1. C Programming Language
Arrays, Pointers and Strings
2000
Instructor : Lee, Kyu-Woong
voice :

2. Arrays array declaration
for use homogeneous data; e.g. grade1, grade2, ..
declaration
type identified[constant integral expression]
expression : positive (size of array)
subscrpt : from 0 to size -1
int a[size];
lowerbound = 0
upperboud = size -1
size = upperbound + 1
good programming style
#define N 100
int a[N];
processing array element
for (i=0; i<N; i++)
sum += a[i];

3. Initialization array initialization external and static array
storage class for automatic, external, or static
not for register
initialization
float f[5] = {0.0, 1.0, 2.0 , 3.0, 4.0 }
when ( # of initializer < # of array element )
remaining elements : zero
ex)
int a[100] = {0};
external and static array
all elements to zero by default
automatic array are not initialized by the system
stating with "garbage values"

4. Initialization declaration without a size
if it is initialized to a series of values,
given the size of the number of initializers
int a[] = { 2, 3, 5, 1} is equivalent to
int a[4] = {2, 3, 5, 1}
useful for charter string
char s[]= "language"
is taken by the system
char s[] = {'l', 'a', 'n', 'g', 'u', 'a', 'g', 'e' , '\n'};

5. Subscipting a[expr] expr : integral expression( subscript or index)
int a[N];
N : symbolic constant
a[i]
0 <= i <= N-1
outside range : runtime error
bracket pair, [ ]
treated as operator
() and [] highest prcedence and left to right associativity

6. Pointers simple variable pointer
stored in a certain number of bytes at a particular memory location or address
pointer
are used to access memory and manipulate address
if v is variable, then &v is the address
& : unary operator ( inverse operator *)
&(p+1) or &(20) : illegal
pointer variable
used to take addresses as values
int *p;
p =0 ; p = NULL;
p = &i; p = (int *) 1776;

7. Pointers *p the value of the variable of which p is the address
direct value of p is the memory location
whereas the *p is the indirect value of p
the value at the memory location stored in p
direct value of P P
p : yyyy
xxxx
int a=10;
int *p;
p = &a;
printf("%o\n", &a); /* xxxx */
printf("%o\n", &p); /* yyyy */
printf("%o\n", p); /* xxxx */
printf("%d\n", *p); /* 10 */
indirect value of P *P
a : xxxx 10
variable : adddress

8. Pointers int a=1, b=2, *p; p = &a; b= *p;
/* is equivalent to b = a; */ 1 2 a b 1 2 a b 1 1 a b

9. Pointers note a pointer can be initialized in a declaration
int i, *p = &i;
initialization of p, not *p
variable of p is type int * and initial value is &i
declaration of i must occur before we take its address p
contains an address or location *p
the value of what is stored at this location
conversions during assignment between different pointers
are not allowed ( In traditional C, it may be allowed)

10. Expressions of Pointer
declaration and initialization
int i=3, j =5, *p = &i, *q = &j, *r;
double x;
expression equivalent expression value
p == &i p == ( &i)
* * & p * ( * (&p))
r = & x r = ( & x) illegal
7 * * p / * q + 7 (((7 * (*p))) / ( *q)
* ( r =&j) *= * p (* (r = (&j) )) *= (*p)

11. Call-By-Reference call-by-value call-by-reference
their values are copied to the corresponding function parameters
the variables in the calling environment are not changed
call-by-reference
the variables used in parameter list in the calling environment are changed as the value of function
addresses of variables must be passed as arguments
pointers must be used in the paramenter list in the function definition

12. Call-By-Reference #include <stdio.h> void swap(int *, int *);
int main(void) {
int i=3, j=5;
swap(&i,&j);
printf("%d %d\n", i,j );
return 0; }
void swap(int *p, int *q)
int tmp;
tmp = *p;
*q = *p;
*p = tmp;
xxxx
i = 3
yyyy
j = 5
p = xxxx, q = yyyy
*p = 3, *q = 5

13. Pointers and Arrays array name
address or pointer value and pointers by itself
constant pointer
pointer variable
different addresses as values
array
address or pointer that is fixed
if a is array
a[i] is equivalent to *(a+i)
if p is pointer
p[i] is equivalent to *(p+i)
we can use array notation with pointers

