Programming Basics: Statements & Functions: Crash Course Computer Science #12

Hi, I’m Carrie Anne, and welcome to CrashCourse
Computer Science! Last episode we discussed how writing programs
in native machine code, and having to contend with so many low level details, was a huge
impediment to writing complex programs. To abstract away many of these low-level details,
Programming Languages were developed that let programmers concentrate on solving a problem
with computation, and less on nitty gritty hardware details. So today, we’re going to continue that discussion,
and introduce some fundamental building blocks that almost all programming languages provide. INTRO Just like spoken languages, programming languages
have statements. These are individual complete thoughts, like
“I want tea” or “it is raining”. By using different words, we can change the
meaning; for example, “I want tea” to “I want unicorns”. But we can’t change “I want tea” to
“I want raining” – that doesn’t make grammatical sense. The set of rules that govern the structure
and composition of statements in a language is called syntax. The English language has syntax, and so do
all programming languages. “A equals 5” is a programming language
statement. In this case, the statement says a variable
named A has the number 5 stored in it. This is called an assignment statement because
we’re assigning a value to a variable. To express more complex things, we need a
series of statements, like “A is 5, B is ten, C equals A plus B” This program tells the computer to set variable
‘A’ equal to 5, variable ‘B’ to 10, and finally to add ‘A’ and ‘B’ together,
and put that result, which is 15, into — you guessed it — variable C. Note that we can call variables whatever we
want. Instead of A, B and C, it could be apples,
pears, and fruits. The computer doesn’t care, as long as variables
are uniquely named. But it’s probably best practice to name
them things that make sense in case someone else is trying to understand your code. A program, which is a list of instructions,
is a bit like a recipe: boil water, add noodles, wait 10 minutes, drain and enjoy. In the same way, the program starts at the
first statement and runs down one at a time until it hits the end. So far, we’ve added two numbers together. Boring. Let’s make a video game instead! Of course, it’s way too early to think about
coding an entire game, so instead, we’ll use our example to write little snippets of
code that cover some programming fundamentals. Imagine we’re building an old-school arcade
game where Grace Hopper has to capture bugs before they get into the Harvard Mark 1 and
crash the computer! On every level, the number of bugs increases. Grace has to catch them before they wear out
any relays in the machine. Fortunately, she has a few extra relays for
repairs. To get started, we’ll need to keep track
of a bunch of values that are important for gameplay, like what level the player is on,
the score, the number of bugs remaining, as well as the number of spare relays in Grace’s
inventory. So, we must “initialize” our variables,
that is, set their initial value: “level equals 1, score equals 0, bugs equals 5, spare relays equals 4, and player name equals “Andre”. To create an interactive game, we need to
control the flow of the program beyond just running from top to bottom. To do this, we use Control Flow Statements. There are several types, but If Statements
are the most common. You can think of them as “If X is true,
then do Y”. An English language example is: “If I am
tired, then get tea” So if “I am tired” is a true statement,
then I will go get tea If “I am tired” is false, then I will
not go get tea. An IF statement is like a fork in the road. Which path you take is conditional on whether
the expression is true or false — so these expressions are called Conditional Statements. In most programming languages, an if statement
looks something like …. “If, expression, then, some code, then end the if statement”. For example, if “level” is 1, then we
set the score to zero, because the player is just starting. We also set the number of bugs to 1, to keep
it easy for now. Notice the lines of code that are conditional
on the if-statement are nested between the IF and END IF. Of course, we can change the conditional expression
to whatever we want to test, like “is score greater than 10” 5 or “is bugs less than 1”. And If-Statements can be combined with an ELSE statement, which acts as a catch-all if the expression is false. If the level is not 1, the code inside the
ELSE block will be executed instead, and the number of bugs that Grace has to battle is
set to 3 times the level number. So on level 2, it would be six bugs, and on
level 3 there’s 9, and so on. Score isn’t modified in the ELSE block,
so Grace gets to keep any points earned. Here are some examples of if-then-else statements
from some popular programming languages — you can see the syntax varies a little, but the
underlying structure is roughly the same. If-statements are executed once, a conditional
path is chosen, and the program moves on. To repeat some statements many times, we need
to create a conditional loop. One way is a while statement, also called
a while loop. As you might have guessed, this loops a piece
of code “while” a condition is true. Regardless of the programming language, they
look something like this: In our game, let’s say at certain points,
a friendly colleague restocks Grace with relays! Hooray! To animate him replenishing our stock back
up to a maximum of 4, we can use a while loop. Let’s walk through this code. First we’ll assume that Grace only has 1
tube left when her colleague enters. When we enter the while loop, the first thing
the computer does is test its conditional…is relays less than 4? Well, relays is currently 1, so yes. Now we enter the loop! Then, we hit the line of code: “relays equals
relays plus 1”. This is a bit confusing because the variable
is using itself in an assignment statement, so let’s unpack it. You always start by figuring out the right
side of the equals sign first, so what does “relays plus 1” come out to be? Well, relays is currently the value 1, so
1 plus 1 equals 2. Then, this result gets saved back into the
variable relays, writing over the old value, so now relays stores the value 2. We’ve hit the end of the while loop, which
jumps the program back up. Just as before, we test the conditional to
see if we’re going to enter the loop. Is relays less than 4? Well, yes, relays now equals 2, so we enter
