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[Powered by Google Translate] [Faili I / O]

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[Jason Hirschhorn, Chuo Kikuu cha Harvard]

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[Hii ni CS50, CS50.TV]

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Tunapofikiria faili, nini inakuja akilini ni Neno Microsoft hati,

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JPEG picha, au wimbo MP3,

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na sisi kushirikiana na kila ya aina hii ya files katika njia tofauti.

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Kwa mfano, katika hati Neno sisi kuongeza Nakala

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wakati na picha JPEG tupate mazao nje edges au retouch rangi.

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Hata hivyo chini ya Hood aina ya files katika kompyuta yetu ni kitu zaidi

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kuliko mlolongo mrefu wa zeros na ndio.

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Ni juu ya maombi maalum ambayo inaingiliana na faili

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kuamua jinsi mchakato huu mlolongo mrefu na sasa ni kwa mtumiaji.

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Kwa upande mmoja, hati inaweza kuangalia saa tu Byte moja,

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au 8 zeros na ndio, na kuonyesha tabia ASCII kwenye screen.

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Kwa upande mwingine, picha bitmap wanaweza kuangalia ka 3,

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au 24 zeros na ndio,

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na kutafsiri yao kama namba 3 hexadesimoli

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kwamba kuwakilisha maadili kwa nyekundu, kijani, na bluu

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katika moja ya pixel ya picha.

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Chochote wanaweza kuangalia kama kwenye kioo, saa yao ya msingi,

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files ni kitu zaidi kuliko mlolongo wa zeros na ndio.

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Hivyo hebu kupiga mbizi katika na kuangalia jinsi sisi kweli kuendesha zeros haya na ndio

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linapokuja kuandika na kusoma kutoka faili.

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>> Mimi itabidi kuanza kwa kuvunja chini katika mchakato rahisi 3-sehemu.

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Next, mimi itabidi kupiga mbizi katika mifano code mbili kwamba kuonyesha sehemu hizi tatu.

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Mwisho, mimi utakagua mchakato na baadhi ya maelezo yake muhimu.

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Kama na faili yoyote aketiye juu ya desktop yako,

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Kitu cha kwanza kufanya ni kufungua hiyo.

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Katika C tunafanya hili kwa kutangaza pointer struct predefined

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kwamba inawakilisha faili kwenye disk.

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Katika wito kazi hii, sisi pia kuamua kama tunataka kuandika au kusoma kutoka faili.

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Next, sisi kufanya kusoma halisi na kuandika.

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Kuna idadi ya kazi maalumu tunaweza kutumia katika sehemu hii,

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na karibu wote wa kwao kuanza na F barua, ambayo inasimamia kwa faili.

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Mwisho, sawa na kidogo X nyekundu katika kona ya juu ya files kufungua kwenye kompyuta yako,

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sisi karibu faili na wito wa mwisho kazi.

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Sasa kwa kuwa tuna wazo jumla ya nini tunakwenda kufanya,

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hebu tuzame katika kanuni.

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>> Katika orodha hii, tuna mbili files C na files zao sambamba kutekelezwa.

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mpango typewriter inachukua moja ya mstari amri hoja,

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jina la hati tunataka kujenga.

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Katika kesi hii, tutaweza kuiita doc.txt.

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Hebu kuendesha programu na kuingia michache ya mistari.

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Hi. Jina langu ni Jason.

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Hatimaye, tutaweza aina "quit."

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Kama sisi sasa orodha yote ya files katika saraka hii,

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tunaona kwamba hati mpya iliopo kuitwa doc.txt.

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Hiyo faili mpango huu tu kuundwa.

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Na bila shaka, hiyo pia ni kitu zaidi kuliko mlolongo mrefu wa zeros na ndio.

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Kama sisi kufungua faili hii mpya,

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tunaona mistari 3 ya maadili ya sisi aliingia katika mpango wetu -

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Hi. Mei jina ni Jason.

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Lakini nini hasa kinachoendelea wakati typewriter.c anaendesha?

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mstari wa kwanza wa maslahi kwa ajili yetu ni mstari 24.

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Katika mstari huu, sisi kutangaza faili wetu pointer.

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kazi kwamba anarudi hii pointer, fopen, inachukua hoja mbili.

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kwanza ni jina la faili ikiwa ni pamoja na ugani faili kama inafaa.

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Kumbuka kwamba faili ugani haina ushawishi faili katika ngazi ya chini yake.

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Sisi ni daima kushughulika na mlolongo mrefu wa zeros na ndio.

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Lakini haina ushawishi jinsi files ni kufasiriwa na maombi yale ni kutumika kufungua yao.

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Hoja ya pili fopen ni barua moja

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kwamba anasimama kwa nini sisi mpango wa kufanya baada ya sisi kufungua faili.

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Kuna chaguzi tatu kwa hoja hii - W, R, na A.

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Tumekuwa wateule w katika kesi hii kwa sababu tunataka kuandika kwenye faili.

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R, kama pengine unaweza nadhani, ni kwa ajili ya kusoma na faili.

