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[CS61C FA20] Lecture 04.3 - C Intro: Pointers, Arrays, Strings: Arrays — Transcript

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0:00and welcome back let's now talk about
0:03c arrays so arrays
0:07are similar to what you've probably seen
0:09in java you
0:10declare them by saying what type it is
0:12and how many you want
0:14so here the first line says int ar of 2
0:16says
0:17i want two integers side to side the
0:20most important thing you understand
0:21about arrays is that
0:22it's a way of asking c for a contiguous
0:25block of memory
0:26i need to have two integers side by side
0:28you can't have them here with a gap in
0:30the middle you have them together
0:32you can also declare and initialize them
0:34at the same time without saying the
0:35space so
0:36ar open square back open close square
0:39bracket equals
0:40two numbers um initializes those two
0:43spots and says there's gonna be two of
0:44them
0:44and the way you access them is the
0:46square bracket notation you've seen from
0:47python
0:48and from java ar of num or num is some
0:50value
0:51and it's zero index it's really
0:52important
0:54so mostly arrays and pointers are
0:56identical actually
0:58they're mostly the same uh so if i say
0:59care star string
1:01or care string open square bracket the
1:03first says the string
1:05is a pointer to a character array to a
1:08i even say character array um and the
1:10second says that
1:12string is an array so first a string is
1:14a pointer
1:15to a contiguous block of characters
1:19and the second is that string is a
1:20character array the neural identical
1:23people often almost the same almost
1:25always even confuse them in the way they
1:26describe them
1:28what's different is that you cannot
1:29increment uh
1:31an array the open square bracket you
1:32can't increment it you can't move the
1:34pointer you can for the pointer but you
1:35can't this so if you have a big array
1:37and you're
1:37moving across you can't increment it if
1:39you have open square brackets
1:40that's really the only it's very subtle
1:42the way you declare declare a filled
1:44array you'll
1:44only do it with the guy on the right not
1:46the guy on the left but you can think of
1:48it
1:48as an array variable points to the first
1:50element i mean there's they're also
1:51subtle some other ways of where they're
1:53stored and i'll talk about that in a
1:54second but
1:55that's the big idea okay so mostly
1:56they're the same i'll say they're the
1:57same
1:59what that means is if you have ar and
2:01it's an array variable
2:04but it looks like a pointer in most
2:05respects so you can say ar of 0
2:07you can also say star ar remember ar was
2:09declared as an array
2:11you can still say star ar you can also
2:13say ar of 0.
2:15you can say ar of 2 you can also say
2:17star
2:18ar plus 2. this is the first time i've
2:20shown you pointer arithmetic and the
2:22plus two says
2:23look whatever ar is pointing to i don't
2:25want that one the zeroth element
2:27i don't want the next one that's the
2:29first element well it's not the first
2:30it's the one element
2:31index i want the two index elements one
2:34two over
2:35so you say well dan how does it know how
2:36wide they are how how much to increment
2:38the pointer
2:39to increment that um so incrementing the
2:42point
2:42how how far to go across to get that
2:44value uh
2:45that's what types are about that's why
2:47you told it what type that
2:48was that pointer was two that array was
2:50for um
2:53they're only the declared space of an
2:55array is only valid they're temporary
2:57while the scope of that procedure is
2:59valid so here's
3:00care starfu and make a new character and
3:02make a new subroutine called foo
3:04pretty cool i'm going to return i'm
3:06going to return uh
3:07a pointer and then i say character of
3:10string of 32 i'm making now some local
3:12storage
3:13uh array of 32 32 characters calling it
3:17string
3:18and i'm going to return that you're not
3:20supposed to do that because
3:21that space was only available in that
3:24subroutine
3:25so only what foo was live there was that
3:27a string available and the moment foo
3:29ends
3:29the system might reuse that so you don't
3:31have access that's a
3:32key error we're going to mention that
3:34many more times but don't
3:35do that don't do what we did here in foo
3:37you don't have access to that once the
3:39procedure subroutine returns
3:43i mentioned before the zero index so if
3:45the race size is n you want to access
3:46zero to n minus one that's your range of
3:48things you can get to
3:49the problem is this particular slide is
3:53talking about
3:55a style thing this is really a style
3:56thing here so this the top line of code
3:59looks right like doesn't this look right
4:00look at it
4:01into i are of 10 for i
4:04is 0 less than 10 i plus plus looks good
4:06right
4:08well the problem is you've written 10
4:10you've kind of hard coded 10
4:12twice and what we want to teach you with
4:15621c is not just
4:16how c works but actually good style and
4:18so
4:19one of the things we want to make sure
4:21inculcate in this class
4:23is that you understand that the single
4:25source of truth is a valuable thing
4:27that's the reason why google drive and
4:29google
4:30documents are so great you have a single
4:31source of truth and 25 people are doing
4:33it rather than passing out
4:34version final final final no version and
4:37oh version 17 right
4:38crazy we look back and we ask ourselves
4:40why do we ever do that
4:42same thing here and see once you realize
4:44that you're going to have 10 hard-coded
4:46you should
4:46the hair in the back of your neck should
4:48go up i should say you know what i
4:49shouldn't do this how do i do this
4:50better well one way to do is you say int
4:52array size is 10
4:53before and ansi days before cnn you have
4:56to have pound signed
4:57array size 10. um pound size pound sign
5:00define
5:01array size 10. you have to def global
5:03define this as a as
5:04a pound defined in a c preprocessor
5:06directive
5:08here int array size equals 10. so into i
5:11a of array size this is a dynamic array
5:13again this came from c9
5:15and now i can say for i equals zero i'd
5:17list an array size
5:18now i only have one ten if i want to now
5:20change it
5:21i want to make a 20. now i change it one
5:23place so look at this and
5:25you know look well young padawan as you
5:27understand the
5:28the ways that that the pro tip for how
5:31to do this you want to really have any
5:33hard-coded value have it there once
5:35abstraction okay so use indirection
5:37indirection can save you a lot of
5:38trouble
5:39and i mean don't have to maintain two
5:40copies of 10.