14. Pointers and Array pointer arithmetic base address of array
a : array , p : pointer
a + i
as its value the ith offset from the base address of array a
ith element of the array
p + i
as its value the ith offset from the value(address) of p
base address of array
initail location in memory where the arrray is stored
the address of the first element (index 0) of the array

15. Pointers and Arrays array a 300 a[0] #define N 100
int a[N], *p, sum =0;
a[0]
304
a[1]
308
a[2]
684
p =a ; is equivalent to p =&a[0]
p=a+1; is equivalent to p=&a[1]
/* variation of summation of array a */
for ( p=a; p < &a[N] ; ++p)
sum += *p;
for(i=0; i < N; ++i)
sum += *(a+i);
p=a;
sum += p[i];
a[96]
688
a[97]
692
a[98]
696
a[99]
★ note : illegal operation
a =p;
++a;
a += 2;
&a;
/* a is array name and constant
pointer */

16. Pointers and Arrays ★ pointer arithmetic and element size
int *p, *q, a[2];
p=a; q = p +1;
printf( "%o\n", p); /* */
printf("%o\n", q); /* */
printf("%o\n", q-p); /* 1 ( because of pointer arithmetic) */
printf("%d\n", (int)q - (int)p); /* 4 */

17. Array as Function Arguments
array as arguments
a formal parameter that is declared as an array
actually a pointer
base address of the array is passed
"call-by-value"
array elements are not copied
#include <stdio.h>
main()
{ int a[10]={0,1,2,3,4,,6,7,8,9};
void func(int []);
func(a); }
void func(int x[]) /* or void func(int *x), or void func(int x[10]) */
{ /* void func(int x[5]) numeric value : no meanig */
int i;
for(i=0; i<10; i++) printf("%d\n", x[i]);

18. Bubble Sort ith iteration : void bubble(int a[], int n)
find the ith smallest number
i.e) find the smallest number in the list of
a[i] ...a[n]
void bubble(int a[], int n) {
int i,j;
void swap(int *, int *);
for ( i=0; i < n-1; ++i) /* no need to find the largest element */
for (j=n-1 ; j>i; --j) /* the largest index : n-1 */
if ( a[j-1] > a[j] )
swap(&a[j-1], &a[j]); }
n=8 i j

19. Program Report write a baseball game
generating the 4 digits by using rand() function
get the 4 digits from user's key input
compare the user's number and random number
if the value of ith digit is the same to the ith digit of random number , then "strike" is displayed
if the value of ith digit is the same to the jth digit of random number , then "ball" is displayed
until ( 4 strike or 40 times)
continue game ? (y/n)

20. Dynamic Memory Allocation
calloc() and malloc()
function prototypes are in stdlib.h
contiguous memory allocation : calloc()
memeory allocation : malloc()
to dynamically create space for arrays, structures and unions
#include <stdio.h>
main() {
char *s;
gets(s);
printf(" string : %s\n, s);
} /* no compile error, but run-time error */
s = malloc(10);

21. Dynamic Memory Allocation
#include <stdio.h>
#include <stdlib.h>
main() {
int *a, n ;
......
a = calloc(n, sizeof(int)); }
use a as an integer array
calloc
two arguments
size_t number of element
size_t size of each element ( size_t : unsigned int)
return
success : points to the base of the array in memory
fail : NULL
initialized with all bits set to zero

22. Dynamic Memory Allocation
malloc
allocates a specified number of bytes of memory
the initial value of the memory is inderterminate
argument
size_t size of memory to be allocated
#include <stdio.h>
main() {
char *s,temp;
gets(temp);
s = malloc( strlen(temp ) + 1) ;
strcpy(s, temp);
printf(" string : %s\n, s); }

23. Strings strings one-dimesional arrays of type char
terminated by the end-of-string sentinel \0 or null character
string constant
treated as a pointer ( base address of the string )
char *p = "abc";
printf( " %s %s \n", p p+1); /* abc bc is printed */
p is assigned the base address of "abc"
p + 1 points to the letter b
"abc" treated as a pointer
"abc"[1] is equivalent to p + 1
*("abc" + 2)

24. String Handling Functions
char *strcat( char *s1, const char *s2);
int strcmp(const char *s1, const char *s2);
char *strcpy(char *s1, const char *s2);
unsigned strlen(const char *s);
const : the character pointed to in memory should not be changed