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Na ni kwa appending na faili.

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Wakati wote w na inaweza kutumika kwa ajili ya kuandika kwa files,

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w kuanza kuandika tangu mwanzo wa faili

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na uwezekano wa overwrite data yoyote ambayo awali imekuwa kuhifadhiwa.

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By default, faili sisi kufungua, ikiwa haina tayari zipo,

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ni umba katika saraka yetu ya sasa ya kazi.

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Hata hivyo, kama tunataka kupata au kuunda faili katika eneo tofauti,

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katika hoja ya kwanza ya fopen,

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tunaweza kutaja Njia ya faili pamoja na jina la faili.

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Wakati sehemu ya kwanza ya mchakato huu ni moja tu mstari wa kanuni ya muda mrefu,

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mara nyingi ni vizuri mazoezi ya pamoja na kuweka mwingine wa mistari

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kwamba kuangalia kuhakikisha kwamba faili ya mara kwa mafanikio kufunguliwa au kuundwa.

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Kama fopen anarudi null, sisi si unataka kuendelea na mpango wetu,

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na hii inaweza kutokea kama mfumo wa uendeshaji ni nje ya kumbukumbu

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au kama sisi kujaribu kufungua faili katika saraka ambayo hatukuwa na permissions sahihi.

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>> Sehemu ya pili ya mchakato unafanyika katika wakati typewriter kitanzi cha.

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Sisi kutumia CS50 maktaba kazi kupata mawazo kutoka kwa mtumiaji,

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na kuchukua wao hawataki kuacha mpango,

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sisi kutumia fputs kazi kuchukua kamba na kuandika ni kwa faili.

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fputs ni moja tu ya kazi nyingi tunaweza kutumia kuandika kwenye faili.

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Wengine ni pamoja na fwrite, fputc, na hata fprintf.

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Bila kujali kazi fulani sisi kuishia kutumia, ingawa,

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wote haja ya kujua, kupitia hoja zao,

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angalau mambo mawili -

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nini mahitaji ya kuwa imeandikwa na ambapo inahitaji kuwa imeandikwa na.

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Katika kesi yetu, pembejeo ni string kwamba mahitaji ya kuwa imeandikwa

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na fp ni pointer kwamba kutuelekeza ambapo sisi ni kuandika.

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Katika mpango huu, sehemu ya pili ya mchakato ni badala ya moja kwa moja.

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Sisi ni tu kuchukua string kutoka kwa mtumiaji

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na kuongeza kuwa moja kwa moja na faili wetu na kidogo-kwa-hakuna uthibitisho pembejeo au hundi ya usalama.

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Mara nyingi, hata hivyo, sehemu mbili itachukua juu ya wingi wa code yako.

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Hatimaye, sehemu tatu ni juu ya mstari 58, ambapo sisi karibu faili.

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Hapa sisi kuwaita fclose na kupita faili yetu ya awali pointer.

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Katika mstari baadae, sisi kurudi sifuri kumuashiria mwisho wa programu yetu.

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Na, ndio, sehemu tatu ni rahisi kama hayo.

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>> Hebu hoja juu ya kusoma kutoka files.

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Nyuma katika saraka yetu tuna faili inayoitwa printer.c.

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Hebu kukimbia kwa faili sisi tu umba -

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doc.txt.

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Mpango huu, kama jina linavyosema, mapenzi tu magazeti nje ya yaliyomo ya faili kupita humo.

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Na kuna sisi kuwa nayo.

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mstari wa kanuni tulikuwa typed mapema na kuokolewa katika doc.txt.

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Hi. Jina langu ni Jason.

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Kama sisi kupiga mbizi katika printer.c,

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tunaona kwamba mengi ya code inaonekana sawa na kile sisi tu kutembea kupitia katika typewriter.c.

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Hakika mstari 22, ambapo sisi kufunguliwa faili,

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na mstari 39, ambapo sisi kufungwa faili,

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ni wawili karibu sawa na typewriter.c, ila kwa ubishi fopen pili.

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Hii wakati sisi ni kusoma kutoka faili,

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hivyo tumechagua r badala ya w.

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Hivyo, hebu kuzingatia sehemu ya pili ya mchakato.

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Katika mstari 35, kama hali ya pili katika kitanzi wetu 4,

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sisi kufanya wito kwa fgets,

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kazi kwa rafiki fputs kutoka kabla.

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Hii wakati tuna hoja tatu.

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kwanza ni pointer safu ya wahusika ambapo string itahifadhiwa.

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pili ni upeo wa idadi ya wahusika kuwa kusoma.

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Na tatu ni pointer faili ambayo sisi ni kazi.

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Utagundua kuwa kwa kitanzi mwisho wakati fgets anarudi null.

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Kuna sababu mbili kwamba hii inaweza kuwa kilichotokea.

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Kwanza, kosa inaweza kuwa imetokea.

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Pili, na uwezekano mkubwa zaidi, mwisho wa faili ulifikiwa na wahusika hakuna zaidi walikuwa kusoma.

