Nested loop to add data to an Array

Question

I'm really struggling to comprehend nested for loops and so have done as followed:

    int arr[][] = new int[10][3];
    int i, j;

    arr[0][0]=21;  
    arr[1][0]=41;  
    arr[2][0]=61;  
    arr[3][0]=81;  
    etc.

... to get the following output:

I understand this is highly inefficient, so have attempted to construct a nested for loop which clearly doesn't work:

for(i=21;i<81;i+=20)
        {
           for(j=0;j<arr[i].length;j++)
           {
              arr[i][j] = i+j;
           }
        }

A guide in the right direction is greatly appreciated. Thanks.

Looks like a nice pair of nested for loops to me. What exactly is your problem? — dave
– dave, Commented Oct 30, 2018 at 2:26

Dang Nguyen · Accepted Answer · 2018-10-30 03:54:56Z

2

This is what I tried, seem it match with your requirement, you can check it.

    int arr[][] = new int[10][3];
    int i, j;
    int value = 21;
    for(i =0; i <10;i++) {
        for(j = 0; j <3; j++) {
            arr[i][j] = value;
        }
        value +=20;
    }

edited Oct 30, 2018 at 3:54

answered Oct 30, 2018 at 2:32

Dang Nguyen

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Comments

Elliott Frisch · Accepted Answer · 2018-10-30 02:33:14Z

The formula you are interested in seems to be ((i + 1) * 20) + 1 which will give you the value at the correct zero based index. Like,

int[][] arr = new int[10][3];
for (int i = 0; i < arr.length; i++) {
    int c = ((i + 1) * 20) + 1;
    for (int j = 0; j < arr[i].length; j++) {
        arr[i][j] = c;
    }
}
System.out.println(Arrays.deepToString(arr));

Outputs (formatted for post)

[[21, 21, 21], [41, 41, 41], [61, 61, 61], [81, 81, 81], 
 [101, 101, 101], [121, 121, 121], [141, 141, 141], 
 [161, 161, 161], [181, 181, 181], [201, 201, 201]]

Note we could use Arrays.fill(int[], int) to fill the array too. Like,

for (int i = 0; i < arr.length; i++) {
    int c = ((i + 1) * 20) + 1;
    Arrays.fill(arr[i], c);
}

marc_s · Accepted Answer · 2022-01-15 13:02:48Z

0

I think this is what you might be looking for. Hope it helps

int minValue = 21, increment = 20;

int arr[][] = new int[4][3];

for(int i = 0; i < arr.length; i++)
{
  for(int j = 0; j < arr[i].length; j++)
  {
    arr[i][j] = minValue;
  }
}

minValue += increment;

edited Jan 15, 2022 at 13:02

marc_s

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answered Oct 30, 2018 at 2:38

changsiang

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4 Comments

Scary Wombat Over a year ago

what is the purpose of maxValue = 81 ?

dave Over a year ago

I don't think j[i].length in the inner for loop will compile.

Scary Wombat Over a year ago

Also i and j are undefined

changsiang Over a year ago

Thanks for all the comments. Correction has been made to the code.

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