconds = index.substring(indexMinutes + 1);
String output = hours + "\n" + minutes + "\n" + seconds;
JOptionPane.showMessageDialog(null, output); |
That way you never have a variable or parameter which you have no clue what it is. With modern IDEs you don't have to worry about typing too much, so don't worry about length (within reason, of course). It is important to be as descriptive as possible so you can remember what each variable is the instant you see it. |
Ah okay that's true, makes more sense.
I'll keep it in mind, thanks for the tip.
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Werelds
Special Little Man
Posts: 15098
Location: 0100111001001100
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| Posted: Sun, 5th Sep 2010 19:11 Post subject: |
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LeoNatan
Banned
Posts: 73193
Location: Ramat Gan, Israel 🇮🇱
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Werelds
Special Little Man
Posts: 15098
Location: 0100111001001100
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| Posted: Sun, 5th Sep 2010 21:05 Post subject: |
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I personally don't mind Java's style, but I'm also used to working in at least 3 different languages for different projects at any given time anyway 
The biggest problem for me is that my fellow students all fail at having a proper coding style. Apart from readability, most aren't even consistent in how they name variables/classes/constants/whatever. Some of them start everything with a capital letter for example, and mix English/Dutch names in their code (i.e. they copy/paste the English bits and don't bother to change it). Really annoying.
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| Posted: Thu, 16th Sep 2010 20:22 Post subject: |
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I've run into a new problem that is bugging me and for the love of it i can't figure it out.
| Code: |
import javax.swing.*;
public class Matteprov {
public static void main (String[] arg) {
String Input;
Input = JOptionPane.showInputDialog("Hur många poäng fick eleven på provet?");
double Input2 = Double.parseDouble(Input);
//String Output;
if (Input2 < 25) {
Output = "IG";
JOptionPane.showMessageDialog(null,"Följande elev får " + Output);
}
else if (25 =< Input2 < 35) {
Output = "G";
JOptionPane.showMessageDialog(null,"Följande elev får " + Output);
}
else if ( 35 =< Input2 < 45 ) {
Output = "VG";
JOptionPane.showMessageDialog(null,"Följande elev får " + Output);
}
else {
Output = "MVG";
JOptionPane.showMessageDialog(null,"Följande elev får " + Output);
}
System.exit(0);
}
}
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The program is supposed to take in a certain value and then determine what grade it is according to the swedish system. The error appears in line 14, saying " illegal start of type". I'm not sure how to fix it, any ideas?
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| Posted: Thu, 16th Sep 2010 20:34 Post subject: |
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Try: else if (25 =< Input2 && input2 < 35)
'&&' is the boolean AND operator.
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| Posted: Thu, 16th Sep 2010 20:42 Post subject: |
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now it says that "> is expected" on the same line. i don't get where to place it..
why am i not getting this error at line 18 as well? its essentially the same function just different values.
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| Posted: Thu, 16th Sep 2010 21:28 Post subject: |
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| Code: |
public class Matteprov {
public static void main (String[] arg) {
double input = Double.parseDouble(JOptionPane.showInputDialog("Hur många poäng fick eleven på provet?"));
String output;
if (input<25){
output = "IG";
} else {
if (input <35){
output = "G";
} else {
if (input <45){
output = "VG";
} else {
output = "MVG";
}
}
}
JOptionPane.showMessageDialog(null,"Följande elev får " + output);
System.exit(0);
}
} |
There is no need to check if input is bigger than 25 and smaller than 35 because if it's not smaller you are already in the else case thus it must be bigger! and if it's not smaller than 35 then just check if it's smaller then 45 and if it's still not smaller you'll have mvg. pretty easy 
And there is no 1<=i<=5 or else if in java as far as i know
Remember to use LOWERCASE variable names! And don't use input or input5  use grade or something more descriptive... |
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| Posted: Thu, 16th Sep 2010 21:56 Post subject: |
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ah okay, i guess i was thinking in mathematical terms more then logical.
also why should i use lowercase variable names?
thanks for the help.
