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[CS61C FA20] Lecture 03.3 - C Intro: C v. Java and C Syntax — Transcript

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0:00and welcome back since so many of you
0:03have come from
0:03a java experience in 61b or some from
0:07another community college or from
0:08another university
0:09let's actually show you that you
0:11probably already know most of what you
0:13need to know for c
0:13there's a couple of details we'll spend
0:14a lot of time talking about the
0:15differences but let's talk about some of
0:17the similarities as well and
0:18do a comparing and contrast kind of
0:20thing so here's a beautiful table
0:22uh java is an object-oriented language
0:25you know that c is mostly a function
0:26language you can do some object stuff
0:28but really that's what uh c
0:30plus plus was for so see the idea the
0:32core id
0:33is a function the job of the core idea
0:34is is uh is a class or an abstract idea
0:38abstract data type you can make apps
0:41data types in c
0:41but it's a lot harder to do we haven't
0:43it's not as clean and simple it's just
0:44screaming you here's an absolute data
0:46type boom there's a factory
0:47instance that's a new object abstract
0:49data type harder to do in c unless you
0:51really do something to
0:52really do it right the way you compile
0:54in so i guess i mentioned before java is
0:57compiled java c
0:58hello.java creates this independent and
1:00machine independent byte code
1:01the way you compile c is gcc hello.c
1:04that could create some machine
1:05independent code machine language code
1:07machine dependent code
1:08c is the machine dependent java java's
1:11output java c's output is machine
1:12independent code
1:13the way you run it is you just have to
1:14say java hello on the java side you say
1:17that out and it out loads it into memory
1:19and then runs the program that's the
1:20more details of what the loader does
1:22we'll talk about that
1:22but 8. out is where you run the program
1:24by the way there's a way to specify the
1:26name of the output file
1:27if your source code is hello.c
1:30dash o put the word put the name of the
1:33executable i don't like to see any 61c
1:35student ever using a dot out they should
1:37say
1:37gcc dash o hello hello.c
1:40if it's just a simple program like hello
1:42world or something fun very small
1:44the point is you shouldn't be having it
1:45out to name your executable then you'll
1:47have
1:47if you have ten active deals what do
1:48they all do i don't know let's run it
1:50all of a sudden
1:50the robot starts moving and this you
1:52know printer starts no name your
1:54executables you know the heck that these
1:55things do
1:56the picture on the right is the you know
1:58you get to job the reason i don't like
1:59java as a first language in apcsa
2:01i advocate for using a blocks language
2:03or something else
2:05python would be great for for apcsa in
2:07high school is
2:08you have to teach the kid on the first
2:09day all the elements of the picture in
2:11java it's a little crazy
2:12and c the amount of things you need to
2:13teach them is not much you know there's
2:15this library that you're including that
2:16has a standard library standard io
2:18libraries you can have printf and some
2:19other things
2:20main we'll talk about the details of the
2:22main program but main
2:24void just says i'm not going to use any
2:26arguments for now just main
2:27i print f you can't you don't use print
2:30print f
2:30uh quotes you have to specify a new line
2:33not a print line for java gives you a
2:35free new line
2:36you have to actually specify the new
2:37line yourself in c and
2:39you have to return a value main returns
2:41a number we'll talk about
2:43functions returning this particular
2:44value and declaring that but main
2:45returns an integer
2:46and so 0 means success just as a little
2:49fyi
2:50zero is always success in unix because
2:52there could be a lot of ways things fail
2:54but only one the way they succeed
2:55so they decided well zero is going to be
2:57the success number and any other number
2:59might be something didn't go wrong it's
3:00kind of fun to see that
3:02so that worked so the return zero was
3:04the right idea when it worked uh if you
3:05didn't work return some other number and
3:07that would be a special code that you
3:08and the person calling you might agree
3:09oh if
