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[CS61C FA20] Lecture 03.4 - C Intro: Basics: C Syntax — Transcript

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0:00and welcome back now let's take a look
0:02at some c
0:03syntax to actually teach you more about
0:05the language that we haven't already
0:08so the first is we talked about before
0:10that there's a new way in c99
0:13to have a boolean type but if you don't
0:15include that you just have to go with
0:16kind of more ansi c
0:18what is true and what's false so what's
0:20false in c is
0:21zero only zero zero is false
0:24that's it turns out null is actually
0:28defined as zero so if you say a pointer
0:30i got a pointer here and if i set the
0:31pointer to null which means it's not set
0:32to
0:33pointing to anything that really means
0:34the pointer is set to zero
0:36um and i mentioned before that standard
0:38bull.h defines official boolean types if
0:39you want to use that
0:40what's true everything else this is a
0:43little different than python because
0:45python has kind of
0:46what what false is for every type so
0:48empty string is false for
0:49strings zero is false for numbers uh
0:53you know empty sequences falls for other
0:54things and anything that's not that is
0:56true
0:57in scheme it's actually closer to what c
0:59does because scheme actually has hashtag
1:01f is false and everything else
1:02even zero is true for scheme even m this
1:05is actually a
1:06this is actually a a matter of debate
1:08you want to get a scheme person riled up
1:10ask them whether the empty list is true
1:13or false
1:13some people say it's not hashtag f some
1:15people say no no empty list it should be
1:17false also so there's a lot of
1:19get get two people who are you know old
1:21scheme folks together and have them
1:23fight over whether
1:24nil nil is a is false or not it's kind
1:26of a fun fun game to do
1:29so in terms of variables you've got to
1:32have type so you know from java you have
1:33types
1:34you have to declare what type a variable
1:36is before you use it and you can't
1:37change it it's locked in for the life of
1:39the program
1:40types can't change so here's an example
1:42of
1:43some of the common types people are
1:44using you saw ins for integer numbers
1:46negative numbers as well
1:47you might say well how big are they
1:49we'll show you a slide how this is all
1:50ambiguous
1:51and this is why we added in types which
1:52declare exactly they are eight bits wide
1:54or 16 bits wide or etc etc
1:57unsign ins for unsigned numbers floating
1:59point for floating float for floating
2:00point numbers
2:01we'll actually have a whole lecture on
2:02how how that actually gets stored that's
2:04a really cool
2:05lecture to see doubles for bigger
2:07floating point numbers
2:08uh characters for single characters for
2:10you know axi characters
2:11uh longs for longer in so i got an in
2:14but i need i need more space for
2:15for a two's complement value i do a long
2:17i need more space than that it's a long
2:19long it's like let's just keep writing
2:20the word long in front of things when
2:21it's almost like you receive a file you
2:23save a file you're working with a friend
2:25or something
2:25back in the days of sneakerwear when you
2:27had a file you called it final and then
2:29you give it to them and they make a
2:30change they call it final final
2:31it's like long long that's a big little
2:33long that's what it is um
2:36python versus java versus c so um
2:40interesting thing about c is see they
2:43decide that
2:44int feels like the the most efficient
2:48way you would store a two's complement
2:51number
2:51so if here's an example if it's a 32-bit
2:54machine
2:55which means the data kind of is the
2:57whole data path is meant for 32 bits
2:59then probably it's going to be 32.
3:00if it's a 64-bit machine built so that
3:03bits are what all
3:04the everything is floating around in
3:05that way that's the width of the data
3:07path
3:08then probably uh ins are going to be 64.
3:10it's really what's called the word size
3:11which is the width of
3:13the majority of the data you're flowing
3:14with um
3:16here's what i don't actually don't like
3:17this slide so much because you have to
3:18remember i don't want you to memorize
3:19these things i mean
3:20the only guarantee is that a long long
3:22is the size of
3:24size i'll try i'll show you in a second
3:25what sizeof does it tells you how many
3:27bits wide this thing is um actually how
3:30many bytes wide this thing is sorry how
3:31many bytes wide this thing is
3:33um so if it's 32 bits the size of we'll
3:35say four okay just remember that then
3:37okay so the only guarantee is that size
3:39of a long long is bigger than equal the
3:40size of a long bigger look at the size
3:42of an inside basically
3:42the size of a short but you don't know
3:44other than that they could all be 64.