5:43here's the other thing arrays don't know
5:46when their bounds are being exceeded
5:47this is c
5:48if there's ever a question of whether c
5:49checks or something the answer is always
5:51no
5:51c doesn't do any of those checks c says
5:53i'm just gonna let you run as fast as
5:54you want because basically i'm gonna
5:56compile it down to raw machine code i'm
5:57not gonna do those checks for you those
5:59sanity checks or make sure you're
6:00between zero and n minus one
6:02no doesn't know that the other downside
6:04is
6:05an array doesn't carry with it it's its
6:07size
6:08so that's a little bit of a bummer so if
6:09you ever pass an array around
6:11you have to pass the array and you got
6:13to pass the size of the array
6:15um or at least the number that you're
6:16going to be searching up to you can pass
6:18it you know a rate of size 100 if you
6:19only pass
6:2010 and then only think it's actually
6:21only 10 long and it won't go past that
6:23but you you need to kind of pass these
6:24two numbers along um there's only one
6:27small
6:28um asterisk to it exception which is
6:30strings
6:31because we've all agreed that zero
6:33character is a special way to terminate
6:34a string
6:35so if i'm passing a string to you you
6:37can keep reading until you get to zero
6:39you don't have to pass the string and
6:40the number now you can also write the
6:41number and then you
6:42you know you'll stop short of that but
6:44strings you don't have to pass because
6:45we've all agreed that zero
6:46is a terminate a null terminator at the
6:48end of a string but every other array
6:51every other array you have to pass in
6:52the array and the size or at least the
6:54size
6:55up two you want to be checking okay
6:57you're going to see some errors
6:58as you're coding in c you wouldn't be
7:01learning if you weren't finding irish
7:02you know you know if you're
7:03you're not learning how to ride a
7:04bicycle if you don't scrape your knee
7:05once or twice you've got to kind of
7:06try whatever if i wiggle the wheels what
7:08if i turn really steep well you hurt
7:10yourself and you're following but if i
7:11pedal when i'm making it
7:12i did this pedaling while i'm making a
7:14turn all of a sudden that pedal hits the
7:16drawer yeah
7:17so you got to try all these things to
7:18know what the bounds and range is so
7:19that when you get to a dangerous
7:20situation you won't know to do that
7:21you won't do that so
7:25segmentation faults and bus arrows are
7:26two kinds of errors that you'll see
7:28the idea of i'll just give you a little
7:30quickie um a segmentation fault says
7:33you're reading and writing to memory you
7:34don't have access to
7:35you're reading outside the segment the
7:37segment kind of is the way that your
7:38your access space a bus error often
7:41means your alignment is wrong
7:43so you know you want to talk about word
7:45alignment a couple of videos ago
7:46i'm going to read an integer if your
7:48integer pointer
7:50doesn't have if you say what's the
7:51address of that of that of that integer
7:53if that integer isn't the address if you
7:56look at it in hex isn't 0
7:574 8 or c you're not word aligned that's
8:00in a 32-bit machine so then if you read
8:03an integer i'm reading an integer but
8:04it's but the actual pointer is
8:055 well the actual you know the actual
8:07address of that integer is 5 well that's
8:10not that's not 4 eight you know 12 or
8:12not a multiple of four so that would be
8:14a bus there
8:15okay if it's if it's in the space over
8:16there you could have it could be
8:18ambiguous where i read it off alignment
8:19but i'm also
8:20off the area i'm supposed to be reading
8:21you can have one of the two one of the
8:23two so
8:23those are two ways if you see those
8:25arrows that's what they mean
8:26segmentation fault means i'm outside of
8:28memory i have control the buzzer and
8:29he's probably not aligned
8:32i mentioned before point arithmetic
8:34which is um if you have a pointer to
8:36something and you say plus a number i
8:37mentioned star a r plus two goes to the
8:39second
8:40uh element in that array however big
8:43they are it just moves over just
8:44strides however big they are that's what
8:46size was four size of was four
8:49so pointer plus n says well if it's a
8:51pointer to something
8:53um it adds here's the thing
8:56size of character is one so it's n times
8:59the size of the character so it says i'm
