I am trying to solve this coding challenge
Result that I need

 1 8 9 16 17
 2 7 10 15 18
 3 6 11 14 19
 4 5 12 13 20

logic for colindex that is even
rowsNumber * colIndex+1 // 4*0+1 which gives 1 for column 0 row 1

logic for colindex that is odd
rowsNumber*(colIndex+1); // 4*(1+1) which gives 8 for column 2 row 1

const rowsNumber = 4;
const columnsNumber = 5;

for(let i=0;i<rowsNumber;i++) {

    for(let j=0;j<columnsNumber;j++) {
        if(j%2 == 0) {
        console.log(rowsNumber*j+1);
    }   else {
        console.log(rowsNumber*(j+1))
    }
  }

}

I am getting following result

1
8
9
16
17
1
8
9
16
17
1
8
9
16
17
1
8
9
16
17

What am I doing wrong?

>Solution :

Your logic is only valid for first row. You’ll also have to consider the rowIndex.
The actual logic is
For even rows : rowsNumber * (colIndex+1) – rowIndex
For odd rows : rowsNumber * colIndex+1 + rowIndex
So the code would be as follows

const rowsNumber = 4;
const columnsNumber = 5;
for(let i=0;i<rowsNumber;i++) {
    for(let j=0;j<columnsNumber;j++) {
        if(j%2 == 0) {
        console.log(rowsNumber*j+1+i);
    }   else {
        console.log(rowsNumber*(j+1)-i)
    }
  }
}

This gives the output as :

1
8
9
16
17
2
7
10
15
18
3
6
11
14
19
4
5
12
13
20