3:46they could
3:46i mean it's a little crazy um i mean i
3:49guess long has to be bigger than 32. i
3:50just don't like this i'd like to use in
3:52types and declare
3:53i need to know i don't want to be i'm
3:54going to be guessing i don't have code
3:56that runs on one machine
3:57and i move to the machine and because of
3:59some way i understand things it doesn't
4:00run anymore no
4:01i don't need to know how wide these
4:02things are i think in types we should
4:04always be using
4:05they could all be 64 bits but i i would
4:07prefer you and encourage you very
4:09strongly to use in types as much as you
4:10can
4:11it gets a little annoying how you printf
4:12and scanf values within types is a
4:15little bit annoying if you use macros
4:16for that
4:17uh not macros but constants for that
4:18it's a little annoying
4:20but do it it's cleaner uh and you'll
4:22have code that then migrates really
4:23cleanly you'll have it work so which
4:24means
4:25you want to live in a 32-bit world even
4:26a 13-bit world if you move to 64 it
4:27won't just
4:28change size integers just to be 32
4:30because you said explicitly i want this
4:31into be 32 bits wide
4:32great um his little comparison about the
4:36three languages python java and c
4:38of the size of what an int is uh python
4:40there are at least 32 bits java they are
4:42exactly 32 bits which is interesting
4:44and c depends on the computer in the
4:46early days ants were 16.
4:48it was a big deal when it became 32
4:50because the machines back in the day had
4:51a data path and a word size it was 16
4:53bits it was two bytes wide
4:54so c has changed and evolves his old
4:57language had that
4:58and i don't like the idea that it
4:59changes use in types and lock it in
5:02you can also turn the lock we're talking
5:04about locking things in you can also
5:05lock in the value of variables
5:06um if you have a variable that you know
5:08is going to change it's going to be a
5:09constant it is reference value it's the
5:11golden ratio it's the days of the week
5:12it's
5:13you know it's the speed of light it's
5:15the law lock that in
5:17and say constant so if you add constant
5:18in front of it it says the value won't
5:19change if you try to change it your
5:21compiler will say hey you can't change
5:22this const
5:22very useful so you can buy those like
5:24your friend in that way so if you know
5:25something's not going to change
5:26lock it in as a value as a constant an
5:29enum is a nice
5:30feature of the language it lets you kind
5:31of have an enumeration so enumerated
5:33constants it gives you
5:34each a value so if i say enum color
5:37red's gonna be zero color
5:38green is gonna be one and blue's gonna
5:40be two if you ever use an enum i
5:42strongly recommend
5:42you not peek under the hood to find out
5:44what those bits are of each one of those
5:46i recommend
5:46you always not know your code should
5:48work even if they were to rearrange
5:50the numbers of those guys if i would say
5:52go back to your code and change
5:53to be blue red green then each of those
5:56will be different values your code
5:57should still work
5:58type functions in c so
6:01all the functions that you declare have
6:03to have a return type
6:04uh they need to you need to explicitly
6:06tell them in advance usually above the
6:08function
6:08what its expected return type is and
6:10then when you're actually using it
6:11you'll
6:12oh i get it your it can match them up
6:14and try to do some kind of matching to
6:15say well you know what you're
6:16you told me it returns an int but you're
6:18actually using it as an unsigned end are
6:19you sure you want to do that
6:20and so it'll give you a warning or give
6:22you an error that which is nice you can
6:23also say look this is supposed to be a
6:25command this would be something that's
6:26supposed to
6:26do have a side effect so the return
6:28value can be void meaning return nothing
6:30um and variables and functions need to
6:32be declared before they're used here's
6:34an example of number of people and
6:35dollars and cents is kind of declared as
6:37ins and a float
6:40a struct is the way to make abstract
6:42data types you can have
6:44a series of fields this feels very much
6:46like python uh where you have a you know
6:48a class and you have
6:50dot fields same idea but you have a lot
6:53more control about what the bit widths
