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hey welcome back to our lecture on list
comprehension

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and what we're going to do an

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and the second part is talk about a

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and a function called zip that we can
use

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to combine two lists into

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a single list and and here's the
signature for Sep shows zip

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as type lester Bay to

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Lester B to list of bears

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of a call might be and here's an example
of zipping through less together

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and and there you can see why this

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function is called Sep it takes two
lists and it takes

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each value and combines them in a bear

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so ship of ABC and want to 34

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is a long be too c3

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and the zip stops as soon as one of the
lists

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is exhausted so in this case you see
there's four elements on the right left

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but only three elements on the left lest
and the resulting list

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has three elements and as I mentioned in
one of the early lectures

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I think the signature of ship

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would be a little bit nicer if it was
not courage if he would ride

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where a blessed of a list gonna be

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to list of pair of a commodity

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and but this is just the way

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ship is defined in the Haskell standard
library

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I'm using ship

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weekend and the fine the following
function

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that takes a list and creates bear shove

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every adjacent value so if we have let's
first look at the

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an example down here before we look at
the implementation because I think when

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you see the example the implementation

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is fairly obvious so what we want to do
is given 1234 we wanted an upgraded

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chain

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like a sliding window of and bears

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want to 23 34 so you see that

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you know did do is change their site
won't do

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23 NN to treat 34 so how can we do that

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and dish function here is

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and I think a beautiful

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example of and using zip

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and at to do that so what you do.

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is I take that list 1234

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then I a good day love this left

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234 and then I ship those two together
show

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now want to get this century starts the
first this is 1234

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and the second place this dude 34 so now
when we ship it we get to jury

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and then when we ship the next wrongly
get sorry I'm Egypt that we get one too

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the next is 23 and the Nexus 34

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write a review if you withdraw

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the picture you get the last you drop
the first element

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shift that one up and then you ship that
too and this

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is exactly what you get alright

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am here's another function that

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and uses the function pairs

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to check whether a list shortage so what
we do is we

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and take the all the adjacent pairs in
the last

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sorry dad it will give us here extra my
wife's

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and then recheck that the first value is
smaller than the second

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the list is sorted if if we look at
every adjacent pair of elements

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the first one is less than the second so
this is a super concise

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I'm way to check whether list is short
show let's just look at some examples

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is wanted 34 shorted yes it is begosh

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one comment to long this lasts into an
annex Paris to commit three

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do is smaller than three and final pair
streak on my for

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artery is less than four so at that list
is sorted

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an is 123 24 sordid

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no because the second pair in

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bears of this blessed is to bear

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32 and their 3 is

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greater than 2 so that will return false

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using ship

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we can also define

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the positions of a.m. an element analyst

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as I mentioned before am lists dog have

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any position unlike erasure in an array
you can

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index into the array an but lists you
cannot read less you can only access the

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elements

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from an left to right so I can take the
hat

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and then the had of the day line and had
said

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and Dale detail a chat room an

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but sometimes its convenience to also
have the position

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of each element analyst so this is a
fairly common

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operation and the way we do that is
using ship

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so what we do as we take the list excess

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we take an the list from zero to an

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and where and if the length of this list
but this

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really can be the infinite less starting
at zero an

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and then we just ship the

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and the alleged am

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wit the positions to get a.m.

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every element here

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alright so if we look at the a.m. and

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am positions here

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we get depositions of 0 well

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does Rd the cities and %eh so what we
did beef what we did is we first zipped

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every element with its position and then
we felt there are two only the values

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that we were looking for because here
where only

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and dresses in the positions %uh 0

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and and so and this think goes into
steps we first

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bear every element to it its position

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and then we felt their odds only the
values that we were looking for

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am up there in Haskell strings are
nothing more than syntactic sugar for

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list of characters

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which means that you can I use
comprehension

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over list as well so for example here's
a string

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and ABC and which

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is really nothing more then the list of
characters

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a be and she and hands all operations on
list

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including comprehension but also map
filter

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Ford are all there lest abrasions that
we'll see

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will work over and strength and

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for jump on the length of abcd as five

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and why is that because that string is
just a list

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of five characters and showed issues is

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the length pension over list we can take
the first three

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an characters a blessed by you using
take three sorry got ABC

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and we can ship a list a Sharia string

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where the list an and this way so for
example every ship

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ABC with 1234 we get the list of pairs

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where the character a Wong character be
too

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and character see with three and many
other languages

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also have strange emblem and

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enter a baller enumerator so you can do

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an the same thing and but in Haskell
really

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there is no special type for less its

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defiant as a list of characters and the
only thing is dead

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there's a special literal syntax just
like

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this index for lists really as we have
seen means Wong

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comes to on three cons for const on the
entry list

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and here's an example

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where we an can't count

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the and number %uh lower case characters

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in a string so what we do as we say

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give me all the characters axe in a
string access

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filter out all the launch that are lower
case

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an and then computer the link

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so for example if we counted the number
of lower case characters

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and a string has car we see that there
are six

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to for 6 and then

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the first character isn't a brick a
symbol

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alright hopefully and

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you've got a little bit of a feeling of

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list comprehension and zip and

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and the fact that strings are less and

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and the exercises there will be plenty
of opportunity

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to and practice with that she next week

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bye bye happy hacking