9:00going to move
9:01that many bytes across to the next guy
9:03so if it's an integer four
9:04if i say pointer plus one a pointer is a
9:06thousand say point is a nice multiple of
9:07four
9:08point thousand point plus one uh this
9:11means now the pointer is going to be if
9:12i ask address it's a thousand and four
9:14order plus one again it'll be a thousand
9:15and eight so it's moving by the size of
9:17that element
9:18so that's why we have to use that's why
9:19those types are useful to know how
9:21big to move when you say pointer plus or
9:23minus point or minus means go backwards
9:25go back in the array so put a plus or
9:26point of minus does that kind of be
9:28useful thing
9:29so you saw this review we saw this a
9:31couple uh lecture maybe last lecture
9:33the way you if i have a subroutine add
9:36one and i have y
9:37declared outside that scope and i pass
9:39in y y is still
9:40y is three initially add one is not
9:42going to affect y it's still going to be
9:44three after the call to add one
9:46the way you fix it is you pass in the
9:48address of y
9:49and now that pointer can manipulate y
9:51and when it's all done y is now four
9:53that's all review
9:55what how about what if i want to change
9:56a pointer so let's look at this code
9:58here let me pull this out
9:59and see if i got this here okay
10:02so i've got increment pointer increment
10:05pointer here all right what color my
10:07drawing here
10:08increment pointer son of a gun
10:14there okay
10:19i have increment pointer
10:23what does it do well let's look at it
10:25let's start here
10:26it's supposed to move the pointer across
10:28so here's a of 3
10:30okay make a nice filled array by the way
10:32you can also i didn't mention
10:33before you can fill the array and also
10:35tell it how big it is
10:37as long as they're consistent nothing
10:39critical so 50 60 70 in this nice array
10:41i have a pointer q we can think of q and
10:44a both point to the beginning of that
10:45array so here's my picture over here
10:47indicating that then i called increment
10:49q increment pointer of q
10:51i really want q to move over so i want q
10:54to be here
10:54and point to there that's what what i
10:56want is for this
10:58there to be my q okay so let's now call
11:00this well what would it do
11:02in star p i pass in the pointer to p now
11:04p and q
11:05are the same thing and i say p equals p
11:07plus one and i hope that p
11:09that q increments okay well
11:13what do you think happens let me try
11:14this erase this then what happens
11:1750 doesn't move at all why
11:21same reason as the slide before you
11:23can't change
11:25q without passing its that's all on
11:28three one two three
11:29address so you pass in queue's
11:32address well now what is the type for
11:36the increment pointer that's receiving
11:37that
11:38it's not a pointer to a int it's a
11:41pointer
11:42to a pointer to an int actually h and by
11:45the way we call it h as we call this
11:47a handle so it's a pointer to a pointer
11:49h is a handle
11:51so now it gets pretty let's try it so
11:54now
11:54q i pass in the address of q now i can
11:58change q
11:59star star h the way you get to that the
12:01way you dereference that handle
12:02is you say star h this is going to move
12:06where
12:06q is one over so let's try this
12:10i run this q moves one over and i print
12:13f star q so this is the same
12:15this didn't change down here the printf
12:16didn't change on there it's just that
12:18key thing was different
12:20and that key different was different i
12:22also changed this as well
12:23okay so now same idea and you're gonna
12:26say well dan look
12:27if i want what if i have a pointer to a
12:28pointer to a point the answer is
12:30yes i'm gonna short circuit you and say
12:32you can have a pointed to a pointer to a
12:34pointer
12:34three stars pointed to a pointer to a
12:36pointer to a pointer four stars
12:38you typically don't have more than two
12:39stars in the code and if you have three
12:42either you know what you're doing it's
12:43really complicated code or
12:45you probably don't need to have it so
12:46think about trying to remove that if you
12:48can but
12:49it gets more complicated once you have
12:50three layers but people have handles all
12:52the time
12:53which is a reasonable thing to do we'll
12:55see in the next video

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