6:54are of that
6:55uh for the most part um so the other
6:58thing i want to explain to you is
6:59typedef
7:00allows you to declare a new type so you
7:02know the types we talked about are
7:03floats and ins and et cetera and cares
7:05you can make a new type that's pretty
7:07cool so i say you know what
7:08i want to bite i know a bite's going to
7:10be eight bits so i'm going to say u and
7:11eight underscore t as byte and now i can
7:14just say
7:14byte b1 and b2 that's really cool so
7:18that's really useful
7:19um the way you can do it a struct is to
7:21say i have a structure
7:22it's going to have two parts to it it's
7:24kind of like a ball of two pieces of
7:25data
7:26together here is uh what's a song what
7:29would a song be well the
7:30length in seconds for how long the song
7:32is and the year is recorded that's a
7:34structure
7:35two wins together i call that a song and
7:37i typed it as a straw
7:38as a song and now i can just say song
7:41song one
7:42and that thing that's going gonna have
7:43two fields how do you get to the fields
7:45dot notation just like python so song
7:48one dot length in seconds set that one
7:49song one dot er recorded do the same
7:51thing for song two so pretty cool so
7:52structs are kind of cool
7:53um we'll talk about them more a little
7:55bit later when we talk about pointers
7:56and how to access those fields
7:58again most of this is from c and java
8:01being the same
8:02within a function it's remarkably close
8:04in terms of its syntax
8:06if looks very similar you've got an if
8:08by without an else
8:09you get an if with an else but that's it
8:11you don't have an l if like you have in
8:12python
8:13um and you'll notice that sometimes you
8:16don't have to have if you only have one
8:18statement you don't have to have
8:19brackets around it that's great except
8:22when you
8:23think it's indented so you're thinking
8:24what's python right because you're in
8:26python mode it's indented it must be
8:27part of the if
8:28and you don't put brackets so you have
8:29one thing that's right but then you put
8:31a second thing it turns out that second
8:32thing even though it's indented is not
8:34part of the if because if without
8:35without brackets
8:36only takes one statement after it one
8:38semicolon that's it so you think it
8:40looks and that's a huge source of bugs
8:42for new programmers where i don't use
8:43the brackets but i have two lines in
8:45there
8:45and i think they're both you know
8:47indented so they're part of the if
8:48they're not part of the f
8:49you need to have brackets so get used to
8:51always putting brackets around your if
8:52even if you only have one
8:54one line i don't think we've done it
8:55always in this code that we're going to
8:57share with you because we had to pack it
8:58tighten the slides but try to do that
9:00just in fact most editors will actually
9:01add the brackets anyway for you if you
9:03go into sublime we'll do it
9:04for you so get in the habit of putting
9:05the brackets around it we also have
9:07while and
9:08do while do the statement while this
9:10expression is test so
9:11if you want to test it before test it
9:13before you go into the
9:14the set then that's the while if you
9:16want to do it before you do the test
9:17test after use the do it's kind of cool
9:21four looks very similar for
9:22initialization a check and an update
9:24then a statement
9:25you have a switch statement which says
9:27here's an expression and then i have
9:28this
9:29cases and if the cases match you do
9:31these statements and here's the problem
9:34you have to have a break statement
9:37after each of the cases so case uh case
9:40it's uh uh one
9:41let's just say hey maybe switch on a
9:43letter you type in some letter
9:45case q because they want to quit well
9:47then do sorry you want to quit
9:49sorry please you want to come back if
9:51you don't have a breaks
9:52a line below that last case it'll roll
9:55into the next case statement you have to
9:56have a break after each of these cases
9:58in there
9:58so that's actually important people have
10:00done this before where they'll just roll
10:02through like why is it running both of
10:03them yeah because it's
10:04case kind of jumps into it and then it
10:06just rolls until it sees a break and
10:07that's a problem
10:09there is a default that says look it's