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| Posted: Thu, 16th Sep 2010 23:27 Post subject: |
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| Razacka2 wrote: | now it says that "> is expected" on the same line. i don't get where to place it..
why am i not getting this error at line 18 as well? its essentially the same function just different values. |
Sorry, Java doesn't know '=<', you need to write '<='.
And you should use lowercase (with camelNotationForMultipleWords) because it's Java convention and other devs expect it when they read your code. Uppercase is "reserved" for classes.
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| Posted: Mon, 25th Oct 2010 22:12 Post subject: |
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I'm wondering if you guys have any ideas on begineer problems and projects I could work on, maybe you know a site that offers these kind of exercises? I'm kind of limited by my creativity right now and unsure of what to code and so on. Just started learning about try, catch and so on so i'm still a beginner.
I checked out projecteuler.net but I'd rather avoid mathematical problems as I've got more then enough of them in coursebook. Don't get me wrong, I have nothing against a problem dealing with maths but i'd like there to be a certain idea (eg, construct a program that decides what insurance you should get based by certain values you put in etc.)
Thanks!
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| Posted: Tue, 26th Oct 2010 00:27 Post subject: |
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Get a good beginner java book and go through it, and try to implement all of the examples in the book yourself.
Once you have basic knowledge of java go ahead and choose a branch of CS you want to work on specifically (security, artificial intelligence(has plenty of sub-fields like data mining, pattern recognition, natural language recognition), computer graphics, networks, algorithms, operating systems, etc.).
Then I suggest you find a programming book meant specifically for that topic and you'll find plenty of projects to work on.
After you're done with books, you can find many scientific papers with new techniques in the field, but I don't suggest you even look at them until you have a pretty good grasp on the basics, as they generally hide the implementation details.
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| Posted: Tue, 26th Oct 2010 16:38 Post subject: |
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| BearishSun wrote: | Get a good beginner java book and go through it, and try to implement all of the examples in the book yourself.
Once you have basic knowledge of java go ahead and choose a branch of CS you want to work on specifically (security, artificial intelligence(has plenty of sub-fields like data mining, pattern recognition, natural language recognition), computer graphics, networks, algorithms, operating systems, etc.).
Then I suggest you find a programming book meant specifically for that topic and you'll find plenty of projects to work on.
After you're done with books, you can find many scientific papers with new techniques in the field, but I don't suggest you even look at them until you have a pretty good grasp on the basics, as they generally hide the implementation details. |
How do you know when to branch into something specifically? Is there a deadend which you reach where you need to pick some subcategory to learn more about in order to do specific things?
Sorry for the tedious questions, just sounded really interesting when you started talking about different branches and picking one.
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| Posted: Tue, 26th Oct 2010 16:56 Post subject: |
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After being a programmer for a few years you'll realize that there isn't much to learn anymore as far as programming itself is concerned. Programming is just a tool used for making something bigger. All languages have the pretty much same underlying concepts and once you truly understand one you understand them all.
And choosing a branch will keep you more productive. If you choose something that sounds cool and interesting to you, you'll want to work on it a lot, and you'll learn a lot of new things. But this is something that will come natural to you in time, and you shouldn't worry about anything than learning the language for now.
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| Posted: Wed, 8th Dec 2010 20:01 Post subject: |
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I'm trying to solve this problem but im unsure of what to do, i asked Leo as well but he seems busy so i don't wish to take his time so i thought i'd try my luck here.
the problem :
"A method to simulate exponential decline is to roll 100 dices and remove all sixes. Then you roll the remaining dices and remove all sixes etc.
Write a program that simulates exponential growth according to the model above.
TIP : Use two for loops and an if loop or whatever you call it.
