F28PL1 Programming Languages Lecture 8 Programming in C - 3

Arrays finite sequence of elements of same type elements accessed by an integer index declaration: type name [int]; allocate int * size for type on stack elements numbered from 0 to int-1 e.g. int a[6]; - allocates 6 * 4 == 24 bytes e.g. char b[6]; - allocates 6 * 1 == 6 bytes e.g. double c[6]; - allocates 6 * 8 == 48 bytes

Array size in type name [int]; size int must be a constant cannot: – decide size at run-time – then declare correct size array must declare “too big” array during use must check not exceeding amount allocated

Array access e.g. int a[4]; a[0] == a+0*4  a a[1] == a+1*4  a+4 a[2] == a+2*4  a+8 a[3] == a+3*4  a+12 a a[0]a[1]a[2]a[3]

Array access: expression name =... exp 1 [ exp 2 ]...; 1.evaluate exp 1 to lvalue 2.evaluate exp 2 to integer 3.return *(exp 1 +exp 2 * size for type) – i.e. contents of offset for exp 2 elements of type from address of 1 st byte exp 1 is usually the name of an array

Array access: assignment exp 1 [ exp 2 ] = expression ; 1.evaluate exp 1 to lvalue 2.evaluate exp 2 to integer 3.evaluate expression to value 4.set exp 1 +exp 2 * size for type to value – i.e. address of exp 2 th element of type from address of 1 st byte

name is alias for address of 1 st byte name is not a variable i.e name == & name int a[3]; printf(“a: %x; &a: %x/n”,a,&a);  a: 80497fc; &a: 80497fc Array name and address a ’s value a ’s address

Array bounds no array bound checking if try to access array outside bounds – may get weird values from bytes outside array or – program may crash

Array type for formal parameter type name [] size not specified same as: type * name

Constant #define name text – pre-processor replaces all occurrences of name in program with text – before compilation use to define constants once at start of program e.g. #define SIZE 127

Formatted file I/O formatted input int fscanf(FILE *,” format”,exp 1,exp 2... ) – exp i == lvalue formatted output int fprintf(FILE *,” format”,exp 1,exp 2...) – exp i == rvalue

Formatted numbers can precede format character with width/precision info e.g. %3d – integer – at least 3 characters wide e.g. %4.2f – double – at least 4 chars wide – 2 chars after decimal point

Example: scalar product calculate V 0 [0]*V 1 [0]+...+V 0 [N-1]*V 1 [N-1] $ scalar vecs.txt scalar product: 217

Example: scalar product file contains: – length of vector - N – 1 st vector – V 0 [0]...V 0 [N-1] – 2 nd vector – V 1 [0]...V 1 [N-1] functions to: – read vector – print vector – calculate scalar product

Example: scalar product #include #define MAX 100 getVec(FILE * fin,int v[],int n) { int i; i = 0; while(i<n) { fscanf(fin,"%d",&(v[i])); i = i+1; }

Example: scalar product printVec(int v[],int n) { int i; i = 0; while(i<n) { printf("%2d ",v[i]); i = i+1; } printf("\n"); }

Example: scalar product int scalar(int v0[],int v1[],int n) { int s; int i; s = 0; i=0; while(i<n) { s = s+v0[i]*v1[i]; i = i+1; } return s; }

Example: scalar product main(int argc,char ** argv) { FILE * fin; int v0[MAX],v1[MAX]; int n; if(argc!=2) { printf("scalar: wrong number of arguments\n"); exit(0); } if((fin=fopen(argv[1],"r"))==NULL) { printf("scalar: can't open %s\n",argv[1]); exit(0); }

Example: scalar product fscanf(fin,"%d",&n); if(n>=MAX) { printf("scalar: %d vector bigger than %d\n",n,MAX); fclose(fin); exit(0); } getVec(fin,v0,n); printVec(v0,n); getVec(fin,v1,n); printVec(v1,n); fclose(fin); printf("scalar product: %d\n",scalar(v0,v1,n)); }

Structures finite sequence of elements of potentially different types each element identified by a field name like a Java object with no methods

Structures struct { type 1 name 1 ;... type N name N ;} name ; name i == field allocate: size of type size for type N on stack fields held left to right NB name != &name – & name is address of 1 st byte in sequence – name is value of byte sequence, depending on type context

Structure declaration: example struct {char * name, float height,int age;} chris; chris nameage height

Structure access in expression exp.name i  *(&exp + size for type size for type i-1 ) i.e. contents of offset of preceding fields from start of structure

Structure access: example struct {char * name,int age,float height;} chris; chris.name = “Chris”; - byte 0 chris.height = 1.75; - byte 0+4 == 4 chris.age = 27; - byte == 12 chris nameageheight Chris\0

Structure type definition struct name { type 1 name 1 ;... type N name N; }; struct name is type of structure defines type does not allocate space struct name 1 name 2 ; allocates stack space associate name 2 with 1st byte of new sequence of type struct name 1

Example: character count count how often each distinct character appears in a file array of struct to hold character and count for each character in file – if character already has struct in array then increment count – if unknown character then add to array with count 1

Example: character count #include #define MAX 255 struct freq {int ch;int count;}; struct freq f[MAX]; int fp; fp is index for next free entry in f chcount 0 MAX-1 fp ‘v’2 ‘,’7 ‘0’4 ‘a’3 ‘ ’17 ‘1’1

Example: character count incFreq(int ch) { int i; i=0; while(i<fp) if(f[i].ch==ch) { f[i].count = f[i].count+1; return; } else i = i+1; search f for entry for ch if found, increment count and return chcount 0 MAX-1 fp ‘v’2 ‘,’7 ‘0’4 ‘a’3 ‘ ’17 ‘1’1 e.g. ch ==‘a’ i

