learn about research we are going to
learn about what recursion is how to do
it and why it is useful this video is
also part of a series you’ll get a lot
more out of this video if you have
watched the previous episodes in this
series first you can find them there so
what is recursion recursion is when a
function calls itself until it doesn’t
that is seriously all the recursion is
recursion is when a function calls
itself until yes a lot of people think
recursion is hard and the reason that
people think that recursion is hard is
that because of some collective insanity
all explanations of recursion on the
internet uses Fibonacci numbers as an
example the only reason and non
mathematician would have heard about
Fibonacci numbers is if they would have
watched the TV series fringe when
somebody tries to explain recursion to
you using Fibonacci numbers you must
murder them I know that sounds rough
because they they might be your friend
and they are they only mean well but
they must die let me draw your attention
to the code we are going to implement
this map count down from it should down
in this case we’re counting down from 10
search should just output 10 9 8 and
then so forth until
one stops we are going to implement this
using recursion I’m going to start by
declaring the function let countdown
from fix our member and there’s the
function this is ECMO script six you
might be watching this from the near
future where this is completely normal
to you on in current time I’ve had some
audience requests to write things in a
coma script 5 instead normally I listen
to audience feedback because I love you
guys
but because I love you guys I won’t
listen to this one because I love you I
want to pressure you to become better
this is one situation where I won’t go
easy on you Eckman script 6 has been
finalized for over a year now it’s time
to get into its really simple if you
just use bobble and gab installed g4
elevator music oh boo boo boo boo boo
Boop done clearing and now you can run
horrible node example gasps okay that
this is my standard string to output run
that again and we have to so quick
Eggman script six crash course lead is
just the new bar you should always use
left instead of barn arrow functions is
just a shorter function syntax hoop is
cool but let’s instead output the number
run that okay 10 so for countdown from
is a function that takes a number and
starts counting down from and that is
counted
what counting down is you take a number
say it then you take that number you
just said minus one and you say that and
now – one run that why thank you
computer for counting down to minus
15,000 if we scroll back up into
infinity okay yeah somewhere here ah
yeah here so you see that it works but
we don’t have any stop condition we’re
gonna fix that but I want to draw your
attention to a little thing it call here
at maximum call stack size exceeded the
call stack it’s the stack of function
calls that your code has made and in
most non functional programming
languages there is a upper limit to how
far you can go functional programming
language has never had this limit
because you use recursion for everything
and then you can have this limit
javascript has this limitation in ACMA
script 5 but it is removed in Atma
script 6 even though that Bobble can’t
simulate it because it’s an engine thing
and this is why atmosphere 6 is so
interesting from a functional
programming perspective because it
removes one of the main limitations to
doing functional programming with
JavaScript this feature is called tail
call optimization by the way because
it’s this big tail of function calls
let’s add that stop condition if num 1 0
right so it just goes through time 10 9
8 7 3 2 1 and when it locks out 1 here
and here it will proceed to this line
and then it will take number 1 minus 1
and it will pass 0 in the counter from
which goes into this this is 0 now we’ll
see that and it will return and thus it
won’t go to 0 and minus 1 and minus 2
and so forth until 60 minus 15,000 and
that is recursion a function that calls
itself until it doesn’t give alight we
have learned what recursion is and a
simple example of how to do it but still
not clear why looking at the example
that we just did that you we could have
done this with just a loop right and yes
you could have
every time you do a loop you can use
recursion instead but it doesn’t work
the other way around because there
aren’t things that recursion can do that
loops cannot the first time I had to use
recursion was when I encountered a
problem that looked a little bit like
this so we had a database a relational
database which had these categories so
uh there was top categories like this
animals category here and the top
categories did not have a parent they
all said no in root categories under
animals there are categories like
mammals if you see here that the the
mammals category has animals as as
parent and we have categories that have
mammals as parents cats dogs and in turn
there’s Chihuahua Labrador which has
dogs as parrot cat
also cats here Persian Siamese so see
that these are the same here Persian is
a subcategory of cats at this time it
was really cool with the this
