Thread: Array related assignment

Hi guys and girls, nice to post again. Once again I have a problem, this time I have to create a.. welll here is what is says

"Create a program that uses a multidimensional array to store student grades. The first dimension should be a number for each student, and the second dimension should be for each student’s grades. Display the average of all the grades earned by each student and an overall average for every student.".

My problem? How do I create a multidimensional array and store something in one and something in the other dimension? Please don't give full code, I don't the whole answer just some help.


THanks!!

Tryin and Readin and surely some Gamin!!

How do I create a multidimensional array

int twoDim[][] = new int[5][]; // create a two dim array with second dim unassigned

Then to add second dim:
twoDim[0] = new int[4];

To put data in:
twoDim[0][0] = 123;

First sorry i forgot to include this link Programming with Java in 24 Hours: Storing Information with Arrays
You can see the full code there, its the one called StudentGrader.java.
And cloud you guys make it a little more clearer please.

Thanks
_________________________________________
Tryin and Readin and surely some Gamin!!

Try writing a small program using the above examples. Make some changes, set one of the elements, display some of the elements.
Come back when you have problems. Post the code and the errors.

I tried but i couldn't do any thing cause I didn't understand the examples.

_________________________________________
Tryin and Readin and surely some Gamin!!

Try copy and paste of any one of the above into a main() method.
Are you able to write a "Hello World" program?
Take that program and insert into it one of the above.

I tried them and here is what I got, I started by the one I understood completely

Originally Posted by helloworld922;12746

[code

int[][] my2dArray = new int[5][3]; // creates a 5x3 array[/code]

My code:

class Test {
    public static void main(String[] args) {
        int[][] my2dArray = new int[5][3];
        int num = 33;
        my2dArray[2][2] = 3 * 33;
        System.out.println(my2dArray[2][2]);
    }
}

My result:
run:
99

Second one

Originally Posted by helloworld922;12746

int [

[ ] my2dArray = new int[5][];
for(int i = 1; i <= my2dArray.length; i++)
{
// initialize an int array and put it into my2dArray
my2dArray[i-1] = new int[i];
}

My code:

class Test2 {
    public static void main(String[] args) {
        int[][] my2dArray = new int[5][];
    for(int i = 1; i <= my2dArray.length; i++){
     // initialize an int array and put it into my2dArray
     my2dArray[i-1] = new int[i];
     System.out.print(my2dArray[4][0]);
         }
    }
}

My result:
Exception in thread "main" java.lang.NullPointerException
at Test2.main(Test2.java:7)
Java Result: 1

Line 7 :

     System.out.print(my2dArray[4][0]);

Have no clue whats wrong, please make it clear. No errors at all till Netbeans runs it.

Here is what i got:
run:
0

But I still don't understand what is happening in the for loop.

Each dimension of an array must be initialized separately.
int[][] twoDim = new int[5][]; // define a two dim array
So now twoDim has 5 empty slots where there needs to be added one dim arrays.
So the code goes thru a loop 5 times to add that one dim array to twoDim
twoDim[i] = new int[4]; // add the second dim array

At the end of the loop twoDim has 5 elements, each of which is one dim array of 4 elements.

Using indexes you can go from element twoDim[0][0] to twoDim[4][3]

Can you tell me what the I in

twoDim[i] = new int[4];

stands for?

I don't see an I in what you posted. I do see an i. Case is important in Java i and I are different.

The variable i in twoDim[i] is an index into the twoDim array. The statement
twoDim[i] = new int[4];
would create a 4 element array and put its address into the twoDim array at element i. i could have a value from 0 to 4 because twoDim[] has five elements

Norm are you sure about the way you initialise an array I haven't worked with them for a while and never have extensively but I was under the impression they could be initialised this way:

int[][] twoDim = new int[5][4];

Originally Posted by Melawe

Here is what i got:
run:
0

But I still don't understand what is happening in the for loop.

It comes up as 0 because you haven't declared it yet. It came up with an error because you were trying to show a value that hadn't been initialised yet(only the first element has been initialised after the first run through.

