# 99 Scala Problems 15 - Duplicate the elements of a list a given number of times

## The problem¶

**P15** (**) Duplicate the elements of a list a given number of times.

Example:

```
scala> duplicateN(3, List('a, 'b, 'c, 'c, 'd))
res0: List[Symbol] = List('a, 'a, 'a, 'b, 'b, 'b, 'c, 'c, 'c, 'c, 'c, 'c, 'd, 'd, 'd)
```

## Initial thoughts¶

Problem 12 already taught me to use `fill()`

to build lists of repeated elements so this could be a good occasion to use it again.

## Solution¶

This problem is a generalization of the previous problem 14 which required to duplicate the elements of a list. There we could build the list into an anonymous function with the `List(e, e)`

syntax.

For this problem we have to use a more general solution, which is the `fill()`

method of the `List`

object (not class), already discovered in [problem 12]. This method is expressed in a curried form, thus the two argument applications.

Again, since the result of `fill()`

is a list, we have to flatten it through `flatMap()`

.

```
def duplicateN[A](n: Int, l: List[A]):List[A] = {
l flatMap { e => List.fill(n)(e) }
}
```

Scala allows us to express the anonymous function in a shorter way, making use of the underscore wild card.

```
def duplicateN2[A](n: Int, l: List[A]):List[A] = {
l flatMap { List.fill(n)(_) }
}
```

## Final considerations¶

Previous problems gave me all I needed to solve this one. I used **anonymous functions** and **mapping**, which are a very important component of Scala. I learned how to **simplify an anonymous function** using the underscore.

## Feedback¶

Feel free to use the blog Google+ page to comment the post. The GitHub issues page is the best place to submit corrections.

## Part 15 of the "99 Scala Problems" series

##### Previous articles

- 99 Scala Problems 01 - Find the last element of a list
- 99 Scala Problems 02 - Find the last but one element of a list
- 99 Scala Problems 03 - Find the Kth element of a list
- 99 Scala Problems 04 - Find the number of elements of a list
- 99 Scala Problems 05 - Reverse a list
- 99 Scala Problems 06 - Find out whether a list is a palindrome
- 99 Scala Problems 07 - Flatten a nested list structure
- 99 Scala Problems 08 - Eliminate consecutive duplicates of list elements
- 99 Scala Problems 09 - Pack consecutive duplicates of list elements into sublists
- 99 Scala Problems 10 - Run-length encoding of a list.
- 99 Scala Problems 11 - Modified run-length encoding
- 99 Scala Problems 12 - Decode a run-length encoded list
- 99 Scala Problems 13 - Run-length encoding of a list (direct solution)
- 99 Scala Problems 14 - Duplicate the elements of a list