WEBVTT

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so Python provides two different ways to
loop around the block of code with for

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loops and while now we’ve seen how a for can be used to iterate through

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an iterable so either the items in a
sequence or the numbers in a range but

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sometimes we actually need to
keep looping as long as some condition

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is true and then stop when it becomes
false, now we do this using a while loop, so

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the basic format of while loop is just a while and
some condition colon and execute a block

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of code let's just look at an example of
a for loop we have done earlier in this section

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and in contrast that to while loop, so
start off by typing

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and we can run that, again we’ve seen that code in this section

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nothing new there, i is now 0 through to 9 as you can see, because we use the range 10 so that's a for loop now with a

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while loop we can produce the same results so what what we can do is i’ll just comment that code out

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and we do something like

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same thing same placement field format, and so if we run that

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to confirm thats the same

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we get exactly the same result that we got in the for loop, you probably wouldn't use a while loop just to

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iterate through a range of values like
we’ve done here but this does give you a

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good example to see what's actually
going so the first thing to note here is

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the condition on line 4 i < 10

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well that controls execution of the block of code
in the loop which is the code

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on lines five and six so in
other words as long as i is less than 10

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then code on lines 5 and 6 is going to be executed but once it becomes equal to or greater than

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10 the condition becomes false and the loops terminates, in other words there no more processing and you saw that

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because when i was less than 10, is now 9 and

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we automatically skipped that point and that was the end but these leads to two other important points to

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consider here, one first the condition
must be initialized before the loop starts of

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you saw me do that on line 3 when initialised i to the value of 0, now with

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the for loop of course we just set i without setting the value first and that's because python knows that

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to give it the first value from the
range and to continue from there but with a

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while loop we had to give i a value first if we didn't get an error that is not defined so thats another

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distinction that you have to do when
using while loops but the

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second thing that is important here is we we need
some way within the loop for the

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condition to become false so just to show you what I mean if we remove this, we remove the

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indent, put them back to the left
hand side so as its no longer indented, and we run that

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you can see we got a loop happening on the right hand
side the code is being executed and

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I'll just stop that you could see there
was no way for the condition to become

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false in other words there was no way
to exit because i was never been incremented

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so this condition would forever stay true, will

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always have i less than 10 in that
situation and you saw an example here to

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the right that I was always 0 so consequently we can never have a false

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condition so the loop’s kept looping
infinitely and just out of interest if

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you want to do that we decided we wanted
to leave infinitely we could have to

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done while true and run it, we will
achieve the same results you can see

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there, i’ll stopped that now, now there are some
scenarios that you'll see loops that just use

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true as a condition but the body of the
loop then contains a way to break out of

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the loop, you can see in this scenario, and i’ll just undo the working example we had,  so you can see that why

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for loop is preferable in this particular and it will be very rare to see a while loop

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use like that because frankly there's
more work more code effectively got four

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lines of code they're doing what two lines of
the for loop of the lines 1 and 2, but there

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are many reasons for looping around the
block of code that don't involve just

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going through items in a sequence which we did in this case so for example in an

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adventure game players to have choose a
direction

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in order to exit from a location, now if they don't choose one of the available exits

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they stay where they are and thats an ideal use of a while loop let's do
something that like, so will just comment

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this code out and say something like

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and will just initialise and empty string and then we can put

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and on the next line we put, so just to confirm that lets run this, please choose a direction we can put

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something like north please choose a direction
because it wasn't in the available

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options you see a on line 7 we got 3 options and we
typed something like south aren’t you glad you got out of there so the interesting

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things about this is we initialize the
variable chosen_exit on line 9

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to set to an empty string and so that meant the
first time we started the while loop chosen_exit

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wasn't in the available exits and
therefore was able to go through and asked

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what the direction it was, if we had initialized that
variable to east lets say and run this you see

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that the loop didn’t get process at all because the first condition while

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chosen_exits not in available exits that
returned false in other words chosen

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exit was unavailable exit so
consequently this line's code was skipped

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altogether we just got this print out there so we initialised it on
line 9 and then it's going to keep

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looping effectively until we’ve got a
valid

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input, or the exit that we’ve chosen is in the list
that we define on line 7

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so probably the most important feature
certainly one of the most important

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features of while loops is that it can be
used in this scenario and that's where

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you can’t determine in advance how many
times you actually need to loop so for

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loop will repeat for each item in a
predetermined sequence whereas as you

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see here in a while loop you don't need to
know ahead of time many times the legal

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execute because of course in this
scenario this could continue sort of

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hundreds of times until the player typed in
a direction that was on our list on line

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7, so one particular application of this would
be reading data from a file now it's

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difficult not impossible but certainly
difficult to know in advance how much

