### Filter, map and reduce

We now have a couple of levels active on Memrise.

The Lists level includes three very powerful and useful functions that we have not yet covered: `filter`

, `map`

and `reduce`

.

###filter

```
def f(x): return x % 2 != 0 # return True if odd
filter(f, [1,2,3])
>>> [1, 3]
```

or

```
filter(lambda x: x % 2 != 0, [1,2,3])
```

###map

```
def f(x): return x*x*x # return the cube
map(f, [1,2,3])
>>> [1, 8, 27]
```

or

```
map(lambda x: x*x*x, [1,2,3])
```

###reduce

```
def f(x,y): return x+y # add two numbers together
reduce(f, [1,2,3])
>>> 6
```

or

```
reduce(lambda x,y: x+y, [1,2,3])
```

`lambda`

just means *this is a function without a name that takes the following arguments*.

Now go and give Memrise a spin.