Java Looping through 2D array - Homework

Question

I have to loop through a 2D array, create and store a random question, and test the user's response. However, I can't figure out how to properly reference the elements. I'm use to the old syntax of (counter; counter < x; counter++).

How do I reference a specific array element with this syntax? It's quite confusing to me. I need to reference the 5th element in the row to see what the user entered to break from the loop and also to loop through and transpose a 1D array into the 2D array's current row.

    for(int arrRow[] : arr)                 //arr is a [100][5] array
    {
        switch(rNum.nextInt(4))             //Creates a random number between 0 and 3 and passes it to a switch statement
        {
            case 0:                         //Generates an Addition question
                arr2 = a.quiz();
                break;
            case 1:                         //Generates a Subtraction question
                arr2 = s.quiz();
                break;
            case 2:                         //Generates a Multiplication question
                arr2 = m.quiz();
                break;
            case 3:                         //Generates a Division question
                arr2 = d.quiz();
        }

        //for (colNum=0; colNum<5;colNum++) //loops through the column in the 2D array and pulls data from returned array
        for(int arrCol : arrRow)
        {
            arrCol = arr2[arrCol];
        }

        if(arrRow[4] == -1)                 //If user enters a -1, breaks from the for loop
        {
            break;
        }
    }
    newTest.printQuestionResult();          //Calls the print function after the user is done or the test is complete
}

You do not need arrCol = arr2[arrCol]; The code for(int arrCol : arrRow) already assigns the value to arrCol — Scary Wombat
– Scary Wombat, Commented Oct 11, 2018 at 1:32
What's arr2 supposed to contain? And what values do arrCol hold that made it the index of arr2? What is clear is arrCol = arr2[arrCol]; is definitely wrong as you are assigning a primitive variable (referenced by value) with new value, which wouldn't change anything. — Jai
– Jai, Commented Oct 11, 2018 at 1:36
Consider your 2D Array like a Data Table that contains 100 Rows (y) with 5 Columns (x) in each Row. Iterate through your Array with that in mind: for (int y = 0; y < arr.length; y++) { for (int x = 0; x < arr[y].length; x++) { System.out.print(arr[y][x] + "\t"); } System.out.println(""); }. — DevilsHnd - 退した
– DevilsHnd - 退した, Commented Oct 11, 2018 at 1:47
@ScaryWombat - Maybe I'm not interpreting the book correctly. My understanding is that "for(arrRow[] : arr)" loops through the rows of the arr array and could be considered as arr[x], where the x value increments each time the loop runs. "for(int arrCol : arrRow)" would then loop through that row's columns. I would then interpret that arrCol would represent arr[x][y] where the y value increments each time that loop runs. Based on that, I further assume that "arrCol = arr2[arrCol]" would be the same as "arr[x][y] = arr2[y]". — RLee
– RLee, Commented Oct 11, 2018 at 4:58
@DevilsHnd - Thanks for that tip. I can get it to work that way no problem, but I'm really trying to understand this unfamiliar syntax from my class's assigned text. They only have a few examples in the book and I just can't seem to grasp the concept right now. — RLee
– RLee, Commented Oct 11, 2018 at 5:04

Jai · Accepted Answer · 2018-10-11 01:58:35Z

2

Your arrCol is an int which is a primitive type variable, so this variable is a value copied from arrRow. If you assign any value to arrCol, it will not be reflected in arrRow.

You should do this instead:

for (int index = 0; index < arrRow.length; i++)
{
    int col = arrRow[index];
    arrRow[index] = arr2[col];
}

I'm not sure what arr2 contains, so I can't be sure if you will encounter ArrayIndexOutOfBoundsException when you read its elements like this.

I would guess that you needed arr2[index] rather than arr2[col].

answered Oct 11, 2018 at 1:58

Jai

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1 Comment

RLee Over a year ago

arr2 is an int array with 5 elements. the case statement calls a randomized quiz and returns a 5 element array to the arr2 array. IE - if an Addition quiz is called it could contain {1,0,2,3,4}: 1 is the left argument 0 indicates it's an addition question 2 is the right argument 3 is the correct answer 4 is the user's response The second for loop pulls the contents of each of arr2's elements into the current row of the main array. If ever the user keys in a -1, it breaks from the first for loop. It works with a regular for loop, but not this new syntax I'm unfamiliar with.

RLee · Accepted Answer · 2018-10-11 14:43:21Z

0

It looks like you cannot use a for-each loop to alter the elements of the array as I was intending to do. I'll have to stick with the typical for loop with a counter.

https://www.geeksforgeeks.org/for-each-loop-in-java/

answered Oct 11, 2018 at 14:43

RLee

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