Lesson 6.1: Structs. This declares a collection of two integer variables to denote the two coordinates of a point in a plane.

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1 Lesson 6.1: Structs Programming in C Prof. Dr. Eike Best Dr. Elke Wilkeit October 12, struct point { 2 int x; 3 int y; 4 }; This declares a collection of two integer variables to denote the two coordinates of a point in a plane. 1 struct point pt; This declares a variable pt of type point. Later, one can access the x coordinate and the y coordinate of pt by pt.x and pt.y, respectively. 1 pt.x = 0; 2 pt.y = 0; This sets pt to the origin of the plane. Nesting is possible: 1 struct rect { 2 struct point lowerleft; 3 struct point upperright; 4 }; We may then set an 8 times 6 screen by: 1 struct rect screen; 2 3 screen.lowerleft.x = 0; 4 screen.lowerleft.y = 0; 5 screen.upperright.x = 8; 6 screen.upperright.y = 6;

2 Lesson 6.2: The Josephus Problem: A Linked List Programming in C Prof. Dr. Eike Best Dr. Elke Wilkeit October 12, 2002 Josephus Flavius was a famous Jewish historian of the first century at the time of the Second Temple destruction. During the Jewish-Roman war he got trapped in a cave with a group of 40 soldiers surrounded by Romans. The legend has it that preferring suicide to capture, the Jews decided to form a circle and, proceeding around it, to kill every third remaining person until no one was left. Josephus, not keen to die, quickly found the safe spot in the circle and thus stayed alive. A linked list is an appropriate data structure for the problem of easily adding and removing members from the circle. Its members are defined as a structure consisting of a number and a pointer to the next member. Passing Arguments to main The numbers N = 41 of persons and M = 3 of counted items may be given to the function main as parameters: int main(int argc, char *argv[]) argc is the number of given arguments plus 1 argv[0] is the name of the executable file, e.g. a.out argv[1]...argv[argc-1] are the arguments given by the user atoi(argv[1]) converts the string agrv[1] to an int. Debug Information The precompiler is used for inserting debug commands or not: #define DEBUG 1 #ifdef DEBUG printf("added node %d \n", number); #endif If DEBUG is defined as above (no matter what value it gets), the print-command is inserted by the precompiler. If the definition of DEBUG ist commented as in the program below, the preprocessor will erase all such print-commands enclosed in #ifdef/#endif. Error Messages The function perror is used in lines 45, 86 and 107 to print an error message to the standard error output. Memory Another new topic is the dynamic allocation of memory in line 84, whenever a new node is created, with malloc. This memory can be freed again with free(), see line 113.

3 1 /* #define DEBUG 1 (unkomment for debug-information) 2 */ 3 4 #include <stdio.h> 5 6 /* structure whose members have a number, item, and 7 * a pointer to their respective successor, next in the circle 8 */ 9 struct circle { 10 int item; 11 struct circle *next; 12 }; struct circle *addnode(int number, struct circle *prev); 15 void killnode(struct circle *prev); /* Solve the Josephus Problem: In which order are they killed? 18 */ 19 int main(int argc, char *argv[]) 20 { 21 int i; 22 int N, M; /* N persons, every Mth is killed */ 23 struct circle first; /* first node in the circle */ 24 struct circle *ptr; /* pointer to current node */ #ifdef DEBUG 27 printf("argc = %d, argv[0] = %s, argv[1] = %s, argv[2] = %s\n", 28 argc, argv[0], argv[1], argv[2]); 29 #endif switch(argc) { /* if arguments are given: use them */ 32 case 1: /* otherwise use default values */ 33 N = 41; 34 M = 3; 35 break; 36 case 2: 37 N = atoi(argv[1]); 38 M = 3; 39 break; 40 case 3: 41 N = atoi(argv[1]); /* convert arguments to int */ 42 M = atoi(argv[2]); 43 break; 44 default: 45 perror("error: too many arguments for main"); 46 exit(1); 47 break; 48 }

4 49 /* initialize first node with successor NULL, then 50 * generate another N-1 nodes for the circle. 51 * pointer points to the last node. 52 * close the circle: the last node points to the first. 53 */ 54 first.item = 1; 55 first.next = NULL; 56 ptr = &first; 57 for (i = 2; i <= N; ++i) { 58 ptr = addnode(i, ptr); 59 } 60 ptr->next = &first; /* as long as there is more than one node: 63 * kill every Mth of them 64 */ 65 printf("\nkill"); 66 while (ptr->next!= ptr) { 67 for (i = 1; i < M; ++i) 68 ptr = ptr->next; 69 printf(" %d", ptr->next->item); 70 killnode(ptr); 71 } 72 printf(" %d", ptr->item); 73 printf("\n"); 74 } /* add a new node following the one prev points to 77 */ 78 struct circle *addnode(int number, struct circle *prev) 79 { 80 struct circle *newnode; /* reserve memory for the new node */ 83 if ((newnode = 84 (struct circle *) malloc(sizeof(struct circle)) 85 ) == NULL) { 86 perror("out of memory"); 87 exit(1); 88 } 89 /* make the new connections between the nodes */ 90 prev->next = newnode; 91 newnode->next = NULL; 92 newnode->item = number; 93 #ifdef DEBUG 94 printf("added node %d \n", number); 95 #endif 96 return newnode; 97 }

5 98 /* kill the node following the one prev points to 99 * by making new connections between the nodes 100 * and freeing the memory 101 */ 102 void killnode(struct circle *prev) 103 { 104 struct circle *tmp; if (prev->next == NULL) 107 perror("error: cannot kill non-existing node!"); 108 else { 109 tmp = prev->next; 110 prev->next = prev->next->next; 111 } 112 if (1 < tmp->item) /* if allocated by malloc */ 113 free(tmp); 114 }