[CS61C FA20] Lecture 04.2 - C Intro: Pointers, Arrays, Strings: Using Pointers Effectively — Transcript

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1. 0:00welcome back so have you scared your net
2. 0:03have i scared you yet are you still
3. 0:04looking forward to playing with pointers
4. 0:05i know that there's a lot of things that
5. 0:07are
6. 0:07rough edged about them but they're
7. 0:08really powerful and they're really fun
8. 0:10let's actually get let's actually talk
9. 0:11about how to use them effectively
10. 0:14so remember before i mentioned that
11. 0:16variables you declare them
12. 0:18into x and now x is always going to be
13. 0:20injured for the rest of the life of the
14. 0:21program but pointers can actually move
15. 0:22around a little bit
16. 0:23if i say pointer uh int star p it means
17. 0:26i'm going to point to
18. 0:27any i can move it to point to this
19. 0:29integer and then next line point it to
20. 0:30that integer so that's actually easy to
21. 0:32do and doesn't require any trouble
22. 0:34you can in fact declare a pointer to a
23. 0:37generic type which is
24. 0:39called you clara you declared a void
25. 0:41star pointer and that means you could
26. 0:42actually
27. 0:43with the more somewhere type casting
28. 0:44which we'll talk about later i can then
29. 0:46move it around now it's pointing to an
30. 0:47end
31. 0:48now i'm going to point it to a float now
32. 0:49point to a care and you've got to really
33. 0:51know what you're doing usually that's
34. 0:52not what you want to do
35. 0:53and so you probably typically not do
36. 0:54that so that's kind of expert mode only
37. 0:56but you can do that if you typecasted
38. 0:58right you never have pointers to
39. 1:00functions the picture the bottom right
40. 1:02shows a pointer to a function that says
41. 1:03i'm declaring this
42. 1:05function pointer fn okay function
43. 1:08pointer to be a pointer
44. 1:10to a function that takes two void stars
45. 1:12two generic pointers
46. 1:14and returns an integer and foo is one of
47. 1:17those guys
48. 1:18okay foo satisfies that so now that
49. 1:20means
50. 1:21that for to call and that declares that
51. 1:25actually
52. 1:25makes the link to it okay address of foo
53. 1:27so that pointer now fn in a way points
54. 1:29to foo
55. 1:30so if i actually want to call make use
56. 1:34of that pointer i'd say star
57. 1:35fn and then open print x and y
58. 1:38so if it's that's the same as calling
59. 1:40foo of x and y is star f n x and y in
60. 1:43fact i'm going to show you example
61. 1:44of doing this to try to write the higher
62. 1:46order function map a little later that's
63. 1:47pretty cool
64. 1:49so we talked about structures before
65. 1:51you've seen that before uh
66. 1:52typed f a point this is like the hello
67. 1:54world of abstract data types make a
68. 1:56point
69. 1:57i can make a point class make a point
70. 1:58structure in c so it has two fields x
71. 2:01and y
72. 2:01i can get access to them and i make two
73. 2:03here i'm making two points
74. 2:05p one and p two each of those themselves
75. 2:07have two fields bloop
76. 2:08p1 and p2 p1 has x and y p2 has x and y
77. 2:12point star p address says i'm going to
78. 2:14make this p
79. 2:15address be a pointer to a point it
80. 2:17doesn't point there yet
81. 2:18the next line says p address now equals
82. 2:20address of p2 now
83. 2:22p address points to p2 so now what can
84. 2:24we do well
85. 2:25the first line dot notation says i can
86. 2:28int h
87. 2:28equals p1 dot x that's fine that just
88. 2:31grabs it before and sets it to h
89. 2:32i can also set you know p one dot y
90. 2:34equals p two dot y nothing special
91. 2:36there i can also use the pointer and add
92. 2:38an arrow notation so i can say p
93. 2:40address arrow the field name so p
94. 2:43address arrow x and then that just
95. 2:46basically follows the pointer
96. 2:47to whatever point it is happens to be p2
97. 2:49but who knows
98. 2:50and then what's that what's the x field
99. 2:52grabs that and stuffs that into h
100. 2:54you can also do something interesting
101. 2:56which is you know that star
102. 2:58p address is kind of like the point
103. 2:59itself so if you have a parens around
104. 3:01star p