3:10it's if it's seven that means file is
3:12not found if it's 12 that means
3:13i couldn't find the printer or something
3:17storage this is what differs this is why
3:20i love to t61c in the third class not in
3:22the first or second class because
3:24you want to be able to have layers of
3:25abstraction one of the layers of
3:26abstraction you have in java is that it
3:28manages the memory for you
3:29you say new it just works it handles it
3:32by your garbage collection hopefully you
3:33learned that in 61b
3:35you have to do this all yourself so all
3:37the
3:38kind of all the safety belts all the
3:41padded rooms are gone you're doing all
3:42the memory management yourself
3:44you can get in trouble another
3:46difference
3:47you've got c in java in terms of
3:49comments so this is old school and cc
3:52uh but in something we'll share with you
3:54c99 within a couple of slides
3:56talks about that actually they allow you
3:57to have the same comments as java so for
3:59now
4:00java and c are the same in comments if
4:01using c99 or later
4:04you have to say final if it's if it's a
4:05constant in java and pound define or
4:07const for a variable that's not a
4:09pound-defined global variable
4:11in c do you have a preprocessor yes you
4:14do in c
4:15you do not in java so we talked about
4:16that a moment ago
4:18you have to have a definition the
4:19beginning of the block and c in old
4:21school and cc
4:22but in new schools c99 and built and
4:24beyond you don't do it you can declare a
4:26variable right before you use it just
4:28like you can in java
4:30naming conventions this is still not
4:32clear there there are lots of people who
4:34have different naming conventions in
4:35java and instill in c as well in fact
4:38a lot of people it's not as clear python
4:41typically universally has the
4:42word with underscores looks like c but
4:44there's still a difference of opinion on
4:46how c is done so sometimes you'll see
4:47even our own projects will have
4:49um camel case or other different
4:50conventions but typically people have
4:52mostly agreed that it's the same
4:54kind of underscores separate words in c
4:56but actually most of the time
4:58seed what they do what they do is
4:59they'll come up with a
5:01short hand so rather than saying um
5:05let's see i don't know uh ascii
5:08ascii to integer you wouldn't write
5:10ascii to integer you say a-t-a-t-o-i
5:14so my point is sometimes the c
5:17the kind of old-school c is to convert
5:21the word into a really tight word like
5:22mv like unix commands like real that's
5:24the old school back in the 70s and 80s
5:26that's how they were thinking
5:27mvcp like a couple of letters for things
5:30so a lot of times a lot of
5:31the the words they should be more
5:33verbose and we've realized it should be
5:35more
5:35verbose but they're shorter in c
5:39the way you access the library is you
5:40import the module in java and you access
5:42the library access library with the
5:44pound sign include
5:45with either the curly list the angle
5:47brackets or the quotes
5:50finally most of the operators you see
5:52are identical you know
5:53assignment arithmetic uh augmented
5:56assignments you know plus
5:57equals minus equals there's a lot more
5:59than you probably haven't seen before
6:01uh bitwise logic that means operating
6:02the actual bits of things not on
6:04kind of logical values but like boolean
6:06logic below is
6:08the actual boolean value true or false
6:09and doing things with that but bitwise
6:11says work on the actual bits themselves
6:12which is nice it kind of works in
6:14parallel very powerful
6:15you can move things around shift them
6:16around with a bit by shift
6:18testing for quality is the same you
6:20group them with parentheses
6:21the relationships orders you can
6:23increment a decrement with plus plus and
6:24minus minus
6:26member selection we'll talk about this
6:27some more this is how you kind of get
6:29into the fields say of a struct we'll
6:32talk about that a little later
6:33conditional evaluation which we call uh
6:34the inline if or ternary if
6:36uh is the same as well has there been an
6:39update to the ncc that i
6:41picture i showed you the beginning yes
6:42there has been it's called c99
6:45so if you hear me saying c99 or beyond
6:47in 1999
6:48a consortium got together and says we
6:50gotta upgrade c he said all right let's
6:51do this before remember
6:52ansi was in ncc spot about 86 1986.
