Data Types. Tour of C. Like Java, Like C. Array Initialization. Array Declaration. Data Types, Arrays, Strings and Pointers in C.

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1 Dt Tyes Tour of Dt Tyes, Arrys, Strngs nd Ponters n chr short nt long flot double long double sgned/ unsgned chr, short, nt, long 2 Lke Jv, Lke Oertors sme s n Jv - Arthmetc nt = +; ++; --; *=2; +, -, *, /, % - Reltonl nd logcl <, >, <=, >=, ==,!= &&,, &,,! Syntx sme s n Jv - f() { } else { } - whle() { } - do { } whle (); - for( = ; <= 00; ++) { } - swtch() {cse : } - contnue; brek; 3 Arrys n 4 Arry Declrton Arry Intlzton The dmenson must be constnt or constnt exresson (needs to be determned t comle tme) #defne BUFFSIZE 26 #defne MAXSIZE 2 nt mn() { nt sze = 6; chr buffer[buffersize]; chr buffer[26]; // elegnt declrton // constnt nteger dmenson // NOT n elegnt declrton // nteger lterl dmenson nt dce[buffsize / 2]; // constnt nteger exresson // used for dmenson #defne MAX nt count[max] = {,, 2, 4, 8}; Provdng too mny ntl vlues cuses comle-tme error: nt count[max] = {,2,3,4,,6}; // ERROR! If the number of ntl vlues lsted s less thn the ccty of the rry, the remnng elements re utomtclly ntlzed to 0. Thus nt count[] = {2}; count [0] [] [2] [3] [4] flot rollvlue[sze];... // NOT vld - sze s not // constnt s equvlent to nt count[] = {2,0,0,0,0}; 6

2 Arry Indces Out-of-Bounds Arry Indces () Logclly, vld ndces for n rry rnge from 0 to MAX-, where MAX s the dmenson of the rry Memory nt A[6]; stnds for A[0], A[], A[2], A[3], A[4] nd A[] Logclly, there s no A[6]! A common logcl error n rocessng rrys s exceedng the vld ndex rnge: #defne MAX 00 nt somearry[max]; somearry[max] = 0; // wll comle // but run tme ERROR! Wht hens when sttement uses n rry ndex tht s out of bounds? A[0] A[] A[2] A[3] A[4] A[] nt ; for( = 0; <= MAX; ++) somearry[] = 0; No utomtc checkng of rry ndces t run tme! 8 Out-of-Bounds Arry Indces (2) Lck of Aggregte Arry Oertons The memory locton somearry[00] my :. Store vrble declred n your rogrm tht vrble wll be ltered. Snce there s no sttement tht drectly ssgns vlue to tht vrble, ths effect seems very mysterous when debuggng. 2. Not be llocted for the use of your rogrm. The result deends on the oertng system you re usng Some oertng systems, such s Wndows9/98 do not crefully montor memory ccesses nd your rogrm my corrut vlue tht ctully belongs to nother rogrm Other oertng systems, such s Wndows NT nd Unx, wll detect tht memory ccess volton hs occurred nd susend or kll your rogrm Aggregte oertons refer to oertons on n rry s whole, s oosed to oertons on ndvdul rry elements. #defne MAX 00 nt x[max]; nt y[max]; There re no ggregte oertons on rrys: Assgnment x = y; Error! omrson f (x == y) Error! I/O rntf( %d, x); Error! Arthmetc: x = x + y; Error! 9 0 Strng Declrtons Unlke Jv, there s no Strng dt tye n Strngs n re smly rrys of chrcters termnted wth 0 (chrcter \0 ) Strngs n chr some[0]; // need to secfy mx sze // one wy: chr msg[6] = { H, e, y,,!, \0 }; // nother wy (lst 2 lces unused): chr msg[8] = Hey! ; // no \0 // or chr msg[ ] = Hey! ; // no \0 memory for 6 chrcters ( lus the null chr \0 ) utomtclly llocted 2