Example: character count incFreq(int ch) { int i; i=0; while(i<fp) if(f[i].ch==ch) { f[i].count = f[i].count+1; return; } else i = i+1; search f for entry for ch if found, increment count and return chcount 0 MAX-1 fp ‘v’2 ‘,’7 ‘0’4 ‘a’4 ‘ ’17 ‘1’1 i e.g. ch ==‘a’

Example: character count if(fp==MAX) { printf("more than %d different characters\n",MAX); exit(0); } f[fp].ch = ch; f[fp].count = 1; fp = fp+1; } if ch not found – check for free entry in f – add ch /1 to next free entry – increment fp chcount 0 MAX-1 fp ‘v’2 ‘,’7 ‘0’4 ‘a’4 ‘ ’17 ‘1’1 i e.g. ch ==‘p’

Example: character count if(fp==MAX) { printf("more than %d different characters\n",MAX); exit(0); } f[fp].ch = ch; f[fp].count = 1; fp = fp+1; } if ch not found – check for free entry in f – add ch /1 to next free entry – increment fp chcount 0 MAX-1 fp ‘v’2 ‘,’7 ‘0’4 ‘a’4 ‘ ’17 ‘1’1 i e.g. ch ==‘p’ ‘p’1

Example: character count showFreq() { int i; i = 0; while(i<fp) { printf("%c : %d\n",f[i].ch,f[i].count); i = i+1; }

Example: character count main(int argc,char ** argv) { int ch; FILE * fin; if((fin=fopen(argv[1],"r"))==NULL) { printf("can't open %s\n",argv[1]); exit(0); } fp = 0; ch = getc(fin); while(ch!=EOF) { incFreq(ch); ch = getc(fin); } fclose(fin); showFreq(); }

Example: character count $ freq freq.c # : 3 i : 48 n : 36 c : 32 l : 8 u : 10 d : 8 e : 27 : 185 < : 4 s : 10 t : 31 o : 12. : 9 h : 22 > : 2 : 54 b : 1 f : 35 M : 4 A : 4 X : 4 2 : 1 5 : 2 r : 23 q : 6 { : 9 ; : 29 } : 9 [ : 10 ] : 10 p : 14 F : 6 ( : 21 ) : 21 = : 19 0 : 5 w : 5 + : 4 1 : 7 " : 8 m : 2 a : 10 % : 4 \ : 3, : 6 x : 2 : : 1 g : 6 * : 3 v : 3 I : 1 L : 3 E : 2 N : 1 U : 1 ' : 1 ! : 1 O : 1

Coercions ( type ) expression 1.evaluate expression to value 2.now treat value as if type does not physically transform expression as if overlaid template for type on value also called cast

Coercions e.g. integer (4 bytes) as array of char (1 byte) int x; char * c; x = 0x c = (char *)(&x); - c now points at x’s space printf(“%x %x %x %x\n”, c[0],c[1],c[2],c[3]); 

Coercions x is 4 hex bytes – stored from most significant to least significant &x returns address of 1 st byte of int (* char) coerces address of 1 st byte of int to address of 1 st byte of array of char – c[0] is 1 st byte of x == 67 – c[1] is 2 nd byte of x == 45 etc

Structure coercion struct {char a;char b;char c;char d;} m; m.a = ‘a’; m.b = ‘b’; m.c = ‘c’; m.d = ‘d’; m 0123 ‘a’‘b’‘c’‘d’

Structure coercion printf(“m: %x; &m: %x\n”,m,&m);  m: ; &m: == ASCII ‘a’ in hex; 62 == ASCII ‘b’ in hex... struct fields held left to right but printing struct as hex: – coerces to int – accesses bytes right to left as most to least significant

String array of char last char is ‘\0’ no length information “ characters ” string constant allocates space for characters ending with ‘\0’ sets each byte to each character returns address of first character

String variables char name [int]; allocates space for int characters cannot assign string constant to name – must copy character by character char * name; allocates space for pointer to characters can assign string constant to name – changes pointer otherwise, must allocate space for characters...

Example: string length #include int slength(char s[]) { int i; i =0; while(s[i]!='\0') i = i+1; return i; }

Example: string length #main(int argc,char ** argv) { char * q; q = "how long is this string?"; printf("%s: %d characters\n",q,slength(q)); } $ slength how long is this string?: 24 characters

Example: string comparison s 0 ==s 1 – same length: n – 0 <= i <= n-1: s 0 [i]==s 1 [i] e.g. “banana” == “banana” s 0 <s 1 – 0<= i <j: s 1 [i]==s 2 [i] – s 0 [j]<s 1 [j] e.g. “banana” < “banish” e.g. “ban” < “band”

Example: string comparison s 0 >s 1 – 0<= i < j: s 1 [i]==s 2 [i] – s 0 [j]>s 1 [j] e.g. “banquet” > “banana” e.g. “bank” > “ban” int scomp(char s0[],char s1[]) – s0 == s1  0 – s0 <= s1  -1 – s0 >= s1  1

Example: string comparison #include int scomp(char s0[],char s1[]) { int i; i = 0; while(s0[i]==s1[i]) { if(s0[i]=='\0') return 0; i = i+1; } if(s0[i]=='\0' || (s0[i]<s1[i])) return -1; return 1; }

Example: string comparison main(int argc,char ** argv) { printf("banana banana %d\n",scomp("banana","banana")); printf("banana banish %d\n",scomp("banana","banish")); printf("ban band %d\n",scomp("ban","band")); printf("banquet banana %d\n",scomp("banquet","banana")); printf("bank ban %d\n",scomp("bank","ban")); } $ scomp banana banana 0 banana banish -1 ban band -1 banquet banana 1 bank ban 1