1. 1 Lecture 9  Arrays  Declaration  Initialization  Applications  Pointers  Declaration  The & and * operators  NULL pointer  Initialization  Readings: Chapter 9, Section 1-8, Chapter 8, Section 1-4

2. 2 What is an array?  A sequence of variables of the same data type  Array elements are indexed and can be accessed by the use of subscripts  Array elements are stored contiguously in memory space  C supports the use of multi-dimensional arrays int student_num[100]; int subject[3]; int mark[100][3]; 99 98 : 4 3 2 1 0

3. 3 1-D Array int subject[3]; Number of elements Variable name Data type of elements subject[0] subject[1] subject[2] Memory Address 1024 1028 1032

4. 4 2-D Array int mark[100][3]; Variable name Data type of elements Number of rows How many elements we have? mark[0][0] mark[1][0] mark[2][0] mark[3][0] mark[0][1] mark[1][1] mark[2][1] mark[3][1] mark[0][2] mark[1][2] mark[2][2] mark[3][2] mark[99][0]mark[99][1]mark[99][2] Number of columns

5. 5 Memory View of 2D Array int mark[100][3]; Number of rows mark[0][0] mark[1][0] mark[2][0] mark[99][2] mark[0][1] mark[1][1] mark[2][1] mark[99][0] mark[0][2] mark[1][2] mark[2][2] mark[99][1] In memory, the array elements are stored contiguously Memory Number of columns

6. 6 Important remarks on array  In an array definition, the size of the array is specified by a constant expression within the square brackets. That value must be a positive integer value.  The indexing of array elements always starts at 0; thus, the legal values for the index of student_num is from 0 to 99

7. 7 Array initialization  An array initializer is a sequence of initializing values separated by commas and enclosed with a pair of braces, e.g., int score[5]={27, 31, 25, 50, 70};  If you list fewer values than the array size, the remaining array elements will be initialized to zero int score[5]={27, 31, 25, 50}; // score[4]=0;

8. 8  Arrays without initializers and other local variables (including those declared inside the main function) are not initialized  If you initialize an array when it is declared, you can omit the size, e.g., int mark[]={34, 87, 72, 65}; is equivalent to int mark[4]={34, 87, 72, 65};

9. 9 Searching an array  Task: Write a program to read in values for an integer array a[0..N-1] (where N is a symbolic constant) and to search for an integer x in a[0..N-1]  Inputs: the values for a[0..N-1], and the target x.  Output:  If x is present in a[0..N-1], output an i such that a[i]=x  If x occurs more than once, output any index i such that a[i]=x  If x is not present in a[0..N-1], output -1

10. 10  Idea (sequential search):  Step through the array from the start to the end, i.e., have an index i goes from 0 to N-1 ;  return the value of i as soon as a[i]=x ;  if x is not found, return -1  Testing the program:  What happens if a[0..N-1] contains the value x more than once?  Can you avoid using the break statement?  Can you modify the program so that you don ’ t need to type in inputs for a[0..N-1] all the time during debugging?

11. 11 #include #define N 10 void main() { int a[N], i, x, position; for (i=0; i<N; i++) scanf(“%d”, &a[i]); printf(“Input your target: “); scanf(“%d”, &x); for (i=0, position=-1; i<N; i++) if (a[i]==x) { position=i; break; } printf(“Target found at position %d\n”, position); }

12. 12 Searching a sorted array  When the array is sorted (say, in ascending order), there is a faster method called binary search  Idea:  Test if x=a[m] where a[m] is the middle element of a[0..n-1], i.e., m = n/2  If x=a[m], we are done  If x<a[m], x will not be present in a[m+1..n-1]. So we just need to search for a[0..m-1], i.e., the search range is reduced by half  If x>a[m], we just need to search for a[m+1..n-1]

13. 13  To search the array a[0..m-1] or a[m+1..n-1], we repeat the same idea by testing x with the middle element of the respective array  To search an unsorted array of size n, we need to look up n array elements in the worst case. E.g., when x is not present  To search a sorted array of size n using binary search, we need to look up log(n) array elements in the worst case

14. 14 #include #define N 8 void main() { int a[N], i, x, position; int lower=0; upper=N-1, m; for (i=0; i<N; i++) scanf(“%d”, &a[i]); printf(“Input your target: “); scanf(“%d”, &x); m=(lower+upper)/2; position=-1; while (lower<=upper && position==-1) { if (a[m]==x) position=m; if (a[m]<x]) lower=m+1; else upper=m-1; m=(lower+upper)/2; } /* while */ printf(“Target found at position %d\n”, position); }

15. 15 Sample runs  a[] = {10, 20, 30, 40, 50, 60, 70, 80} x = 65  iteration 1: lower=0, upper=7, m=3  iteration 2: lower=4, upper=7, m=5  iteration 3: lower=6, upper=7, m=6  iteration 4: lower=6, upper=5  a[] = {10, 20, 20, 40, 50, 60, 70, 80} x = 20  iteration 1: lower=0, upper=7, m=3  iteration 2: lower=0, upper=2, m=1

16. 16 What is a pointer?  A pointer is the memory address of a variable  More precisely: the address of the first byte of the sequence of memory locations storing a variable  E.g., Assume an integer x is stored in address 1000 - 1003; then address 1000 is a pointer to x 1000 1001 1002 1003

17. 17 Pointer variable declaration  A pointer variable is a variable that can store a pointer  Pointers are classified by the type of variables they are pointing to  Pointer variable definition: E.g. double *p; declares p to be a pointer variable that can hold a pointer that points to a double variable  The compiler reserves space to store the pointer variable; however, no space is reserved to store the double variable pointed to by p

18. 18 Pointer variable declaration (Cont ’ )  This pointer variable cannot hold a pointer that points to a variable of different type  More examples: int *ptr1, *ptr2, v, w; The variables v and w are not pointer variables  We can give a name to a data type using typedef E.g.: typedef int* IntPtr; IntPtr ptr1, ptr2;

19. 19 Addressing: The & operators  The & operator returns the address of a variable: ptr1 = &v; We say that pointer variable ptr1 points to variable v  We call ‘ & ’ the address-of operator  E.g. int v = 5, *ptr; ptr = &v; 1000 5 5 ptr 1000v v

20. 20 NULL pointer value  A special value that can be assigned to any type of pointer variable  Is a symbolic constant defined in several standard library headers, e.g.,  Same as 0  When assigned to a pointer variable, that variable points to nothing

21. 21 Dereferencing: The * operators  The * operator returns the variable pointed to by a pointer: int v = 5, *ptr; ptr = &v; *ptr = 42; /* v now stores 42 */  The asterisk * is the same as the symbol for multiplication; here it is called the dereferencing operator  Dereferencing a 0 pointer usually causes fatal error: int *ptr = 0; *ptr = 42;

22. 22 Initializing pointer variables (I)  Pointer variables can be initialized to 0, NULL or an address  E.g. int *ptr1 = 0; /* same for NULL */ means ptr1 is a pointer to an integer and that initially ptr1 points to nothing.  Note: integer 0 is converted to a pointer value of the appropriate type.

23. 23 Initializing pointer variables (II)  Example: int a, *p = &a; /* note the order */ means p is a pointer to an integer and that the initial value of p is the address of a  DO NOT read it as “ the integer pointed to by p is initialized to the address of a ”  Although an address is an integer, a pointer cannot be manipulated as an integer

24. 24 Operations on pointers  Assignment: p = q;  Different from: *p = *q; 84 99 Before 84 99 After p q p q 84 99 Before p q 99 After p q