3 Memory Reresentton chr some[0]; [0] [] [2] [3] [4] [] [6] [] [8] [9] Redng Into Strng () #defne MAX_BUFFER 20 chr buffer[max_buffer]; chr msg[6] = { H, e, y,,!, \0 }; [0] [] [2] [3] [4] [] scnf( %s, buffer); gets(buffer, MAX_BUFFER); // or H e y! 0 chr msg[8] = Hey! ; Wht f the rry s not lrge enough to hold the nut? [0] [] [2] [3] [4] [] [6] [] [8] [9] H e y! 0 chr msg[] = Hey! ; [0] [] [2] [3] [4] [] - chrcters wll be stored nto memory loctons st the end of the rry - wll result n run-tme memory ccess volton error! H e y! Redng Into Strng (2) Functons for Mnultng Strngs Better: #defne MAX_BUFFER 20 chr buffer[max_buffer]; fgets(buffer, MAX_BUFFER, stdn); fgets s smlr to gets, but: - t tkes thrd rgument, n our cse stndrd nut - f stores nto buffer no more thn MAX_BUFFER chrs (extr chrcters re gnored), so memory volton error won t occur rovdes lrge number of functons for mnultng strngs. Four mortnt ones: strlen(s) // returns the length of s strcy(tos, froms) // coy froms to tos (tos must be lrge enough) strcm(s, s2) // returns 0 f s == s2 // returns n nteger < 0 f s < s2 // returns n nteger > 0 f s > s2 strtok red the Sun mnul ges to fnd out wht ths functon does 6 Wht re Ponters? A onter s vrble tht holds the ddress of nother vrble (object) Ponters n Suose tht we hve n nteger vrble nt ; nd wsh to hve onter ont to ths vrble. Thus we need to know the memory ddress of. How do we know the ddress of?... & s the ddress of. The oertor ADDRESS-OF oertor. & s clled the 8

4 Ponters Mde Esy () Ponters Mde Esy (2) We cn declre tht onter Ptr onts to n nt by syng nt * Ptr; Suose tht we hve: nt = ; nt j = ; (&) 00 (&j) 80 We cn mke Ptr ont to by ssgnng to Ptr the memory locton where s stored. Thus j Ptr We cn lso ntlze Ptr t the ont of declrton: nt ; nt * Ptr = &; Here s common error: Ptr nt ; nt * Ptr = ; // ERROR: s not n ddress Ptr = &; sets Ptr to ont to. Ptr 9 20 Declrng Ponters Dereference * When declrng severl onter vrbles n one sttement - the stersk does not dstrbute throughout the sttement: nt *, q; equvlent to nt * ; nt q; nt *, * q; equvlent to nt * ; nt * q; The vlue of the dt beng onted t s obtned by usng the oertor * If s onter vlue, then * refers to the vrble onted to by. Snce reference s nother nme for ddress, the oertor * s clled dereference oertor Dereference Exmle Note the Dfference nt ; nt * = &; * = 0; 0 Equvlent to = 0; (*)++; 02 Equvlent to ++; tr tr2 j Intl stte tr tr2 After j tr tr2 After j tr = tr2; *tr = *tr2; A dereferenced onter behves exctly lke the vrble t onts to. strtng from ntl stte strtng from ntl stte 23 24

5 Unntlzed Ponters Puttng t ll Together... Suose tht we hve the followng declrtons: nt ; nt * Ptr; *Ptr = 00; Ptr Wht s the vlue of Ptr? Undefned. Wht could hen? - Ptr could hold n ddress tht does not mke sense t ll, cusng your rogrm to crsh f dereferenced. - Ptr could ont to n ddress whch s ccessble. Then the ssgnment *Ptr = 00; would ccdentlly chnge some other dt, whch could result n crsh t lter ont. Ths s tough error to detect snce the cuse nd symtom my be wdely serted n tme. 2 wth the hel of smle exmle: nt, vlue; nt * Ptr; // declres Ptr to be onter to n nteger = 0; // Ste Ptr = &; // Ste 2 vlue = *Ptr; // Ste 3 Ste Ste 2 Ste 3 Ptr Ptr Ptr vlue vlue vlue 0 26 The null Ponter The vlue of onter cn be: - some grbge (onter unssgned) - the ddress of some vrble (eg., nt * = &; ) - the constnt 0 (the null onter, onts to bsolutely nothng) someponter = 0; Arrys nd Ponters Ths sttement does not cuse someponter to ont to memory locton zero; t gurntees tht someponter does not ont to nythng The null onter s secl onter vlue tht rogrm cn test for: f (someponter == 0) Arrys nd Ponters An rry nme s bsclly constnt onter onsder the declrton: chr [3]; &[0] 000 &[] 004 &[2] 008 & [0] [] [2] The comler lloctes three ntegers for the rry object. These re referenced s [0], [], [2] nd occuy contguous block of memory. The vlue of s exctly &[0], the ddress of the frst nteger n the rry 29 Arrys nd Ponters - Exmles onsder the followng declrtons: nt [] = {, 2, 3}; nt * ; = &[2]; You cn use the ndex [] oertor wth onter: [0] = ; [] = 23; Indexng cn be used wth ny onter, but t only mkes sense when the onter onts to n rry 2 3?? 2 3?? 2 23? 30

6 Arrys re NOT Ponters Other resources Declrng n rry sets sde sce for ts elements chr []; [0] [] [2] [3] [4] Declrng onter vrble sets sde only sce to hold the vrble A very good tutorl on onters nd rrys n : htt://w.netcom.com/%etjensen/tr/onters.htm chr * ; You cn chnge onter vrble, but not the ddress of n rry chr b[6]; = b; // OK = b; // ERROR! b b[0] b[] b[2] b[3] b[4] b[] 3 32