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data is there read from a file in your computer
now the while loop can be used to keep

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repeating until there's no more data left
now will see this soon when we look at file input

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and output which is in a future section
on this course let's just enhance our

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example a little bit because just like
with the for loop we can also use break

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and continue to break out of the loop or to
cause the loop to stop the current iteration

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and to test the condition again, so breaks can provide a simple way

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to quit in other words without choosing
a valid exit so what we could to there is

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we could do something like, now if we run that

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we still got the scenario if we enter something wrong like west, that’s not going to work but if we typed quit you can see game over

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was printed out and then we exited the
game with a break, so the break exited out

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of the while loop in that scenario and again if we just run again to confirm that it still works with a

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valid one, east, that still works, but
we probably shouldn't print the final

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message if the player chooses quit , so just like a for loop we can also use

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else, the one you saw in the for loop
section, so otherwise down here we can put

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else, and this is going to be executed just like the for loop, if the code hasn’t been broken out in other words

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a break hasn't been
processed so if we run, again will type quit, oops I put a space there, quit, we’ve now got game

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over and you can see that because of the else
the statement wasn't printed and we can just put

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a valid one like east, we should find it does actually show that

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message on the screen, now on the early lecture on if statements if you recall

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we created a very simple guessing game
that's what the player had to guess a number

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between 1 and 10, know there are a
number of problems with that game one

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problem was that it only allowed two
guesses if he wanted to allow more than

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two guesses but we have to do, would
involved with be the nesting of more and more

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if test, every time they guess it correctly was employed and the other problem was 5 was always the correct number which make the game

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a bit boring not that its terribly engaging to begin with but the

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first problem we talked about their can
be solved using a while loop in the other words

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if we want to allow more than two guesses and
you'll be doing that in the challenge

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next, but before we do that let's take digress
slowly and address the second problem to

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produce a more interesting game that's because resolving the situation where

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the correct number to guess was always 5 so to fix the problem the regarding correct number being 5

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what we need to do is get the computer to
generate a random number in the chosen

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range, from one to ten in the example
of this game but of course we can extend that

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now Python has got a lot of features built in such as the ability to do
arithmetic, lists, ranges, and tuples and

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of course more than that, and you’ve seen a lot of
that already as we go through the

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course, and lists, iterators, etc you're going to be dealing

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with in the next section so there's
enough built in fact for us to be able

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to write a random number generator if
you want to do this but great news is that

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one has already been written for us and
ships with python in a module called

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Python so we're gonna be looking at
creating modules and packages much later

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in the course but for now what we’re gonna do is
used to one that comes with it, and because

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it's an external module we need to import
it into our program using the import

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statement, now don't worry too much about this now all you

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really need to know to use it is as we
have to import it which tells Python

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which location to grab that particular piece of code so that we can

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actually take advantage of that and use
it in our

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game let's now go ahead and type in that
game, I’m going to comment out this code again

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like so, so instead of typing lets go ahead and
i’ll just paste in here first and

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we'll go through it because it's
essentially exactly the same as the

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previous game with a few extra lines of code so you can see
what I've done there so we started off

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got import random, thats the
command which tells Python we want to take

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advantage of this pre written code to
generate random numbers and you can see

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on line 21 , first thing we started with the
highest to find a variable called

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highest which is set to a value of 1 and on line 21 we got the answer which is a random

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number between one and the highest
number which in this case 10 some in other

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words answer should be a number between
one and 10 each time and you can see

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we the modified program to also
display, put the numbers, please guess

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number between one and we can put
the number and whatever however we wish

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at the moment its set to highest and that means you
could change this number if you wanted to

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to make that sort of twenty fifty a
hundred or whatever you want to do

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and then we got the code which is essentially the
same as what it was before going to the guess scenario

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but of course it's still a little bit limited
in that its only going to give two chances to try

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it and probably in most cases you need at least 4 so lets try running this to

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confirm that it works, please guess a number between 1 and 10, we can try 5

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first please guess higher, 8, Sorry you have not guessed correctly, try again and of course it didn't help us

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because it didn’t tell us what the number was either
try again and see what we can get lucky, five

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again, please guess higher, try 8, sorry you have not guess correctly, will try one more time, 5 lower

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this time, will try 3

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sorry you have not guessed correctly, so you can see that the point is that
that number has now changed anyway we

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had two times it said the number is
higher the second time was lower

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so clearly its generating a random number each
time, but of course the game method exists is very

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frustrating and the reason that it's
frustrating of course is that we only got

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two chances so what we probably need is a
minute

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three or maybe four guesses to be sure of getting that number correct so that's going to give

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us a good challenge to try and so what
we'll do is we'll look at that challenge

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in the next video