text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
|---|---|---|
>> Meanwhile, we've seen this before,
printf or print formatted, that's | 596.97 | 4.16 |
going to be a function that comes with
C, along with a whole bunch of others, | 601.13 | 4.49 |
that will from time and time
again, in order to do exactly | 605.62 | 4.52 |
as its name suggests, print something. | 610.14 | 2.31 |
What do we want to print? | 612.45 | 1.05 |
Well, we'll see that
by enclosing characters | 613.5 | 2.27 |
like these-- hello world,
backslash n in double quotes, | 615.77 | 2.91 |
we can tell printf exactly
what to print on the screen. | 618.68 | 4.36 |
>> But in order to do
that, we unfortunately | 623.04 | 3.39 |
need to take something that is
already cryptic to us humans, | 626.43 | 3.58 |
but at least it's somewhat readable--
sharp include, standard io.h, int, | 630.01 | 4.5 |
main, void, printf, all of the magical
incantations we just saw on the screen. | 634.51 | 4.83 |
But we actually have to
go more arcane still. | 639.34 | 3.13 |
We first need to translate the code
that we write into machine code. | 642.47 | 4.67 |
And recall from last week that machines,
at least the ones we know here, | 647.14 | 4.23 |
at the end of the day only
understand zeros and ones. | 651.37 | 3.08 |
>> And my God, if we had to write these
zeros and ones to actually program, | 654.45 | 3.65 |
it would very, very quickly
take the fun out of anything. | 658.1 | 3.16 |
But it turns out, per last week,
that these patterns of zeros and ones | 661.26 | 3.89 |
just have special meaning. | 665.15 | 1.25 |
In certain contexts,
they might mean numbers. | 666.4 | 2.1 |
>> In some contexts, they might mean
letters, or colors, or any number | 668.5 | 3.34 |
of other abstractions there upon. | 671.84 | 2.87 |
But just as your computer has
a CPU, Central Processing Unit, | 674.71 | 3.74 |
or the brains inside of your computer. | 678.45 | 1.94 |
It's usually Intel
inside, because that's | 680.39 | 1.85 |
one of the biggest companies
that makes CPUs for computers. | 682.24 | 2.66 |
>> Well, Intel CPUs and others
simply have decided in advance | 684.9 | 4.01 |
that certain patterns of zeros and
ones shall mean specific things. | 688.91 | 5.06 |
Certain patterns of zeros and ones
will mean, print this to the screen, | 693.97 | 3.07 |
or add these two numbers, or
subtract these two numbers, | 697.04 | 2.67 |
or move this piece of data from
my computer's memory over here, | 699.71 | 3.6 |
or any number of other very low level,
but ultimately useful, operations. | 703.31 | 4.56 |
But, thankfully, we humans are not going
to need to know this level of detail. | 707.87 | 5.152 |
Indeed, just like last time, where we
abstracted again, and again, and again, | 713.022 | 3.208 |
building from very low level
primitives like zeros and ones | 716.23 | 2.7 |
to higher level concepts
like numbers, and letters, | 718.93 | 2.23 |
and colors, and more,
so can we as programmers | 721.16 | 3.17 |
stand on the shoulders of
others who have come before us | 724.33 | 2.75 |
and use software that other
people have written before us-- | 727.08 | 4.18 |
namely programs called compilers. | 731.26 | 3.08 |
>> C is a language that
is usually compiled, | 734.34 | 3.43 |
which means converted from
source code to machine code. | 737.77 | 4.36 |
In particular, what this means
is that if you've got your source | 742.13 | 3.1 |
code that you yourself write, as we soon
will in just a moment on the screen, | 745.23 | 4.3 |
and you want to convert it
ultimately to machine code-- | 749.53 | 3.61 |
those zeros and ones that
only your Mac or your PC | 753.14 | 3.96 |