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Katika kesi wewe wanashangaa, majukumu mawili ya kufanya kuwepo kwamba kuruhusu sisi kuwaambia

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ambayo sababu ni sababu ya pointer hasa hili null.

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Na, si ya kushangaza, kwa vile wao kufanya na kufanya kazi na files,

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wote kazi na kuanza ferror feof kazi pamoja na f barua.

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>> Hatimaye, kabla ya sisi kuhitimisha, moja ya haraka ya note kuhusu mwisho wa kazi ya SVG,

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ambayo, kama tu zilizotajwa, imeandikwa kama feof.

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Mara nyingi utapata mwenyewe kwa kutumia wakati na kwa matanzi kwa kuendelea kusoma njia yako kupitia files.

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Hivyo, itabidi njia ya kumaliza matanzi haya baada ya kufikia mwisho wa files haya.

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Wito feof kwenye faili pointer yako na kuangalia kuona kama ni kweli

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ingekuwa kufanya tu.

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Hivyo, kitanzi wakati na hali (! Feof (fp)) inaweza kuonekana kama ufumbuzi kikamilifu sahihi.

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Hata hivyo, wanasema sisi moja line kushoto katika Nakala faili wetu.

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Tutaweza kuingia wakati wetu kitanzi na kila kitu kazi nje kama ilivyopangwa.

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On duru ijayo kupitia, mpango wetu kuangalia kuona kama feof ya fp ni kweli,

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lakini - na hii ni hatua muhimu kuelewa hapa -

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itakuwa si kweli bado tu.

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Hiyo ni kwa sababu madhumuni ya feof si kuangalia

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kama wito ijayo kazi kusoma nitawatwangeni mwisho wa faili,

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lakini badala ya kuangalia kama au mwisho wa faili tayari kufikiwa.

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Katika kesi ya mfano huu,

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kusoma mstari wa mwisho wa faili wetu unaendelea vizuri kikamilifu,

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lakini haina mpango bado kujua kwamba tumekuwa hit mwisho wa faili wetu.

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Siyo mpaka ni gani moja ya ziada kusoma kwamba counters mwisho wa faili.

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Hivyo, hali sahihi itakuwa zifuatazo:

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fgets na hoja yake ya tatu - pato, na ukubwa wa pato, na fp -

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na yote ya kwamba si sawa na null.

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Hii ni mbinu ya sisi alichukua katika printer.c,

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na katika kesi hii, baada ya exits kitanzi,

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unaweza kuita feof au ferror kuwajulisha mtumiaji kama hoja maalum kwa exiting hii kitanzi.

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>> Kuandika ili na kusoma kutoka faili ni, katika msingi wake mkubwa,

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rahisi 3-sehemu ya mchakato.

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Kwanza, sisi kufungua faili.

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Pili, sisi kuweka baadhi ya mambo ndani ya faili wetu au kuchukua baadhi ya mambo nje ya hiyo.

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Tatu, tuna karibu faili.

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sehemu ya kwanza na ya mwisho ni rahisi.

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sehemu ya katikati ni ambapo mambo Tricky uongo.

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Na ingawa chini ya Hood tuko daima kushughulika na mlolongo mrefu wa zeros na ndio,

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ni msaada gani wakati coding kuongeza safu ya uchukuaji

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kwamba anarudi mlolongo katika kitu ambacho karibu zaidi liknar kile sisi ni kutumika kuona.

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Kwa mfano, kama sisi ni kufanya kazi na faili 24-bit bitmap,

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tutaweza uwezekano kuwa kusoma au kuandika ka tatu kwa wakati mmoja.

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Katika kesi ambayo, inaweza kuleta hisia ya kufafanua na ipasavyo jina

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struct kwamba ni 3 ka kubwa.

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>> Ingawa kazi na files inaweza kuonekana ngumu,

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kutumia yao inaruhusu sisi kufanya kitu kweli ya ajabu.

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Tunaweza kubadili hali ya dunia ya nje ya mpango wetu,

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tunaweza kutengeneza kitu ambacho anaishi baada ya maisha ya mpango wetu,

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au tunaweza hata kubadili kitu kwamba iliundwa kabla ya mpango wetu kuanza mbio.

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Kushirikiana na files ni sehemu ya programu kweli nguvu katika C.

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na mimi nina msisimko kwa kuona nini wewe kwenda kujenga na hayo katika code ijayo.

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Jina langu ni Jason Hirschhorn. Hii ni CS50.

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[CS50.TV]

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>> [Kicheko]

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Sawa. Moja kuchukua. Hapa sisi kwenda.

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Tunapofikiria faili - >> Oh, ngoja. Sorry.

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[Kicheko] Sawa.

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Hey huko.

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Tunapofikiria faili -

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00:11:17,610 --> 00:11:20,710
Wakati unafikiri ya faili - Sawa. Niambie wakati uko tayari.

187
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Oh, kubwa.

188
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Ingawa kusoma kutoka teleprompter inaweza kuonekana - hapana. Yangu mbaya.