CODEWITHSUNDEEP
elixir
Josh Petitt
Josh Petitt

Reputation: 9589

How to make list of integers with a start, length and increment in Elixir?

I would like to generate a list of integers. I have the starting value, the increment value and the length of the list.

I know this should be simple, but I can't crack it. I have tried list comprehensions, Stream functions, etc.

Here is what I've tried and what didn't work:

A range allows me to choose the start and end, but not the increment

1..3 |> Enum.to_list()

This list comprehension works, but is it the "best" way?

start = 1
length = 3
increment = 2
for i <- 0..length-1, do: start + i*increment

Upvotes: 5

Views: 1492

Answers (4)

Mario Olivio Flores
Mario Olivio Flores

Reputation: 2805

Here is a very concise alternative in pure Elixir.

> Enum.to_list(50..150//5)
[50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135,140, 145, 150]

> Enum.to_list(3..30//5)
[3, 8, 13, 18, 23, 28]

The // part is called range step operator: https://hexdocs.pm/elixir/1.14/Kernel.html#..///3

Upvotes: 2

tkowal
tkowal

Reputation: 9299

You can also use :lists.seq(From, To, Increment).

:lists.seq(start, length*increment, increment)

Upvotes: 3

michalmuskala
michalmuskala

Reputation: 11288

The answer by Gazler is a great one.

If you'd like to get a little bit more fancy you can use Stream.iterate/2 and Enum.take/2 for a similar result:

start = 1
length = 10
increment = 3
Stream.iterate(start, &(&1 + increment)) |> Enum.take(length)
#=> [1, 4, 7, 10, 13, 16, 19, 22, 25, 28]

Upvotes: 5

Gazler
Gazler

Reputation: 84190

You can do this with a comprehension:

for x <- 1..10, do: x * 3
[3, 6, 9, 12, 15, 18, 21, 24, 27, 30]

In the above example, the following are the values you specified:

start = 1
length = 10
increment = 3

You will need additional parentheses for a negative range:

for x <- -1..(-10), do: x * 3 
[-3, -6, -9, -12, -15, -18, -21, -24, -27, -30]

Upvotes: 5

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