1
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>> [Daqq tal-mużika]

2
00:00:02,350 --> 00:00:05,444

3
00:00:05,444 --> 00:00:06,360
Doug LLOYD: Kull dritt.

4
00:00:06,360 --> 00:00:07,770
Tip ta 'suġġett stramba, right?

5
00:00:07,770 --> 00:00:09,050
Numri magic.

6
00:00:09,050 --> 00:00:12,012
What fenka hu jfisser meta hu
jitkellem dwar in-numri magic?

7
00:00:12,012 --> 00:00:14,220
Ukoll, xi wħud mill-programmi
li konna bil-miktub fil CS50

8
00:00:14,220 --> 00:00:16,660
s'issa kellhom xi stramb
numri tip ta 'jintefa fihom.

9
00:00:16,660 --> 00:00:19,680
Forsi għal raġunijiet aħna ma
kompletament jifhmu dritt issa.

10
00:00:19,680 --> 00:00:23,950
Per eżempju, fil-problema Mario, aħna
limitat l-għoli tal-piramida fuq 23.

11
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Aħna espliċitament qal inti
ma tistax tmur ogħla minn 23.

12
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>> Imma dak ma 23 jfisser?

13
00:00:28,702 --> 00:00:30,410
Ukoll, jekk inti taqra l-
spec b'attenzjoni, inti

14
00:00:30,410 --> 00:00:32,493
tista raw li l-
raġuni għandna limitat dan fid-23 ta '

15
00:00:32,493 --> 00:00:36,160
huwa minħabba l-għoli standard
ta 'tieqa terminal huwa 24.

16
00:00:36,160 --> 00:00:38,860
U hekk jekk ikollna l
piramida jkun taller minn dan,

17
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jista 'tagħmel dan ħaġa stramb
fejn hija tmur barra l-iskrin.

18
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U inti taf, dak li ma
Dan ifisser fil-kuntest, id-dritt?

19
00:00:45,140 --> 00:00:48,880
>> T-tifsira tal-23 immedjatament ovvji
lil xi ħadd li tħares lejn programm tiegħek

20
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u forsi għandu differenti
tieqa terminal daqs?

21
00:00:51,550 --> 00:00:52,330
Probabbilment le.

22
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Jidher bħal, OK.

23
00:00:53,080 --> 00:00:55,005
Well, għaliex hi biss inqas minn 23?

24
00:00:55,005 --> 00:00:56,880
B'mod ġenerali, huwa tip
ta drawwa ħażina fil-fatt

25
00:00:56,880 --> 00:00:58,940
jiktbu kostanti fil-kodiċi tiegħek.

26
00:00:58,940 --> 00:01:02,190
Meta tagħmel dan, meta inti fil-fatt tagħmel
jiktbu kostanti fis kodiċi tiegħek,

27
00:01:02,190 --> 00:01:05,630
kultant s imsejħa użu
numri magic, li huwa xi ħaġa

28
00:01:05,630 --> 00:01:08,030
aħna ġeneralment jridu jippruvaw u jevitaw.

29
00:01:08,030 --> 00:01:12,830
>> Per eżempju, ejja tagħti ħarsa
fil din il-funzjoni sempliċi hawn.

30
00:01:12,830 --> 00:01:15,726
Ovvjament hemm l-ebda data
tip C imsejħa karta jew gverta.

31
00:01:15,726 --> 00:01:16,600
Hekk biss jkollhom miegħi.

32
00:01:16,600 --> 00:01:18,910
Huwa xi ftit ta '
pseudocode mħallta fil hawn.

33
00:01:18,910 --> 00:01:21,050
Din hija funzjoni msejħa
jittrattaw biljett li apparentement

34
00:01:21,050 --> 00:01:26,570
tieħu gverta kif parametru tagħha, u
rieda output lili karta waħda.

35
00:01:26,570 --> 00:01:30,990
>> U jien tagħmel xi ħaġa hawn fejn I
jkollhom loop li tmur 0-52,

36
00:01:30,990 --> 00:01:33,394
u I jittrattaw karta.

37
00:01:33,394 --> 00:01:35,310
Well, konna ltqajna magic
Numru fil hawn, id-dritt.

38
00:01:35,310 --> 00:01:38,790
Inti tara dak in-numru magic hija?

39
00:01:38,790 --> 00:01:42,280
Jew aktar importanti, do you
tara x'inhi l-problema hawn?

40
00:01:42,280 --> 00:01:44,310
Partikolarment jekk dan
huwa biss funzjoni waħda

41
00:01:44,310 --> 00:01:48,030
fil-fajl tagħha stess fil-
folder li fih

42
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mazz ta 'differenti
fajls, li kull wieħed minnhom

43
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ma ħaġa oħra għal gverta ta 'karti.