105. 3:02address you can then use dot notation
106. 3:04now almost everybody 99.9
107. 3:06of people if they have a pointer to a
108. 3:08structure you use the arrow notation but
109. 3:10you could also use the dot notation
110. 3:12as well if you say p1 equals p2
111. 3:16that says pointer p1 gets the values of
112. 3:19p2 that's actually pretty interesting so
113. 3:20that actually copies about it does not
114. 3:22say sharing that says get the values of
115. 3:24p1 and put them in now p1p2 have exactly
116. 3:26the same x's and y's
117. 3:27that's not a that's not a pointer that's
118. 3:29not a sharing of the same storage that's
119. 3:31two different storages
120. 3:33each of now which have the same values
121. 3:35that's really important that's different
122. 3:36than a lot of other languages
123. 3:37kind of cool you have a null pointer the
124. 3:41pointer of
125. 3:42all that point remember the null was a
126. 3:44special thing and a pointer
127. 3:46that's null means it points to nothing
128. 3:48it's the only kind of save thing you can
129. 3:49point
130. 3:49you can declare it as okay how do you
131. 3:51initialize something you have to have a
132. 3:53kind of a sentinel value and null is the
133. 3:55sentinel value of pointers
134. 3:57null means zero which really means i can
135. 3:59never have anything stored at zero
136. 4:01remember this infinite array of memory
137. 4:03well zero's reserved never have anything
138. 4:05valuable there
139. 4:06okay so that's important if you ever try
140. 4:08to read or write a null pointer that's
141. 4:09bad if you're trying to follow no point
142. 4:10or get its value
143. 4:11you can't do that so that's trouble so
144. 4:13but it's a that's a way
145. 4:15you can protect you say well it's p
146. 4:16equal equal null then if it is i'm not
147. 4:18going to touch it it means i didn't get
148. 4:19some storage or
149. 4:20it's null it's not it's not valid i
150. 4:22can't use it to read or write any values
151. 4:24you also remember that 0 is false so now
152. 4:26i can actually say if p
153. 4:28and q are pointers i can say you know if
154. 4:30if not p
155. 4:31meaning if not p that means only if
156. 4:35if p was zero that's not true if p is um
157. 4:38so if not p
158. 4:39that's only true when p is zero i'll
159. 4:41make sure i say this right so that's
160. 4:42cool that says if not p that's a way to
161. 4:44say you know p
162. 4:44equal equals null is the same thing as
163. 4:46saying if not p if i say if q
164. 4:48that's true only if q is a good pointer
165. 4:51so
166. 4:51you can use the fact that pointers are
167. 4:53zero when they're bad when they're point
168. 4:55to null and
169. 4:56non-zero when they're good point to some
170. 4:57actual data as
171. 4:59a useful test in your code
172. 5:02you can point to variable things as i
173. 5:05mentioned before you can have pointers
174. 5:06to shorts or to integers or characters
175. 5:09modern machines are bite addressable the
176. 5:11olden days they were word addressable
177. 5:12now they're by the address so i can get
178. 5:14access to every byte okay that's kind of
179. 5:15useful
180. 5:16and a c pointer is just a way of
181. 5:18thinking of an abstract abstracted
182. 5:19memory address it's our way of
183. 5:21interfacing with memory
184. 5:23when you have type declarations the way
185. 5:24of telling the c compiler this is how
186. 5:26big these things are and this is how
187. 5:28much data should be
188. 5:29moving back and forth from two pieces
189. 5:31but we actually want what's called
190. 5:34word alignment and word alignment means
191. 5:35that most machines are really fast if
192. 5:38whatever the word size is we mentioned
193. 5:40before word size is kind of
194. 5:41the generic size of your
195. 5:44address space so if i have a 32-bit
196. 5:47machine that means my address space is
197. 5:4832 bits i can access two to the 32
198. 5:50different memory
199. 5:51points and it also means that the data
200. 5:54channels
201. 5:55are almost always going to be 32 bits
202. 5:56moving around it doesn't mean i can't
203. 5:58move a 64-bit value just may not do it
204. 5:59twice
205. 6:00if my main channel if i made like 32
206. 6:02it's like 32 lanes okay i can still send
207. 6:0364 cars