I assume you are using the loop in helloworld's post

for(int i = 1; i <= my2dArray.length; i++)
{
    my2dArray[i-1] = new int[i];
}
System.out.println(my2dArray[4][0]);

whats happening is creating an array that looks like

#
##
###
####
#####

where # represents a space in the array if that makes sense

if you said

int[][] my2dArray = new int[5][5];

on the other hand it would look like this(I say look but maybe that's not the best way of putting it but I find it's good to visualise it)

#####
#####
#####
#####
#####

I try to write a short program to test before posting. The following compiles and executes.

public class TestArrayDef {
 
  public static void main(String[] args) {
     int twoDim[][] = new int[5][];
     twoDim[0] = new int[2];
     twoDim[0][0] = 123;
     System.out.println("twoDim[0][0] =" + twoDim[0][0]);
 
     int[][] my2dArray = new int[5][5];
  }
}

Running: D:\Java\jre6_10\bin\java.exe -classpath D:\JavaDevelopment;. -Xmx512M TestArrayDef

twoDim[0][0] =123

0 error(s)

Okay thanks everyone!! Cloud you guys help me with one more thing please, storing data in one dim and storing some other data in the second dim.

Thanks!

Well looking at your original post you say:

Originally Posted by Melawe

"Create a program that uses a multidimensional array to store student grades. The first dimension should be a number for each student, and the second dimension should be for each student’s grades. Display the average of all the grades earned by each student and an overall average for every student.".

So from my understanding you just need a 2D array of two "columns" so it would look something like this

1 65
2 50
3 80
4 55

Where the first column is the student number and the second is the grade. Am I correct in this assumption? If so you just need a 2D array of ints right?

Although this part has me a bit in doubt "Display the average of all the grades earned by each student and an overall average for every student"

Yep thats what it says.

Originally Posted by helloworld922

To initalize it, you have two three choices in Java.

Shouldn't that be 'three choices'?

Don't forget array initializers:

int twoDim[][] = { 
   { 1, 2, 3 }, 
   { 4, 5, 6 } 
};

Sorry guys I didn't reply in a while. I'm trying some things out, nothing worked yet but here is what I was doing and what I got. I thought it would be easier to do if I knew how to do it in one dim arrays, then all i had to do was squeeze them to in a two dim array.

class NewAry {
    public static void main(String[] args){
        int[] stud = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
        int[]grd= new int[14];
        int grd[0]= 60;
        System.out.println(int grd[0]);
    }
}

DLorde could you tell me what the lines of code in your post will do and how I can modify it to suit my needs more?
Thanks!

Originally Posted by dlorde

Shouldn't that be 'three choices'?

Don't forget array initializers:

int twoDim[][] = { 
   { 1, 2, 3 }, 
   { 4, 5, 6 } 
};

When I tried it out i got this result:

run:
[[I@3e25a5

class JForumtest{
    public static void main(String[] args){
        int twoDim[][] = {
            { 1, 2, 3 },
            { 4, 5, 6 } };
        System.out.println(twoDim);
    }
}

Excuse "t" in the file name.

[[I@3e25a5

That's what the JVM will give you when you ask for a String representation of an object that does not have a toString() method to convert it for human viewing. Arrays are objects. The String representation for twoDim: [[I says two dimensions of int
and the @3e25a5 gives the object's location in memory. Not very useful info for you, but sometimes it is when debugging.

Their are some methods that you can pass an array reference to and it will generate nicer looking output. Look at the Arrays class.

Could you tell me whats wrong in this

class NewAry {
    public static void main(String[] args){
        int[] stud = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
        int[]grd= new int[14];
        int grd[0]= 60;
        System.out.println(int grd[0]);
    }
}

And what the error message is when I run the NewAry program(code is in first post page 3)?

Drop the int from the println statement like so

       System.out.println(grd[0]);

EDIT:
And the line before that you only need to declare the type of the array once.

grd[0]= 60;