hierarchical BA dhtml menus you know
menus that work sort of like the Windows
Start menu where you hover a folder and
when you do it expands into that folder
and then you hover a subfolder that and
that expands into a new subfolder and so
on so I wanted to represent this
database as as that so in order to
represent that we need to make this in
just tree structure pass to the dhtml
menu we want an outfit that looks
something like this boom so this is a
tree structure animals contains another
tree with mammals here and mammals
contains to a tree with two properties
dogs and cats and these in turn have
Chihuahua and Labrador we don’t really
need modes here I think ah and since
they don’t have any subcategories they
just have a head no its comment that out
yes because this is just to remember
where we’re going this is by the way a
good trick whenever you’re programming
to always so always think about what
what it is you’re doing what is my end
goal instead of just starting to code I
think one of the most common and
mistakes in programming is to start
coding too early take some time to think
about your problem and where you’re
going in that will save you a lot of
time scrolling up a bit and we will also
I’ll make three categories run that
oh okay I made a mistake in my example
data häringe it is supposed to be an
array first curly bracket that good
error here make tree is not a fun yes we
have to define matrix let make tree be a
function undefined
yeah because make sure he doesn’t return
anything so yes return start by
returning an empty object so now we want
to start assigning the sub properties
here you want to create this animals
thing to do that need to break this out
into a variable so we can manipulate we
are gonna call this more than call back
out no part so tree on hold notes in
computer science we are we getting our
categories from we are getting them here
as and we are now going to filter the
cat words array ah we are looking for
the top body looking for the root
element alright so this will now be a n
an array of the categories that has
parents and no and at this moment it
will will just be this because it has
filtered out all of the others because
they don’t have for each subcategory we
are going take the ID of the category
which in this case in this first loop is
going to be animals here so idea here is
animals and we’re going to assign that
it’s the same thing here right it’s this
ah and that it’s now going to get the
subtree uh and we gonna make that by
calling make sure you recursive and it’s
gonna get the same categories could be
some a new line but it’s not gonna get
the same parent it’s going to get c ID
okay let’s run that okay that is maybe
the right result it’s a bit messy let me
yes this is the best tarik ever json
stringify and then you add as a second
argument to string five you pick no and
you two and two is four in divisions and
now is for magic things and go now we
have a nice tree and it’s pretty much
what we want we’re still up here with
the the objects but that’s not that
important let’s look through what just
happened so in the first loop when we
call make tree we are passing in
categories categories is going to be all
of the categories and we’re going filter
them here I’m going to filter out the
ones that have the same parent that’s
the one we pass in here
ah and that is the same thing as what we
passed in here so now it’s a route ah
and for every such category we are going
to for each it and in this for each this
is clear we’re gonna unsign property on
the node with the same ID as each
category with animals here you can
assign this node animal here with the
return value of ourselves but this time
we are
not as me known as a parent category we
are passing in animals here so now we
are making a tree with the categories
that have animals as approach and they
in turn will pull make tree and make
trees that has mammals as a parent
category and they in turn will form a
tree where the cats and dogs making
trees that include Chihuahua elaborately
and this could just go on and on and on
but it ends because it runs out of
things to make freezer and to a certain
degree you could do this with with loops
if you do nested for loops like the
outer for loop has I as an integer and
then the inner for loop as J and you go
in and in and in until you run out of of
consonants or sanity but that only works
if it’s a very limited amount of nesting
sometimes you need to make trees that
are a hundred level sleep and then you
have can use use and that’s recursion we
have learned that the recursion is just
a function that calls itself until it
doesn’t we’ve looked at an example of
how to use it to count down from ten and
we have looked at an example of why you
need recursion why you can’t solve
everything loops I am the one recording
is show but it is the audience that
makes a show I need to hear from you
either act mpj me on twitter or youtube
comment down below comment with a
reflection or or a question or something
that you’re confused about or something
funny or nice or what you would like the
next episode to be about speaking of
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