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Thread: Can someone explain the logic behind looping through multidimensional arrays?

  1. #1
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    Default Can someone explain the logic behind looping through multidimensional arrays?

    Let say i have this code:
    public class ArrayDemo {
        public static void main (String[]args) {
     
            int [][] numbers = {{5,3,2},{7,5,3}};
            int sum=0;
            int index1;
            int index2;
     
            for (index1 =0; index1< numbers.length; index1++) {
            for (index2 =0; index2< numbers[index1].length; index2++) {
                 sum += numbers[index1][index2];
     
            }
        } 
           System.out.println("Sum: " + sum);
           System.out.println("Length: " + numbers.length);
     
    }}
    this is output
    output.PNG
    What does the statement
     for (index2 =0; index2< numbers[index1].length; index2++)
    mean?
    And why does the length = 2?
    Please i need an explanation in layman's terms.Thanks


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    Default Re: Can someone explain the logic behind looping through multidimensional arrays?

    Please break down each element of the for loop you've highlighted and tell us what you think it means. For example, write your explanation for each of the following:

    index1:

    index2:

    numbers:

    numbers[index1]:

    numbers[index1].length:

    index2++:

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    Default Re: Can someone explain the logic behind looping through multidimensional arrays?

    ehm,
    index1 = the index of the first element (row) in the array, say numbers[0][1], no?
    index 2 = the index of the first element (column) in the array
    numbers = the array
    numbers[i]= the current index the loop is in
    numbers[i].length (no idea, this is in fact the source of my confusion)
    index2++ = no idea.

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    Default Re: Can someone explain the logic behind looping through multidimensional arrays?

    Good job, and that little exercise pointed to the source of your confusion.

    Some disagree with the description of arrays in Java as "multi-dimensional." They insist, for example, that a 2D array in Java, myArray[x][y], is 'x' arrays of 'y' arrays, and 'y' doesn't have to be the same for each 'x'. That's why the notation you're confused about is important.

    Imagine the array was defined this way instead of the way in your original post:

    int [][] numbers = {{5,3,2},{7,5}};

    In this case, y = 3 elements in the first row but only 2 elements in the second row. Then to get the iteration right in nested for loops, the inside loop - the one that iterates through each column element - has to account for this difference in length:
    numbers.length = 2      // for 2 rows
    numbers[0].length = 3  // for 3 elements in the first row (3 columns)
    numbers[1].length = 2  // for 2 elements in the second row (2 columns)

    Hope this helps.

  5. The Following User Says Thank You to GregBrannon For This Useful Post:

    ojonugwa (August 27th, 2013)

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    Default Re: Can someone explain the logic behind looping through multidimensional arrays?

    Thanks.That explains a lot

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    Default Re: Can someone explain the logic behind looping through multidimensional arrays?

    thanx to all nice sharing . this thread help me lot of..

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