CS 100Lecture 221 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

CS 100Lecture 222 Today’s Topics More C Data structures Type definitions Arrays Struct and array parameters Strings in C

CS 100Lecture 223 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);

CS 100Lecture 224 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 CCorresponding Java typedef struct {public class Pair { int a;int a; char ch;char ch; } Pair;}

CS 100Lecture 225 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.

CS 100Lecture 226 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) }

CS 100Lecture 227 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

CS 100Lecture 228 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

CS 100Lecture 229 Example with malloc #include Pair * p; /* p is a pointer to a Pair*/ p= (Pair *) malloc( sizeof(Pair) ); p->a= 17; … free(p);

CS 100Lecture 2210 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.

CS 100Lecture 2211 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!

CS 100Lecture 2212 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

CS 100Lecture 2213 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); }

CS 100Lecture 2214 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.

CS 100Lecture 2215 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); }

CS 100Lecture 2216 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); }

CS 100Lecture 2217 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); }

CS 100Lecture 2218 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: h e l l o \0

CS 100Lecture 2219 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.

CS 100Lecture 2220 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!