Simulation of a dice can be written as : (int)(6 * Math.random() + 1)
So if we write it down in pseudo code:
1. Roll a certain amount of dices ( 100 in our case)
2. Write out the amount of rolled dices in a text window (JOptionPane.showMessageDialog)
3. Count the amount of sixes that have been rolled.
4.Remove the same amount of dices as the amount of rolled sixes.
5. Do step 1-4 10 times.
6. write out the result.
"
This is how far i've gotten :
| Code: |
import java.util.StringTokenizer;
import javax.swing.*;
public class Proguppgift866 {
public static void main(String[] arg) {
Integer dice =(int) (6 * Math.random() + 1);
String total = "";
for(int i = 1; i <= 100; i++) {
Integer dice1 =(int) (6 * Math.random() + 1);
total = total + " " + dice1;
}
JOptionPane.showMessageDialog(null,"100 dices have been thrown");
StringTokenizer t = new StringTokenizer(total);
int n = 6;
int dices = 100;
while (t.hasMoreTokens()) {
if(t.nextToken().equals(6))
dices = dices -1;
}
}
}
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Any ideas?
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| Posted: Wed, 8th Dec 2010 20:20 Post subject: |
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First nest all of it in a for loop that loops ten times. Then replace "100 dices have been thrown" with a more dynamic message that uses a variable of your code to print the actual amount.
This should point you in the right direction.
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| Posted: Wed, 8th Dec 2010 20:52 Post subject: |
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| me7 wrote: | First nest all of it in a for loop that loops ten times. Then replace "100 dices have been thrown" with a more dynamic message that uses a variable of your code to print the actual amount.
This should point you in the right direction. |
I see, what do you think about the removal of dices that equal 6? I am a bit unsure of how to do that step. Also how am I gonna remove dices for every step when i loop everything together 10 times?
Because the end result window will look something like this:
Exponential decline :
100
84
73
60
50
44
etc for ten steps.
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| Posted: Wed, 8th Dec 2010 21:10 Post subject: |
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I'd do it this way:
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int dices = 100;
for (int i = 0; i < 10; i++)
{
System.out.println("There are " + dices + " dices");
int remove = 0;
for (int j = 0; j < dices; j++)
{
int dice = (int) (6 * Math.random() + 1);
if (dice == 6)
{
remove++;
}
}
dices = dices - remove;
}
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Werelds
Special Little Man
Posts: 15098
Location: 0100111001001100
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| Posted: Wed, 8th Dec 2010 21:14 Post subject: |
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Start by storing the rolled values into an array rather than appending them to a string
Allows you to easily get rid of specific values, or a specific number of values, etcetera. A string is rarely the datatype you want to be working with, that's usually the last conversion you do in order to display it to the user, and even then it's often done implicitly
I'd personally get some recursion going for stuff like this, but the quick and dirty way is indeed to nest several for loops. One big one that goes around 10 times, then a couple of ones inside that to roll x times, followed by one that loops over all the results to check for sixes.
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| Posted: Wed, 8th Dec 2010 21:16 Post subject: |
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I don't know what an array is, seems like that is unfortunate in my case. I'll try what you wrote me7. thanks.
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| Posted: Wed, 8th Dec 2010 22:20 Post subject: |
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Another problem: "A sum that converges very slowly towards pi can be written like this: pi = (4/1) - (4/3) + (4/5) - (4/7) + .... Write a program which determines how many terms you need to add to reach pi with 6 correct decimals ( 3.141592). TIP: use a while loop. set (diff>0.0000005) in the while loop"
I solved it like this:
| Code: |
import javax.swing.*;
public class Proguppgift855 {
public static void main(String[] arg) {
int n = 0;
double s = 0;
while (Math.abs(Math.PI-s) > 0.0000005) {
s = s + (Math.pow((-1), n) * 4)/(2*n+1);
n= n+1;
}
JOptionPane.showMessageDialog(null, n );
System.exit(0);
}
}
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What do you guys think? Did I do it right or is there another cleaner way to do it?
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