44
00:01:51,670 --> 00:01:57,310
Forsi shuffles minnhom, jew jittratta idejn
ta 'ħames karti minflok karta waħda.

45
00:01:57,310 --> 00:01:59,420
>> Inti tara dak l-
problema tista 'tkun hawn?

46
00:01:59,420 --> 00:02:03,220
Inti tara l-għadd magic
Stajt injettat fil-kodiċi?

47
00:02:03,220 --> 00:02:04,390
Huwa 52, id-dritt.

48
00:02:04,390 --> 00:02:06,440
>> Bħal, intuwittivament inti probabilment taf, OK.

49
00:02:06,440 --> 00:02:09,740
Bħal gverta standard ta
karti fih 52 cards.

50
00:02:09,740 --> 00:02:12,570
Iżda fil-programm tagħna, huwa biss
tip ta 'floating madwar fil hemmhekk.

51
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Huwa simili kollha f'daqqa hemm 52.

52
00:02:15,280 --> 00:02:18,290
>> Mod wieħed biex issolvi din
problema hija li dan isir.

53
00:02:18,290 --> 00:02:22,724
Aħna espliċitu ħafna issa
ssejjaħ out-daqs gverta kif 52.

54
00:02:22,724 --> 00:02:25,390
Hija tagħti hija ftit aktar intuwittivi
li jfisser meta fil-linja għal

55
00:02:25,390 --> 00:02:28,650
aktar tard aħna mbagħad ngħidu, i
huwa inqas mid-daqs gverta.

56
00:02:28,650 --> 00:02:32,666
Huwa biss tidher aħjar milli qal 52.

57
00:02:32,666 --> 00:02:34,290
Issa dan ma attwalment tiffissa l-problema.

58
00:02:34,290 --> 00:02:38,460
Hija ma tagħti xi simbolika
li jfisser il-kostanti.

59
00:02:38,460 --> 00:02:40,820
Iżda ma tip ta 'fatt
jintroduċu problema oħra

60
00:02:40,820 --> 00:02:43,770
li jistgħu ma jkunux immedjatament apparenti.

61
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Anki jekk dan il-varjabbli
huwa ddikjarat globally--

62
00:02:45,859 --> 00:02:47,650
Tiftakar dak li
ifisser li meta aħna niddikjaraw

63
00:02:47,650 --> 00:02:50,500
varjabbli globalment versus lokalment?

64
00:02:50,500 --> 00:02:53,340
Anki jekk aħna tiddikjara varjabbli
globalment, dak li jekk hemm

65
00:02:53,340 --> 00:02:55,500
funzjoni oħra fil
suite tagħna ta 'funzjonijiet

66
00:02:55,500 --> 00:02:59,750
li jittrattaw manipulazzjoni karta
li involontarjament bidliet daqs gverta,

67
00:02:59,750 --> 00:03:02,727
jew dan iżid lilha mill
1 jew jonqos sal-1.

68
00:03:02,727 --> 00:03:04,060
Dan jista 'jespliċitaw trouble, id-dritt?

69
00:03:04,060 --> 00:03:08,261
Speċjalment jekk aħna qed jittrattaw ma 'sett
ta 'karti fejn jsir ċaqliq-gverta mimlija

70
00:03:08,261 --> 00:03:08,760
hija meħtieġa.

71
00:03:08,760 --> 00:03:12,804
Jekk id-daqs gverta hija naqset
b'1, per eżempju, sa 51,

72
00:03:12,804 --> 00:03:14,970
aħna mhux qed attwalment jsir ċaqliq
l-karti kollha possibilment.

73
00:03:14,970 --> 00:03:16,500
Aħna qed tħalli wieħed minnhom out.

74
00:03:16,500 --> 00:03:21,680
U li l-valur jista 'forsi jkun
mbassra jew sfruttati minn attur bad.

75
00:03:21,680 --> 00:03:24,920
>> C jipprovdi dak li sejjaħ
direttiva Preprocessor, li

76
00:03:24,920 --> 00:03:27,764
hija magħrufa wkoll bħala makro għall
ħolqien kostanti simboliku.

77
00:03:27,764 --> 00:03:30,180
U fil-fatt, inti stajt diġà
raw direttiva Preprocessor,

78
00:03:30,180 --> 00:03:32,916
anki jekk int ma smajtu
imsejħa li ma #include.

79
00:03:32,916 --> 00:03:37,150
Huwa eżempju ieħor ta makro
jew direttiva Preprocessor.

