# For, Map and Reduce
## What's a list comprehension?
The command 'for' in Elixir is a list comprehension. The result of a for is a list.
For instance, in the example below, i goes from one to ten. The result is a list containing each value of i multiplied by 10.
```elixir
for i <- 1..10 do
i * 10
end
```
You could do the same using Enum.map/2.
```elixir
1..10
|> Enum.map(fn x -> x * 10 end)
```
If, instead, you wanted result of the sum of all the values of the list, you would have two options.
The first one is to usem Enum.sum() to sum all values of the resulting list.
```elixir
1..10
|> Enum.map(fn x -> x * 10 end)
|> Enum.sum()
```
The second option is to use Enum.reduce/2:
```elixir
1..10
|> Enum.reduce(fn x, accum -> x * 10 + accum end)
```
What if I wanted to multiply all values of the resulting list? The following solution would not work.
```elixir
1..10
|> Enum.reduce(fn x, accum -> x * 10 * accum end)
```
Because this is the value of 10 *... * 100:
```elixir
10 * 20 * 30 * 40 * 50 * 60 * 70 * 80 * 90 * 100
```
The correct way is:
```elixir
1..10
|> Enum.reduce(1, fn x, accum -> x * 10 * accum end)
```
What's the difference between Enum.reduce/2 and Enum.reduce/3?
## Back to for
For allows you to have more than one generator (the 'i <- 1..10' part):
```elixir
for i <- 1..3, j <- ["Brasil", "Mexico", "Angola"] do
{:number, i, :country, j}
end
```
You can also add filters:
```elixir
require Integer
for i <- 1..3,
j <- ["Brasil", "Mexico", "Angola"],
Integer.is_even(i),
String.starts_with?(j, "B") do
{:number, i, :country, j}
end
```
There are many more things that you can do with for, map and reduce. Explore Elixir's docs to learn more!