understands-- you've got a first
feed that source code in as | 757.1 | 4.13 |
input to a special
program called a compiler, | 761.23 | 5.11 |
the output of which we
shall see is machine code. | 766.34 | 2.634 |
And, indeed, last time we talked
about, really, at the end of the day, | 768.974 | 2.916 |
problem solving. | 771.89 | 0.72 |
You've got inputs. | 772.61 | 0.75 |
And you've got outputs. | 773.36 | 0.958 |
And you've got some kind
of algorithm in the middle. | 774.318 | 2.242 |
>> Algorithms can surely be
implemented in software, | 776.56 | 3.27 |
as we saw with pseudocode last week
and as we'll see with actual code | 779.83 | 3.07 |
this week. | 782.9 | 0.59 |
And so a compiler really just
has a set of algorithms inside | 783.49 | 2.94 |
of it that know how to
convert the special keywords, | 786.43 | 3.63 |
like main, and printf,
and others that we just | 790.06 | 2.12 |
saw into the patterns of zeros and
ones that Intel inside and other CPUs | 792.18 | 5.44 |
actually understands. | 797.62 | 2.4 |
So how do we do this? | 800.02 | 2.44 |
Where do we get a compiler? | 802.46 | 2.01 |
>> Most of us here have a Mac or a PC. | 804.47 | 1.93 |
And you're running Mac OS, or
Windows, or Linux, or Solaris, | 806.4 | 2.752 |
or any number of other
operating systems. | 809.152 | 1.708 |
And, indeed, we could
go out onto the web | 810.86 | 1.708 |
and download a compiler
for your Mac or your PC | 812.568 | 3.142 |
for your particular operating system. | 815.71 | 1.65 |
But we would all be on
different pages, so to speak. | 817.36 | 2.257 |
We'd have slightly
different configurations. | 819.617 | 1.833 |
And things wouldn't work all the same. | 821.45 | 1.76 |
And, indeed, these days
many of us don't use | 823.21 | 2.07 |
software that runs only on our laptops. | 825.28 | 2.236 |
Instead, we use something
like a browser that | 827.516 | 1.874 |
allows us to access web-based
applications in the cloud. | 829.39 | 3.54 |
And later this semester,
we will do exactly that. | 832.93 | 2.7 |
We will write applications or
software using code-- not C, | 835.63 | 4.03 |
but other languages like Python and
JavaScript-- that run in the cloud. | 839.66 | 3.2 |
>> And to do that, we ourselves
during the semester | 842.86 | 3 |
will actually use a cloud-based
environment known as CS50 IDE. | 845.86 | 6.03 |
This is a web-based programming
environment, or integrated development | 851.89 | 4.14 |
environment, IDe, that's built atop some
open source software called Cloud 9. | 856.03 | 4.58 |
And we've made some pedagogical
simplifications to it | 860.61 | 2.356 |
so as to hide certain features in
the first weeks that we don't need, | 862.966 | 2.874 |
after which you can
reveal them and do most | 865.84 | 1.93 |
anything you want with the environment. | 867.77 | 1.63 |
>> And it allows us, too, to
pre-install certain software. | 869.4 | 3.07 |
Things like a so-called CS50
library, which we'll soon see | 872.47 | 2.86 |
provides us in C with some
additional functionality. | 875.33 | 3.88 |
So if you go to, ultimately, CS50.io,
you'll be prompted to log in, | 879.21 | 5.182 |
and once you do and create
an account for free, | 884.392 | 1.958 |
you will be able to access an
environment that looks quite like this. | 886.35 | 5.8 |
>> Now, this is in the default mode. | 892.15 | 1.61 |
Everything is nice and
bright on the screen. | 893.76 | 1.89 |
Many of us have a habit of
working on CS50 piece that's | 895.65 | 2.291 |
quite late into the night. | 897.941 | 1.209 |
And so some of you might prefer to
turn it into night mode, so to speak. | 899.15 | 3.25 |
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