80
00:03:37,150 --> 00:03:41,290
>> Il-mod biex jinħolqu kostanti simboliċi,
jew tagħti isem lil kostanti

81
00:03:41,290 --> 00:03:43,740
sabiex ikollha aktar
li jfisser, huwa kif ġej.

82
00:03:43,740 --> 00:03:47,030
#define, isem, sostituzzjoni.

83
00:03:47,030 --> 00:03:49,140
Verament importanti aside
hawn tassew mgħaġġla.

84
00:03:49,140 --> 00:03:54,180
Tpoġġix b'waqfa u virgola fi
l-aħħar tal #defines tiegħek.

85
00:03:54,180 --> 00:03:57,310
Allura huwa #define, isem, sostituzzjoni.

86
00:03:57,310 --> 00:03:59,540
>> Meta program tiegħek hija miġbura,
x'inhu dak li attwalment jiġri

87
00:03:59,540 --> 00:04:01,740
huwa l-kumpilatur jekk tmur
li jmorru permezz-kodiċi tiegħek

88
00:04:01,740 --> 00:04:06,770
u jissostitwixxi kull każ ta
il-kelma "name" ma kwalunkwe inti

89
00:04:06,770 --> 00:04:08,860
jitqiegħdu kif sostituzzjoni.

90
00:04:08,860 --> 00:04:13,060
Analogu, jekk #include huwa tip
tal simili għal ikkupjar u pasta,

91
00:04:13,060 --> 00:04:15,700
allura #define huwa tip ta
simili biex isibu u jissostitwixxi,

92
00:04:15,700 --> 00:04:19,180
jekk inti stajt qatt jintużaw li karatteristika fil-
kelma ipproċessar programm, per eżempju.

93
00:04:19,180 --> 00:04:26,345
>> Għalhekk, per eżempju, jekk I
#define pi bħala 3.14159265,

94
00:04:26,345 --> 00:04:28,720
jekk int aħjar matematikament
inklinati u f'daqqa waħda

95
00:04:28,720 --> 00:04:31,640
tara 3.14159265 jtajru
madwar fil-kodiċi tiegħek,

96
00:04:31,640 --> 00:04:33,517
inti probabilment taf li jitkellem dwar pi.

97
00:04:33,517 --> 00:04:35,850
Imma forsi nistgħu tagħtiha a
ftit tifsira aktar simbolika.

98
00:04:35,850 --> 00:04:39,850
U nistgħu minflok ngħidu #define
pi bħal dik f'gidma ta 'numri

99
00:04:39,850 --> 00:04:42,110
li jien mhux ser iżommu
qari aktar u aktar.

100
00:04:42,110 --> 00:04:45,560
>> U x'inhu jiġri mbagħad fl
jikkompilaw żmien huwa meta l-programm huwa

101
00:04:45,560 --> 00:04:48,530
kkompilata, l-ewwel ħaġa li
se jiġri huwa se jmorru permezz

102
00:04:48,530 --> 00:04:51,520
u din se tissostitwixxi kull darba
jara kapital P, kapital I,

103
00:04:51,520 --> 00:04:55,610
dan ser litteralment tibdilha
3.14 u l-bqija, sabiex inti

104
00:04:55,610 --> 00:04:58,090
ma għandekx tip li kull
ħin waqt program tiegħek xorta

105
00:04:58,090 --> 00:05:00,631
għandu l-funzjonalità li inti
jistennew, għaliex inti qed jaħdmu

106
00:05:00,631 --> 00:05:05,090
ma manipulazzjoni, multiplikazzjoni,
diviżjoni, tkun xi tkun mill pi.

107
00:05:05,090 --> 00:05:08,230
>> Inti mhux limitati għal dan
sostituzzjoni għal numri biss.

108
00:05:08,230 --> 00:05:12,279
Per eżempju, I jistgħu #define
kors kif l CS50 sekwenza.

109
00:05:12,279 --> 00:05:14,070
F'dan il-każ, meta l-
programm hija miġbura,

110
00:05:14,070 --> 00:05:16,236
#define se jgħaddu mill-
kodiċi, jissostitwixxu kull darba

111
00:05:16,236 --> 00:05:19,900
jara "kors" ma 'l-CS50 sekwenza.

112
00:05:19,900 --> 00:05:21,720
>> Int ser ikollok avviż hawn
ukoll li I spiss

113
00:05:21,720 --> 00:05:26,090
#define kollha tiegħi definit simboliku
kostanti, biex ngħidu hekk,

114
00:05:26,090 --> 00:05:28,130
huma dejjem fil-limiti kollha.

115
00:05:28,130 --> 00:05:28,960
Huwa konvenzjoni.

116
00:05:28,960 --> 00:05:30,170
Mhuwiex meħtieġ.