6:56so they decided to do the by the way i
6:58think i still think that they should
6:59have used uh
7:00why why why why i mean 99 we keep
7:03working with this for another 100 years
7:05we're like do you give me 99 in 1999 or
7:072000
7:08it's a little crazy that they didn't
7:09think about the y2k problem and i
7:10already tried to have four digits for
7:11their but they didn't
7:12remember this is still the very abridged
7:15way of naming things so 99 is fine all
7:17right
7:17i guess i'll be fine let's see how we
7:19maybe they'd say oh c won't last 100
7:20years who knows
7:21um the way you can find out what version
7:23of c you're running is this little print
7:24left if you say printf
7:25percent ld long decimal and you have
7:27this constant standard c version it'll
7:29tell you what number that is and that
7:30that is the number which is 1999.01 it's
7:33just a big number
7:34but that's the actual date of that the
7:36release of that version you see
7:38you're working with actually the
7:39compiler you're working with really
7:41references there's a wikipedia link to
7:42that here's some things that you'll see
7:44that are different from ncc so if you
7:46see old school c you're probably not
7:47going to see a lot of old school c
7:48probably the code did it but maybe you
7:49get a job that you see
7:50old ncc you need to know that's ncc and
7:53maybe
7:54hopefully you'll have a c99 or above
7:56compiler that you're working with
7:58but if you see old school c that's doing
7:59something weird you'll see that's
8:01because of these reasons that you had to
8:02have declarations at the top
8:04now you can have declarations anywhere
8:05thanks to c89 declarations can be
8:07anywhere java like comments we talked
8:08about
8:09this is a big deal boy all the code i
8:11wrote from 86 to 99 was so bad because
8:14you couldn't have a variable length
8:16array you had to have that be a constant
8:18and there's nothing you could say
8:19hey user how big a radio you want oh 100
8:21okay make an array
8:22we'll talk about what a raise are later
8:24then of a hundred you couldn't do that
8:25you had to hard code the number into the
8:27code itself you couldn't ask dynamically
8:29what the size something is in an array
8:30that was a big deal so that third bullet
8:32down i'm like
8:33very happy another thing you're going to
8:35see in a couple slides
8:36is how big are things how big is an
8:39integer
8:40well it used to depend uh and in type
8:43said
8:43enough with this depending it should
8:45just be clear you should be able to
8:46specify how wide integers are i think
8:48you've said that you've seen that before
8:49in the number representation you can say
8:50i want it to be 8 or 16 or 32 or 64.
8:53um so they have that so please use n
8:55types rather than in
8:56i recommend and standard bull lets you
8:58actually have booleans we never had
8:59booleans at all we always had zeros and
9:01ones
9:01and we now actually have booleans that
9:02you can use if you use the standard
9:04boolean
9:04library has there been an upgrade to 79
9:07yes there has been it's called c11 and
9:09now actually the newest one is called
9:10c18 which actually is mostly c11 which
9:12has bug fixes
9:13and here's the two numbers you get 20
9:162011 and 2017 for the longs
9:18there's references here now is just one
9:20reference for all c standard revisions
9:22you now have multi-threading
9:24multi-threading support which is amazing
9:26you now have unicode you really we
9:27talked about unicode the last series of
9:29lectures we can now support unicode in c
9:31that was a big deal
9:32get-ass was a huge security hole for so
9:34many years they got that out of there
9:36and some more some more things that are
9:37not so complex values if you care about
9:39that
9:40some type generic macros some other
9:41things like that pretty cool stuff so
9:42there's a lot of things
9:43check out the wikipedia if you want to
9:45use those you probably won't use most of
9:46those in this class but it's pretty
9:47exciting
9:48as i mentioned before you've got this
9:50main before we had a vote in there
9:51saying you know what i don't need any
9:52arguments i'll just say void
9:54if you want to be able to process
9:55arguments you have to say the following
9:56into arg c
9:57care star argv open close square
10:01brackets what that means is
10:02rxc says how many arguments do you have
10:05however
10:06it starts arguing by the way and the
10:08index of that starts we
10:09you know i haven't told you this but c
10:10is zero indexed so the first guy in that
10:12array is index is zero just like python
10:15and unlike scratch and snap is which is
10:17one index the next one is a one the next
10:19is a two
10:20so here arg c is two why is argc
10:23two because there's two things there
10:25sort and my
10:27file interesting so arc v arc v
10:30of zero is sort it's the name of the
10:33executable it's the name of the program
10:34itself
10:36arc v of 1 is my file so that's kind of
10:38cool right so you can still go from 0
10:40up to but let up to less than in some
10:42loop and you'll process through all the
10:44things that were passed into you
10:45you don't you can't ever get to the arg
10:47v of two because if
10:48it's an array it has two guys it's zero
10:50one uh
10:52zero and one you only have two guys zero
10:54and one you can't get to arg v of two
10:55if you actually uh if you had handed arc
10:57c of two
11:00so why do you might think to yourself
11:03why would you ever care about getting
11:04the name of the program
11:06think about that i'm gonna leave that
11:08out there and see if you can figure out
11:09why you might actually want the name of
11:10the program
11:11in the program itself think about that
11:16we'll see the next video

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