C Programming Lecture 22 Review Highlights

1. Announcements • P5 due Thursday • Final Exam: Tuesday August 10, 8AM - 10AM, Olin 155 • Want references? Remember to drop off a picture and note. • Check your grades on the door of Upson 5141 Lecture 22

2. Today’s Topics • More C • Data structures • Type definitions • Arrays • Struct and array parameters • Strings in C Lecture 22

3. Recall Pointer Parameters // Swap contents of variables pointed to by x and y void swap (int * x, int * y) {int tmp; tmp= *x; *x= *y; *y= tmp;} Example of a call on procedure swap: int j= 17; int k= 24; swap(&j, &k); Lecture 22

4. Type Definitions and structs • C provides a way to group a bunch of variables together, in what is called a struct. • A typedef is similar to a class in Java, but the typedef doesn’t have methods C Corresponding Java typedef struct { public class Pair { int a; int a; char ch; char ch; } Pair; } Lecture 22

5. Use of types/structs • Pair abc, xyz; xyz.ch= ‘q’;abc.a = 5; • Declaration of a struct variable in C actually creates the structure. No separate allocation is needed, as it is in Java (using new …). • struct variables may be local to a function,just like ints, chars, and doubles. Lecture 22

6. Struct parameters Structs in C behave exactly like ints and chars in Java. In declaring a variable of some struct type, no reference is created. A struct variable is passed by value. typedef struct { … } Pair; void fv (Pair p) {p.a= 42;} void main (void) { Pair pr; pr.a= 17; pr.ch= ‘?’; fv(pr) } Lecture 22

7. Struct Pointer Parameters If you want to change a component of a struct variable, pass a pointer to the struct typedef struct { … } Pair; // As before void fr (Pair * p) {(*p).a= 42;} void main (void) { Pair pr; pr.a= 17; pr.ch= ‘?’; fr(&pr) } Abbreviation. In C, instead of writing ( * p) . a one can write p ->a Lecture 22

8. malloc -- Dynamic Storage Allocation • The standard library function malloc is used to obtain a block of storage. • It is an extremely low-level function ---its argument is the number of bytes or words to be allocated (typically obtained using operator sizeof). • It returns an untyped pointer, which must be cast to the desired type. • Allocated storage must be explicitly freed by calling procedure free. There is no automatic garbage collection Lecture 22

9. Example with malloc #include <stlib.h> Pair * p; /* p is a pointer to a Pair*/ p= (Pair *) malloc( sizeof(Pair) ); p->a= 17; … free(p); Lecture 22

10. C Arrays • C arrays are syntactically similar to arrays in Java, but the semantics (meaning) is different. • Declaring an array in C actually allocates it. • Declaring an array of structs allocates a block of storage with space for all the structs in the array. • The array name is effectively a pointer to the first element of the array. • Array indices start at 0. • There is no built-in function to determine the length of an array. Lecture 22

11. Array examples int b[10]; Pair v[10]; int k; for (k= 0; k < 10; k= k+1) b[k]= 0; v[0].a= 10; v[2].ch= ‘!’; EVIL SYNTAX THAT C ALLOWS: 1[b] = 33; x = 5[b]; UGLY UGLY UGLY! Lecture 22

12. Arrays as parameters • When an array name is used as a parameter, the “value” of the parameter is a pointer to its first element. • Therefore, array parameters are always pointers. • The called function refers to the argument array directly. No copy of the array is made. • An & is usually not needed or appropriate for an array argument Lecture 22

13. Example of array parameter /* Set s[0..n-1] to 0.0 */ void zero (double a[ ] , int n) { int k; for (k= 0; k<n; k= k+1) a[k]= 0.0; } void main(void) { double b[100]; zero(b, 100); } Lecture 22

14. Arrays and Pointers • Because array names are treated as pointers, C programmers tend to use pointers and arrays somewhat interchangeably. • Procedure zero could have been written with a pointer parameter, as shown on the next slide. • It could still be called with an array name as argument. Lecture 22

15. Array parameter as pointer /* Set s[0..n-1] to 0.0 */ void zero (double * a , int n) { int k; for (k= 0; k<n; k= k+1) a[k]= 0.0; } void main(void) { double b[100]; zero(b, 100); } Lecture 22

16. Arrays of structs as parameters An array of structs is treated like any other array. If a parameter is an array of structs, a pointer to the begin-ning of an array (i.e.an array name) is the appropriate argument. /* Store 0’s and blanks in pa[0..n-1] */ function zap (Pair pa [ ], int n) { int k; for (k= 0; k<n; k++) {pa[k].a= 0; pa[k].ch= ‘ ’;} void main( void ) { Pair ray[42]; /* initialize ray[0..16] to 0’s and blanks */ zap(ray, 17); } Lecture 22

17. Structs containing arrays as parameters Structs may contain arrays --but remember that struct arguments will be copied unless pointers are used. typedef struct { int a[10]; } Ray; void isCopied(Ray r) { r.a[2]= -1; /* Doesn’t change the argument */ } void notCopied(Ray * r) { r.a[2]= -1; /* Changes the argument */ } void main (void) { Ray a; isCopied(a); notCopied(&a); } Lecture 22

18. h e l l o \0 Strings in C • In C, a string is an array of characters. • The array contains the characters in the string followed by an extra char (byte) containing a binary zero. • Example: “hello” is represented by an array: Lecture 22

19. More on Strings • Because strings are arrays, and because array names are basically pointers, the assignment and comparison operations manipulate pointers, and not the strings themselves. char s[ ]= “hello”; char t[ ]= s; /* s and t point to same place*/ s[1]= ‘?’; /* s and t are now “h?llo” */ • Standard library strlib.h contains routines to copy string values, catenate them, compare them, etc. Lecture 22

20. END OF NEW MATERIAL • Tomorrow, Friday: review, review, review • Check your grades on the door of 5141 Upson this week! • Remember: Last chance to see me about your final grade is Wednesday before 2PM • Final is Tuesday at 8AM, Olin 155. Bright and early! Lecture 22