the loop again! 2 plus 1 equals 3. So 3 is saved into relays. Loop again. Is 3 less than 4? Yes it is! Into the loop again. 3 plus 1 equals 4. So we save 4 into relays. Loop again. Is 4 less than 4?…. No! So the condition is now false, and thus we
exit the loop and move on to any remaining code. That’s how a while loop works! There’s also the common For Loop. Instead of being a condition-controlled loop
that can repeat forever until the condition is false, a FOR loop is count-controlled;
it repeats a specific number of times. They look something like this: Now, let’s put in some real values. This example loops 10 times, because we’ve
specified that variable ‘i’ starts at the value 1 and goes up to 10. The unique thing about a FOR loop is that
each time it hits NEXT, it adds one to ‘i’. When ‘i’ equals 10, the computer knows
it’s been looped 10 times, and the loop exits. We can set the number to whatever we want
— 10, 42, or a billion — it’s up to us. Let’s say we want to give the player a bonus
at the end of each level for the number of vacuum relays they have left over. As the game gets harder, it takes more skill
to have unused relays, so we want the bonus to go up exponentially based on the level. We need to write a piece of code that calculates exponents – that is, multiplying a number by itself a specific number of times. A loop is perfect for this! First lets initialize a new variable called
“bonus” and set it to 1. Then, we create a FOR loop starting at 1,
and looping up to the level number. Inside that loop, we multiply bonus times
the number of relays, and save that new value back into bonus. For example, let’s say relays equals 2,
and level equals 3. So the FOR loop will loop three times, which
means bonus is going to get multiplied by relays… by relays… by relays. Or in this case, times 2, times 2, times 2,
which is a bonus of 8! That’s 2 to the 3rd power! This exponent code is useful, and we might
want to use it in other parts of our code. It’d be annoying to copy and paste this
everywhere, and have to update the variable names each time. Also, if we found a bug, we’d have to hunt
around and update every place we used it. It also makes code more confusing to look at. Less is more! What we want is a way to package up our exponent
code so we can use it, get the result, and not have to see all the internal complexity. We’re once again moving up a new level of
abstraction! To compartmentalize and hide complexity, programming
languages can package pieces of code into named functions, also called methods or subroutines
in different programming languages. These functions can then be used by any other
part of that program just by calling its name. Let’s turn our exponent code into a function! First, we should name it. We can call it anything we want, like HappyUnicorn,
but since our code calculates exponents, let’s call it exponent. Also, instead of using specific variable names,
like “relays” and “levels”, we specify generic variable names, like Base and Exp,
whose initial values are going to be “passed” into our function from some other part of
the program. The rest of our code is the same as before,
now tucked into our function and with new variable names. Finally, we need to send the result of our
exponent code back to the part of the program that requested it. For this, we use a RETURN statement, and specify
that the value in ‘result’ be returned. So our full function code looks like this: Now we can use this function anywhere in our
program, simply by calling its name and passing in two numbers. For example, if we want to calculate 2 to
the 44th power, we can just call “exponent 2 comma 44.” and like 18 trillion comes back. Behind the scenes, 2 and 44 get saved into
variables Base and Exp inside the function, it does all its loops as necessary, and then
the function returns with the result. Let’s use our newly minted function to calculate
a score bonus. First, we initialize bonus to 0. Then we check if the player has any remaining
relays with an if-statement. If they do, we call our exponent function,
passing in relays and level, which calculates relays to the power of level, and returns
the result, which we save into bonus. This bonus calculating code might be useful later, so let’s wrap it up as a function too! Yes, a function that calls a function! And then, wait for it…. we can use this
function in an even more complex function. Let’s write one that gets called everytime
the player finishes a level. We’ll call it “levelFinished” – it needs
to know the number of relays left, what level it was, and the current score; those values
have to get passed in. Inside our function, we’ll calculate the
bonus, using our calcBonus function, and add that to the running score. Also, if the current score is higher than
the game’s high score, we save the new high score and the players name. Finally, we return the current score. Now we’re getting pretty fancy. Functions are calling functions are calling
functions! When we call a single line of code, like this
the complexity is hidden. We don’t see all the internal loops and
variables, we just see the result come back as if by magic…. a total score of 53. But it’s not magic, it’s the power of
abstraction! If you understand this example, then you understand the power of functions, and the entire essence of modern programming. It’s not feasible to write, for example,
a web browser as one gigantically long list of statements. It would be millions of lines long and impossible
to comprehend! So instead, software consists of thousands
of smaller functions, each responsible for different features. In modern programming, it’s uncommon to
see functions longer than around 100 lines of code, because by then, there’s probably
something that should be pulled out and made into its own function. Modularizing programs into functions not only
allows a single programmer to write an entire app, but also allows teams of people to work
efficiently on even bigger programs. Different programmers can work on different
functions, and if everyone makes sure their code works correctly, then when everything
is put together, the whole program should work too! And in the real world, programmers aren’t
wasting time writing things like exponents. Modern programming languages come with huge
bundles of pre-written functions, called Libraries. These are written by expert coders, made efficient
and rigorously tested, and then given to everyone. There are libraries for almost everything,
including networking, graphics, and sound — topics we’ll discuss in future episodes. But before we get to those, we need to talk
about Algorithms. Intrigued? You should be. I’ll see you next week.

