Name: Hello (World) Abstraction! - Computerphile
Uploaded: 2020-03-27T17:01:04.000Z
Duration: 14 min 6 s
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The imperative

Well, let me begin by asking you a question

Modal verbs - A2

Okay, what do you think the simplest program we could write

Not being a programmer but having done a lot of computerphile videos.

I suspect we're going for hello world here

I mean, it's probably the classic one everyone start with if we... we can even write it on a piece of paper

I'm going to use C to talk about this. You could write it in any programming language of your choice

Let me guess you walked into Dixon's and wrote a rather rude version of hello world. I didn't you, probably did

Oh, so the scrolling on the screen thing your basic is effectively hello world

So I'd go some prints something and then yeah, that's the more advanced version. But yeah, I think so

We could write it and see we have our main function

we have to have a few arguments if we're doing this properly because otherwise we'll get

Complaints in the comments and then we can do a printer

Backslash Ames we want to go to a new line at the end

A head of files and we've written Heather world

These pens are bought because they're a bit quieter and yet there's always a compromise somewhere isn't there's no other perfect pen

Now we've got pens that you can't actually read so there's a hello world program

And on the face of it it is one of the simplest programs you can write you can understand the instructions

That's going on and it says print whatever string it is. Hello world and it print it out. You compile it and you run it

But actually when you think about what's going is it a lot a lot of functionality is being hidden behind there

And one of the things you see a lot in computer science is that we build computer systems by using layers of abstraction

So we're making use of a code library. So what we've already got here is a level of abstraction in that

Printf does a lot of things it can do more than just print out a single line of text

we can actually get it to print out numbers integers as

Text so we can read them on the screen and it will convert them from the integer

to think so that's code libraries hiding a whole layer of

Instructions so we can think about this in terms of we get another bit paper. Let's turn it upside down and use the Australian version

We can think about we write our program and we're writing it

We're writing it in C and we're making use of that C standard library to actually make it do things

The computers don't understand C. They understand their machine code. So actually we already have to convert this into

The machine code for the program so we convert it into the machine code for the CPU that we're running on in this case

It's an x86 thing, but we could run it on a six eight thousand. We could run another Zed 8006 five or a two

PowerPC whatever type of computer you wanted to so we've already got on two layers of abstraction there

We've got without stretching away how the hardware works

Because as soon as you write it and see we can then compile it to run on different CPUs

How we actually print something out so we can use printf to print a number we can use it to print text

But we don't have to know how to convert a number

Okay. Well, that's a very good thing printf time to print formatted and that's what I was talking about

So you can say I want to print this number and you can also say I want it formatted to look like this

So I want this many of leading zeroes. I want this many decimal places afterwards and so on you can format the output

Hence the app so we've got a program here

We've got a hello world program and we want to print out the variable with a 42 in it

And we can put a % D in there. Just say we want to print out a number and

Of course they have not made any mistakes and if we run that thing it prints out the number 42

And again, this is a good example of that abstraction. If I were to print out that number with four leading zeros, I can say

And it prints it out 0 0 for 2 all that sort of complexity well do I print out that number

Okay, well had open time ago divide it by 10

The remainder will be that did you I need to put it last than the rest of the number and then I drew that again

That's all abstracted away from me as a programmer. I can think about what I want to write

At a high level, which is our different out hello world followed by the number

With two leading zeros, and we could write that so we've already abstracted that away

But that standard library then still needs to print that out. So it needs to take each of the characters its generated and

the serial teletype that you're dealing with or however it is you're outputting these things because if you think about it a

I'm sitting at my computer here typing the commands in to run that program just written on this system

but I could have logged in remotely using SSH or something to another computer system and run the program there and

Then it would have had to do something different

So in this case, it's printing it on my local screen if I'd logged in to a different machine

You would have to run the program there send the characters. He wants to print having encrypted them

to my machine and then print them out sue got a different thing so

Even the C standard library is built to print things out but it's built on and I was stretching in this case that abstraction

Is the operating system so we can add another

Layer of abstraction, which is the operating system and most operating systems. Certainly the modern ones. We get UNIX windows mac

o s-- viscose whatever one you were to do will provide some level of abstraction so you can just say print this and

It will then send it to the right thing and do the right thing based on what you go

So I hello world program are suddenly gone from being one

Line that prints things out, but that's then opened a can of worms because it's using a library

It's also being converted to machine code we can see that so we can tell the compiler to just produce the machine code for us

We get the machine code version here. And as you can see that's quite a lot more lines of text that we've got

There we can see there the call to printf and we're doing various things to get that

How that works is perhaps another video on how we call the function in the machine code

So let's backtrack a bit from printf because as we've talked about that's a more complicated

Implementation it can format the text and let's just think about how we can just print a single string to the screen

so let's rewrite this program to use put's which is another

C standard library function that can print out. I think this one can't do formatted text

So remove our additions and so we now got a slightly simpler

Version of hello world that still does the same thing and then it prints out hello world

It also prints an extra new line because puts my app as a standard output

It adds a new line to use it got slightly from bit of functionality

But we can start to think about how that works. So what that is given as input is

Arranged in memory suddenly. Our program is getting more complicated because we're actually having to think about how things are stored in there as

Basically a series of memory locations one after the other with the H followed by the e

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Hello (World) Abstraction! - Computerphile

stretch

basically

prone

character