问题描述:

This is probably very basic question.

Suppose I have an array with 19 items `[0,1,2,3,4..19]`

. How could I selecting only the ones with the index count that are multiples of a given number i.e. 2?

**Updated:** Supposed this code is intended to use for columns. How to manage to get indexes that would be of first column given that all are multiple of one?

Select elements which are multiples of a given number (works if your elements equal the index):

```
ary.select { |element| element % 2 == 0 }
```

In this special case, you can also use symbol to proc:

```
ary.select &:even?
```

If your elements are different from the indices, group in pairs of 2 and use the first element:

```
ary.each_slice(2).map { |slice| slice[0] }
```

Check out Enumerable#select (`Array`

includes `Enumerable`

). You can do this:

```
1.9.3p392 :001 > a = [1,2,3,4,5,6]
=> [1, 2, 3, 4, 5, 6]
1.9.3p392 :002 > a.select {|n| n % 2 == 0}
=> [2, 4, 6]
```

`select`

will filter the array, picking out anything for which the block returns true. In this case, using the mod (`%`

) operator to find elements divisible by 2.

If I understand you correctly you want to take only the even indices. Try using each_index.

```
arr = (0..19).to_a
arr.each_index { |x| puts arr[x] if x % 2 == 0 }
```

This prints:

```
0
2
4
6
8
10
12
14
16
18
```

If you want the even elements of the array you could use `find_all`

too.

```
arr.find_all { |e| e.even? }
```

```
(1..19).to_a.select{|e| e.%(2).zero?}
```

This is slightly faster than using `== 0`

.