117
00:05:30,170 --> 00:05:33,900
Ir-raġuni ġeneralment nies se jużaw
kapitali kollha meta jkunu qed #defining

118
00:05:33,900 --> 00:05:37,590
huwa biss li jagħmilha verament ċara li
dan l-element partikolari ta 'kodiċi tiegħi

119
00:05:37,590 --> 00:05:38,820
huwa kostanti definit.

120
00:05:38,820 --> 00:05:43,730
Jekk kien lowercase, huwa possibbli li
jista 'jkun konfuż ma' varjabbli.

121
00:05:43,730 --> 00:05:46,120
U li probabbilment
mhix ħaġa tajba li tagħmel.

122
00:05:46,120 --> 00:05:48,910
>> Allura dan partikolari
soluzzjoni hija ħafna aħjar

123
00:05:48,910 --> 00:05:50,550
minn jew ta 'dawk ta' qabel.

124
00:05:50,550 --> 00:05:59,950
Jekk I ewwel #define daqs gverta 52, imbagħad
issa użu tiegħi ta '52, jew id-daqs gverta hawn,

125
00:05:59,950 --> 00:06:01,850
huwa ħafna aktar intuwittivi u ħafna aktar sikuri.

126
00:06:01,850 --> 00:06:03,280
Inti ma tista 'timmanipola kostanti.

127
00:06:03,280 --> 00:06:05,259
Inti ma tistax tgħid 52 plus plus.

128
00:06:05,259 --> 00:06:06,800
Li mhux se jissarfu sa 53.

129
00:06:06,800 --> 00:06:09,390
Inti ma tistax tbiddel 52 sa xi ħaġa.

130
00:06:09,390 --> 00:06:12,470
>> Tista 'bidla varjabbli
li l-valur huwa 52,

131
00:06:12,470 --> 00:06:14,870
li kienet l-ewwel jiffissaw kellna qabel.

132
00:06:14,870 --> 00:06:17,000
U inti tista 'żżid
dak il-varjabbli sa 53.

133
00:06:17,000 --> 00:06:21,100
Imma inti ma tistax tgħid 52 plus plus u
jkollhom li għarrieda jduru 52 fis 53.

134
00:06:21,100 --> 00:06:23,350
52 huwa dejjem 52.

135
00:06:23,350 --> 00:06:28,860
U għalhekk inti ma tistax tbiddel involontarjament
daqs gverta hawn billi timmanipula dan,

136
00:06:28,860 --> 00:06:29,940

137
00:06:29,940 --> 00:06:32,390
>> Ieħor sekondarji tajba
effett ta 'dan għalkemm huwa

138
00:06:32,390 --> 00:06:38,310
li huma inti taf li ma
pajjiżi kollha madwar id-dinja

139
00:06:38,310 --> 00:06:40,690
użu gverta ta 'karti ta' daqs 52?

140
00:06:40,690 --> 00:06:45,630
Per eżempju, huwa verament komuni fil
Ġermanja li jużaw daqs gverta ta '32,

141
00:06:45,630 --> 00:06:48,020
fejn dawn strixxa out xi
tal-karti ta 'valur inqas.

142
00:06:48,020 --> 00:06:50,960
U f'dan il-każ, I
riedu port suite tiegħi

143
00:06:50,960 --> 00:06:55,390
ta 'funzjonijiet li jittrattaw
manipulazzjoni karta lill-Ġermanja.

144
00:06:55,390 --> 00:06:59,440
I jistgħu fl-ewwel istanza aħna
wera, ikollhom imorru u tissostitwixxi

145
00:06:59,440 --> 00:07:03,570
każijiet kollha ta '52 fil-kodiċi tiegħi ma 32.

146
00:07:03,570 --> 00:07:07,940
>> Iżda hawnhekk, jekk I #define daqs gverta
kif 32 fuq nett tal-kodiċi tiegħi,

147
00:07:07,940 --> 00:07:11,730
jekk I bżonn biex jibdlu, nista
biss jmorru u l-bidla li ħaġa waħda.

148
00:07:11,730 --> 00:07:15,010
Recompile kodiċi tiegħi, u kollha ta '
f'daqqa dan tinfirix permezz.

149
00:07:15,010 --> 00:07:18,850
Fil-fatt, nistgħu nbiddlu gverta
daqs għal xi valur li rridu.

150
00:07:18,850 --> 00:07:22,500
>> Nista jinteressawk fil-
logħba tal Pickup daqs gverta?

151
00:07:22,500 --> 00:07:23,430
>> Jien Doug Lloyd.

152
00:07:23,430 --> 00:07:25,840
U dan huwa CS50.

153
00:07:25,840 --> 00:07:27,772