10:10everything else but the cases have cases
10:12but the cases are checked through the
10:13default case c also has a strange syntax
10:16called a go to
10:18do not use a go to at least in 61c go to
10:21is reserved for
10:22kind of expert c programmers who know
10:24what they're doing you can basically
10:26make c look like basic where you go here
10:28and you have all these labels and you
10:29can go there and go there and go there
10:31and it's
10:32it's it's it's non-local control flow
10:35which means jump around just randomly
10:37don't do that it's like the worst code
10:39you'll ever have
10:40so don't use a go-to until you get to
10:41have like until you upgrade to badge
10:44level 17
10:45of the badges you've earned for
10:46programming at c it's bad it's bad case
10:48but he has a go too so you might see
10:50some go-tos and some
10:51code that you might inherit we're going
10:53to end this this lecture with a look at
10:55actual c code stop talking about the
10:57format actually show me how this works
10:59well let's read it together and in fact
11:01i have
11:02i think i have this set to be a pen
11:06check this out so i can actually
11:07highlight each of these lines and we can
11:08talk about them
11:09so you saw the easiest of my includes of
11:11that that's nothing new before let me
11:13make this white just
11:14just to give you some juice here all
11:17right
11:17so we declare some we declare some
11:21values
11:22angle do this is this is kind of old
11:23school ansi style because i'm declaring
11:25it at the
11:25top before i actually really use it i
11:27could have declared it just at the
11:28moment
11:28i literally could have declared it int i
11:30could have said int
11:32right here in front of angle degree and
11:33i wouldn't have to do any more than that
11:35so a lot of people declare it locally
11:36rather than here but this is like more
11:38old school style to do this let me clear
11:40that up
11:41so we can take a computer table of prime
11:44of sine of sine function okay so first
11:46here's pi pi is going to be a double
11:48that means it's a float but it's a
11:49little bit more space for than that
11:50we'll talk about the details in a slider
11:52in a lecture or two
11:53um i set uh a tan eight hand is in
11:57math.h
11:58so i could just get access to that you'd
12:00be like well dan you mean in python i
12:01used to have this dot notation if i
12:03import math i have to say math.atan
12:05nope all of those in the same shared
12:07namespace welcome to one namespace
12:10have a good time um
12:14i'm going to print this out print what
12:15pi is i'm going to print out this little
12:18table header that's
12:19nothing special there and now here's my
12:21first in my first actual
12:23control structure while angle degree is
12:26less than or equal to 360.
12:29that's fine um you never this is like an
12:32fyi i'm going rogue a little bit you
12:33never want to have one of these test
12:35cases be based on a floating point value
12:37because the floating point value
12:38sometimes can be 0.001 sometimes 0.99999
12:41and that means the number of iterations
12:42can actually be off by one so you always
12:44want to have kind of a while loop be
12:46checking out an integer value not a
12:47floating point value this is an
12:48important thing
12:49so i'm doing this while which is on
12:51angle degree and angle degrees in int so
12:52yay thumbs up on that one
12:54so what the angle radian is a little
12:56expression say what the value is call
12:58sine i print this out the value of angle
13:00degree and value
13:01i increment angle degree and i stay in
13:02my loop until i'm done return zero
13:04so pretty simple piece of code but
13:06that's an actual working piece of code
13:07you can copy and paste this and have it
13:08run and print little sign tables in fact
13:10there's my sign table that's what you'd
13:12get there's a little pie and some little
13:14angles and signs kind of nice and it
13:15goes on till 360 degrees all right
13:17that's c we're a little lot more but
13:19we're kind of getting our hands dirty i
13:21love it enjoy by the way grab
13:22you better be if you haven't if you've
13:23been watching up to now you better go
13:25grab your editor and copy this stuff in
13:27come on i told you that let's have some
13:28fun we'll see the next video

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