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Python has this function called 'filter',

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and filtering is really just a way to apply a function and

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to remove data that you don't want, just like the name intends.

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So if you wanted every second number, every even number,

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just vowels, you could do that.

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And you could apply a filter against some sort of data type.

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So just like the 'map' function, it takes a function name ,and

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then some sort of iterable, something that we can loop over,

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and if you were to run something like this, you're going to

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see that there is some sort of memory point allocated to it.

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And so you can't actually store this in a variable because

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it's just going to be a piece of memory.

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So let's go ahead and create an example here where we just

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want all the even numbers.

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So 'def get_even_numbers()', and then it's going to take a 'num'

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and this is not called 'get',

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let's call it 'filter_even_numbers',

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and we're going to give this a Docstring "Returns only even

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numbers". And what we can do here, is we can return whatever

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that 'num' is going to be,

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modulus, '% 2 = 0'.

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Now, the thing with the 'filter' function here, is if it returns

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True, then it's going to keep the data, whatever this value is

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going to be, but if it returns False, it's going to discard it.

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We'll see this in action in just a little bit.

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And with this modulus, what this is saying is, if there's

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a number, let's say 5 divided by 2, is there a remainder?

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And if there is no remainder, if that remainder is 0,

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then return True.

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And that's what this modulus, this '%' sign is, that's

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called modulus, and so that really just gets the remainder.

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So let's go ahead and run that.

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And let's create a list of numbers in here.

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So 'nums', we've got 1-2-3-4-5-6, all the way to whenever

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we want to stop, so 14, I guess.

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And now what we can do, is we can loop through this,

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so we can say 'for num in filter',

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it's going to take the name of the function, 'filter_even_numbers',

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and then the 'nums' itself.

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So the 'nums' is referring to the iterable, it's a list.

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So we can loop through it,

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it's called iterable.

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And 'filter_even_numbers' is referring to this function without

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the parentheses. It knows to run this function on its own internally

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so we don't need to execute it right now.

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And then we could simply print the number.

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Now, what this is going to do is, it's going to put number

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1 in here. Is 1 divided by 2,

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does that have any sort of remainder?

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What about 2?

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2 divided by 2, does that have a remainder?

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3 divide by 2, does that have a remainder?

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4 divided by 2, and it'll just do that over and over and over again.

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So let's go ahead, print this,

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and now we have just even numbers.

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2-4-6-8-10-12, and 14.

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Now, what happens if we just run this on its own?

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'filter(filter_even_numbers())',

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with 'nums' in here. Again, we get a memory location just like

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the 'map' function gave us.

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We can't even store this as a variable.

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So let's say 'evens =', again, it's just, it's saying that

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this is a 'filter', and is pointing to a piece of memory,

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and we can't do anything with that.

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But just like with 'map', what we can do, is cast this to a

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'list', and if we do that, there's some magic behind the scenes

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here. It will automatically give us a list of all the numbers.

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And so essentially, what we've done is we've created a function

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called 'filter_even_numbers', and we gave it something to loop

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through, and it looped through 1-2-3-4, all the way up to

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13 and 14,

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and it applied this logic to it every single time.

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Now, if this logic came back True, we had a number.

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If the logic came back False, it was stripped out.

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Now, where is this useful?

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Actually, even numbers or odd numbers it is quite useful.

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I've seen that quite a bit.

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You can also strip out characters that aren't a vowel or

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they are a consonant,

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or if you're dealing with a big website, you could even filter

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through users that have a certain permission.

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So you could get all these users, and then you can loop through each one,

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you can check to see if they're an admin, and just get a list

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of admin users, and just return User IDs,

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you could do something like that as well.

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So this gets to be very, very useful down the road.

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Let's go ahead and give 'filter' a shot,

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give 'map' a shot.

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