66 comments on “Programming Basics: Statements & Functions: Crash Course Computer Science #12”

  1. CrashCourse says:

    Hey guys! The fine folks at Thought Café brought our Grace Hopper game to life! Check it out here and tell us your best score! – brandon (366) <– not too shabby

  2. Knowledge Magnet says:

    4:08 It is different in python 3

  3. Catherine Ma says:

    1:53 JavaScript in the form of jQuery! lol

  4. Juan Macedo says:

    that moment when your middle name is andre,

  5. Vincent Shury says:

    It's not magic Its SCIENCE in bill Nye voice is all I thought of

  6. Starla Pearson says:

    Who dislikes these videos?? "OH GOD KNOWLEDGE EW GROSS"

  7. Sandya Thiyyari says:


  8. Sandya Thiyyari says:

    I found the only way to understand computing

  9. David Hughes says:

    7:33 I have to say the level of abstraction images shown in the series CS videos are amazing, from physical to two different logical diagrams, even the music changes. To the crew who designed that I say well done! I've tried to make my fair share of logical diagrams and it can be quite difficult.

  10. David Hughes says:

    5:03 I've found in computer science there are many times when reading from right to left makes more sense. I'm a mediocre "programmer" (I dare to use that word) at my absolute best but the reading right to left trick I eventually discovered late in life helps me quite a bit.

  11. CaddyWhumpusGamer says:

    I have wanted to get into programming for awhile now and found this video. It is by far the best I have found. I am looking forward to watching many more Like this one. Liked and subbed. keep up the good work.

  12. Bella H says:

    This was way too fast for someone who doesn't know anything about programming and has just started!

  13. steven stallings says:

    \global vars
    Int main (int num, char* str[])
    \ formal vars ——^
    \ local vars
    { \ subs calls
    Return(results); }
    \ subs

  14. Grace Zhao says:

    Couldn't give you enough like!!!! Your video is extremely helpfu!!

  15. Jessica Patrick says:

    Everything makes sense until I'm given a word problem and I have to try and figure out what the assignment is asking me to do.
    <– Computer Science Newbie

  16. Mugdha Kurkure says:

    4:07 What is going on with the condition for shell script????

  17. Jose A. Fernandez IV says:

    Its was the mark 2 that had a bug in it

  18. Joe Davies says:

    this is so helpful and fun to watch. i love it 😉

  19. Chris Sanders says:

    i wish i saw this video two years ago…..a lot of things would have made so much more sense lol i loved it

  20. lydia pooley says:


  21. lydia pooley says:


  22. DDavis says:

    Do you use like an computer app to start programming

  23. skatf says:

    This has helped me break down coding to a basic level which I desperately needed! Thank you so much.

  24. Not Available says:

    Love all CC videos thank you. In my opinion I do think the definitions are a bit rushed and it may be difficult for a new coder to watch this video and then explain what a function is. I'd describe a function as a nested block of code that performs some logic and can be called repeatedly at other points. This saves a coder from retyping those rules over and over again and they can simply reference the function they created.

  25. Victoria x’o says:

    Where is part 1?

  26. Ivo Vo says:

    I just wonder if she learn everything as fast as she speak

  27. al - Ashraf says:

    If i master this i can start to program anything ??

  28. man of the woods says:

    Is there a way to slow this video down a bit? I feel stressed

  29. Jen farmer says:

    This video quality wouldve been greatly improved with a paper bag for her head.

  30. williams robin says:

    thanks Carrie Anne

  31. Alesandro Alberto says:

    To be honest, this approach of teaching is too hard to understand for non-native person.