208. 6:04just have to do it twice and that's
209. 6:06internal to you
210. 6:08if you happen to have things be word
211. 6:10aligned that means everything has to be
212. 6:12aligned to whatever the size of your
213. 6:13word is if a word and the machines we're
214. 6:15going to use is
215. 6:1632 bits that's a four bytes which means
216. 6:19things have to be aligned on four byte
217. 6:21values so that means you can sort a zero
218. 6:24but you can't store anything
219. 6:26let's say if it's i can store a
220. 6:27character anywhere that's true
221. 6:29but if it's a larger value like an
222. 6:31integer or a float or a short or
223. 6:33something you can't
224. 6:34i'm sure it's aware because it might be
225. 6:36half let's talk about ins for now so if
226. 6:37i have an int
227. 6:3932 machine i can't store the int like
228. 6:41this i have to store the end exactly
229. 6:43aligned with that
230. 6:44so word alignment says you have to be
231. 6:46aligned to 0
232. 6:47which means the address of the of the
233. 6:49end has to be 0
234. 6:50or 4 or 8 or
235. 6:53c if you look at the hex value the hex
236. 6:55digit
237. 6:57the nibble that's going to be 0 4
238. 7:008 or c and then zero again so if i
239. 7:03actually ask you what the address is if
240. 7:05it's word line it's always gonna be zero
241. 7:06four eight or c
242. 7:07it's kind of a cool thing here's an
243. 7:09example on the right of a nice picture
244. 7:10that shows a short which in this case is
245. 7:12going to be
246. 7:1316 bits wide and there's a pointer to
247. 7:15that short it happens to live there
248. 7:18this is an integer uh this is x
249. 7:20appointed to x and you see that it lives
250. 7:22at 48.
251. 7:23again that's a multiple of four so we're
252. 7:24good there and there's a character that
253. 7:26can live anywhere so that's kind of a
254. 7:27nice thing okay
255. 7:30sizeof is an operator you're going to
256. 7:31see used a lot later when we talk about
257. 7:33reserving memory from up from preserving
258. 7:36space from your memory system
259. 7:38and size of any type is going to tell
260. 7:39you how many bytes wide that type is
261. 7:42so size of car is one by the definition
262. 7:45of c999
263. 7:46but remember you don't know how big size
264. 7:47of events are so you actually can go to
265. 7:49a new machine a great program to write
266. 7:50on your first machine you're just
267. 7:52you know encountering someone drops a
268. 7:53new machine i just invented a new
269. 7:54machine
270. 7:55first thing you write is printf
271. 7:56sizeofint how big are the ins there
272. 7:58how big are the doubles how big are the
273. 7:59longs that little table i showed you
274. 8:01before
275. 8:02had like long long long inch shorts do a
276. 8:04whole table that says
277. 8:05and this machine shorts are printf
278. 8:09percent d size of short it's our
279. 8:12print percent d size of ins and you keep
280. 8:16doing this so actually be kind of neat
281. 8:17to play with that as a
282. 8:19as you as you move that as you ported we
283. 8:20learned the word porting that code to
284. 8:22different machines be kind of fun to see
285. 8:23what the output of that was
286. 8:25this is why in types exist so you have
287. 8:27to kind of guess what the size of those
288. 8:28guys are but sizeof is really useful to
289. 8:30find out how big they are
290. 8:31you can also pass in size of a struct
291. 8:33type and a struct type says oh i make
292. 8:35this struct that has an integer and it
293. 8:37has a character
294. 8:38that's four standard machine and this is
295. 8:41five
296. 8:41if i say sizeobstruct is going to tell
297. 8:43me five or tell me eight
298. 8:45does it actually make sure the struct is
299. 8:47word aligned
300. 8:48so does that mean even if i only use a
301. 8:50character does it actually get a use up
302. 8:51that whole int even though i only use
303. 8:53one of those four bytes it's going to
304. 8:54use a ball for because it has to be word
305. 8:56aligned so
306. 8:56you can ask size of an int plus a
307. 9:00character to see whether it's
308. 9:01they actually make sure you word align
309. 9:02all of your structs as well okay
310. 9:04we'll see size of a little bit later and
311. 9:06we'll learn more in the next video