25. String Handling Function
char *strcat(char *s1, const char *s2) {
register char *p = s1;
while (*p) /* while ( *p != '\0')
++p;
while ( *p++ = *s2++) ;
return s1; }

26. Multidimensional Array
arrays of any type
including arrays of arrays
a[n][m]
the n array of m array
n ×m elements
base address : a[0][0]
col 1 col 2 col col m
row 1
row 2
row 3
....
row n
a[0][0]
a[0][1]
a[0][2]
a[0][m]
a[1][0]
a[1][1]
a[1][2]
a[1][m]
a[2][0]
a[2][1]
a[2][2]
a[2][m]
a[n][0]
a[n][1]
a[n][2]
a[n][m]

27. Multidimensional Arrays
array elements
are stored contiguously one after the other
linear order
row-major order
example :
address of a[i][j]
base address of a + ( ( i × j ) + j) × sizeof( element )
base address of array a[3][5] = 100
address of element a[2][3]
100 +{ 2 × } × sizeof(element)
efficiency
for (i=0; i<n; i++)
for(j=0; j<m; j++)
sum += a[i][j]
for (j=0; j<m; j++)
for(i=0; i<n; i++)
sum += a[i][j]
is more efficient than

28. Multidimensional Arrays
numerous ways to access elements
expressions equivalent to a[i][j]
*(a[i] + j )
( *(a+i) ) [j]
* ( ( *(a+i)) + j )
* ( &a[0][0] + 5*i + j)
three dimensional array
int a[7][9][2];

29. Multidimensional Arrays
initialization
int a[2][3] = { 1,2,3,4,5,6};
if there are no inner braces
then a[0][0], a[0][1], a[0][2] ...
remaning elements : zero
int a[2][3] = { { 1,2,3 }, { 4,5,6} }
int a[][3] = { {1,2,3} , {4,5,6} }
if the first bracket pair is empty
the number of inner brace pairs
all sizes except the first
must be given explicitly

30. Multidimensional Arrays
matrix multifly
#define L 4
#define M 5
#define N 3
typedef double matrix_a[L][M]
typedef double matrix_b[M][N]
typedef double matrix_c[L][M]
.....
void multiply(matrix_a a, matrix_b b, matrix_c c) {
int i,j,k;
for( i = 0; i < L; i ++)
for (j =0; j<N; j++)
{ c[i][j] = 0.0;
for ( k=0; k < M; k++)
c[i][j] += a[i][k] * b[k][j] ; }

31. array of pointers array of pointers char *s[3]; s[0] "abcde\n"
equivalnet to
char * ( s[3] );
s[0]
"abcde\n"
s[1]
"xyz\n"
s[2]
"test\n"
swap(&s[1], & s[2]);
swap(char **p, char **q) {
char *tmp;
tmp = *p;
*p = *q;
*q = tmp ; }

32. arguments to main() main function int argc : argument count
char *argv[] : argument value
#include <stdio.h>
main(int argc, char *argv[]) {
int i;
for ( i=0; i<argc; i++)
printf(" argument %d : %s\n", i, argv[i]); }
%a.out test.c test.dat test.out
argument 0 : a.out
argument 1 : test.c
argument 2 : test.dat
argument 3 : test.out

33. Function as Arguments pointers to functions example
can be passed as arguments
example
call :
sum_square(sin,2,13)
double sum_square(double f(double), int m, int n) {
int k; double sum = 0.0;
for ( k = m ; k < = n ; k++)
sum += f(k) + f(k) ;
return sum; }

34. Function as Arguments equivalent header note :
double sum_square( double (* f)(double), int m int n)
" f is a pointer th function that takes a single argument of type double and returns a double "
note :
double *f(double) : function f that takes an argument of type double and returns a pointer to double

35. Example #include <stdio.h> #define M 5 #define N 5 main() {
int MAT[M][N] = {
{0,1,2,3,4},
{10,11,12,13,14},
{20,21,22,23,24},
{30,31,32,33,34},
{40,41,42,43,44} };
int i, j;
for(i=0; i<M; i++)
for(j=0; j<N; j++)
printf("MAT[%d][%d]: %3d\t", i, j, MAT[i][j]);
printf("\n"); }