  32. A I says:

    “If you understand this example…..” hahahahaa.. I’m watching this wondering how so many people talk about coding like its just this little thing they learned, when from the outside as a beginner it looks like something unreasonably complicated. I sure hope it gets easier and clearer. Smh. Edit: she just mentioned libraries, so now it seems less scary and overwhelming

  33. Jose Vidbaz says:

    "Adding numbers is boring, let's make Video Games instead". Totally love this course!

  34. Isaac Ndidi says:

    You're honestly a great teacher

  35. 蔡德灿 says:

    9:20 I was ready 4 the mongtage, but no

  36. Ruchir Rawat says:

    the comment section is like a subreddit

  37. azrul nizam says:

    Imagine writing a programme without a libraries.

  38. admanvothing says:

    This series is incredible, I feel like all I need is homework to make this an entry level college course

  39. Okalt OKe says:

    can you just talk a bit slow.

  40. Brandonbraun says:

    she reminds me of mark corrigan from peep show

  41. Xentimex says:

    I find the speaker awesome

  42. Peekaymaja says:

    This episode not fun because I know all this stuff. Previous episodes were more fun.

  43. abbas fadhil says:

    things are getting easier

  44. ClassicalPHX says:

    But you can want raining (provided its a gerund); but then again language is more malleable (poetic?) than code I guess 🙂

  45. bill Niko says:

    thanks for your words about programming, if you do some lecture about computation theory the mother theory to mental,psychology,computer,programming,artificial intelligence and etc it will be more much better.

  46. bill Niko says:

    1:51 this is probably my wrong understanding about programming below
    the program starts at the first statement and runs down one at a time until it hits the end.
    this maybe not absolutely execute of all programming.I saw so many program if there are some special symbols calls the specific statement the specific statement will executes immediately,not following the up down sequence executing rule.
    so good describes of this case is : if condition have been turned on then program starts and runs down until it hits the end
    I don't know, this understanding may comes from illusion completely.

  47. Ashish Pothi says:

    Anyone notice the creeper on the shelf behind her?

  48. bill Niko says:

    a = 5
    "a" is variable name
    "5" is variable value
    "=" is assignment that assign variable value to variable name to create variable symbol,or assign any to any.
    you can use any symbol to represent any thing or symbol

  49. bill Niko says:

    if "X" is true
    then do "Y" (do some operating to variable symbol)
    a golden words of computational theory.

  50. daktarbari says:


  51. Richard S. says:

    Hi, does anyone know of a more programming-oriented course online that would be as digestible as this series?

  52. bill Niko says:

    thanks for your series that most clearly describe about programming I found

  53. bill Niko says:

    a function equals to a conditional abstracted variable symbol that hidden procedural program sort of as a word hidden definition.we call you create a function successfully by package program into a function as define function. it will receives value gives to specific named variable from where calls it then processes,gets the result variable value and returns the result value to function,but send value into function and function returns value isn't always an absolutely necessary needed for every function.

  54. Cockeyed Optimist says:

    speaks 100 miles per hour, makes my head shatter…at 3/4 playback speed, she made far more sense

  55. MightyMirth says:

    2:22 got distracted following the bugs and forgot to listen hahaha

  56. Debarshi Majumdar says:

    Hi… can anyone please explain me if if the "levelfinished" function needed additional information passed in the brackets ().. "highscore" and "playername"

  57. ClumsyCora says:

    Before I always avoided computer science topics. I always felt overwhelmed even though I belong to the hard sciences. Didn't bother me, I mean, my career should be totally independent from this. But lately at work, I'm dealing with so much data gathering and excel won't cut out–it's an inefficient nightmare. After that, I just knew I had to do something. I'm so happy to at least consider making myself literate and in this journey, this crash course series has been so enjoyable, my younger self would be so surprised. Kudos to you miss and your whole team. The effect is surely exponential. To be honest, i think that education for computer literacy should be as important and free as reading and writing in the future.

  58. CRAZ ZYY says:

    For i=1 to 10
    Next i

  59. Loz D says:

    1:45 Them be funny looking noodles

  60. Loz D says:

    This series is exactly how programming should be introduced to kids (or anyone for that matter) 👍

  61. THE Redstone BR says:

    Perfeito para programadores na Unity3D 😀

  62. Aee Bee Cee says:

    If a receives a letter with x
    Then a write a letter with y send to z

  63. DaveyDoo says:

    From an outsider whos never coded at 4:10 then python looks the least complicated, is that true in general?

  64. Menelik Z says:

    Am I the only one who thinks she has a motor mouth? Slow it down a bit lol so that I can learn something.

  65. MooSword says:

    Great teaching! Thanks for the video

  66. Renaissance Dreemurr says:

    7:59 I see someone is a Flufflepuff fan.

  67. Fredrik Hertzberg says:

    Those were NOT noodles

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