Slide 1Upcoming schedulefree: releases memoryMemory corruptionStructured dataUsing structstypedefStructs as parametersPointers to structsThe -> operatorCopy by assignmentStruct as return valueComparing structsComparing structs, cont'dStructs and inputArrays of structsStructs with pointersAlternativeLinked data structuresManipulating a linked listExerciseQuestions?David Notkin Autumn 2009 CSE303 Lecture 13This space for rentUpcoming scheduleCSE303 Au09 2Today10/23Monday10/26Wednesday10/28Friday10/30Monday11/2Finish-up WednesdaySome specifics for HW3Social implications FridayMemory management MidtermreviewMidterm•structured data•struct, typedef•structs as parameters/returns•arrays of structs•linked data structures•stacks•linked listsfree: releases memory•free(pointer);•Releases the memory pointed to by the given pointer–precondition: pointer must refer to a heap-allocated memory block that has not already been freed–it is considered good practice to set a pointer to NULL after freeingint* a = (int*) calloc(8, sizeof(int));...free(a);a = NULL;Memory corruption•If the pointer passed to free doesn't point to a heap-allocated block, or if that block has already been freed, bad things happen–you're lucky if it crashes, rather than silently corrupting somethingint* a1 = (int*) calloc(1000, sizeof(int));int a2[1000];int* a3;int* a4 = NULL;free(a1); // okfree(a1); // bad (already freed)free(a2); // bad (not heap allocated)free(a3); // bad (not heap allocated)free(a4); // bad (not heap allocated)Structured data•struct: A type that stores a collection of variables–like a Java class, but with only fields (no methods or constructors)–instances can be allocated on the stack or on the heapstruct Point { // defines a new structured int x, y; // type named Point};Using structs•Once defined, a struct instance is declared just like built-in types (e.g., int, char) except preceded by struct–this allocates an instance on the stack–name fields of a struct using the . operatorstruct Point { int x, y;};int main(void) { struct Point p1; struct Point p2 = {42, 3}; p1.x = 15; p1.y = -2; printf("p1 is (%d, %d)\n", p1.x, p1.y); return 0;}typedef•Tell C to acknowledge your struct type's name with typedeftypedef struct Point { int x, y;} Point;int main(void) { Point p1; // don't need to write 'struct' p1.x = 15; p1.y = -2; printf("p1 is (%d, %d)\n", p1.x, p1.y); return 0;}Structs as parameters•when you pass a struct as a parameter, it is copied–not passed by reference as in Javaint main(void) { Point p = {10, 20}; swapXY(p); printf("(%d, %d)\n", p.x, p.y); return 0; // prints (10, 20)}void swapXY(Point a) { int temp = a.x; a.x = a.y; a.y = temp; // does not work}Pointers to structs•structs can be passed using pointers–must use parentheses when dereferencing a struct* (because of operator precedence)int main(void) { Point p = {10, 20}; swapXY(&p); printf("(%d, %d)\n", p.x, p.y); return 0; // prints (20, 10)}void swapXY(Point* a) { int temp = (*a).x; (*a).x = (*a).y; (*a).y = temp;}The -> operator•We often allocate structs on the heap–pointer->field is equivalent to (*pointer).fieldint main(void) { Point* p = (Point*) malloc(sizeof(Point)); p->x = 10; p->y = 20; swapXY(p); printf("(%d, %d)\n", p->x, p->y); // (20, 10) return 0;}void swapXY(Point* a) { int temp = a->x; a->x = a->y; a->y = temp;}Copy by assignment•One struct's entire contents can be copied to another with =–struct2 = struct1; // copies the memoryint main(void) { Point p1 = {10, 20}, p2 = {30, 40}; p1 = p2; printf("(%d, %d)\n", p1.x, p1.y); // (30, 40) // is this the same as p1 = p2; above? Point* p3 = (Point*) malloc(sizeof(Point)); Point* p4 = (Point*) malloc(sizeof(Point)); p3->x = 70; p3->y = 80; p3 = p4; printf("(%d, %d)\n", p3->x, p3->y);}Struct as return value•We generally pass/return structs as pointers–takes less memory (and time) than copying–if a struct is malloc-ed and returned as a pointer, who frees it?int main(void) { Point* p1 = new_Point(10, 20); ... free(p1);}// creates/returns a Point; sort of a constructorPoint* new_Point(int x, int y) { Point* p = (Point*) malloc(sizeof(Point)); p->x = x; p->y = y; return p; // caller must free p later}Comparing structs•relational operators (==, !=, <, >, <=, >=) don't work with structsPoint p1 = {10, 20};Point p2 = {10, 20};if (p1 == p2) { ... // error•what about this?Point* p1 = new_Point(10, 20);Point* p2 = new_Point(10, 20);if (p1 == p2) { ... // true or false?Comparing structs, cont'd•the right way to compare two structs: write your own#include <stdbool.h>bool point_equals(Point* a, Point* b) { if (a->x == b->x && a->y == b->y) { return true; } else { return false; }}int main(void) { Point p1 = {10, 20}; Point p2 = {10, 20}; if (point_equals(&p1, &p2)) { ...Structs and input•you can create a pointer to a field of a struct–structs' members can be used as the target of a scanf, etc.int main(void) { Point p; printf("Please type your x/y position: "); scanf("%d %d", &p.x, &p.y);}int main(void) { Point* p = (Point*) malloc(sizeof(Point)); printf("Please type your x/y position: "); scanf("%d %d", &p->x, &p->y);}Arrays of structs•parallel arrays are conceptually linked by their index–parallel arrays are usually bad design; isn't clear that they are related–you should often replace such arrays with an array of structsint id[50]; // parallel arrays to storeint year[50]; // student data (bad)double gpa[50];typedef struct Student { int id, year; double gpa;} Student;Student students[50];Structs with pointers•What if we want a Student to store a significant other?typedef struct Student { // incorrect .. Why? int id, year; double gpa; struct Student sigother;} Student;•When to stop the recursion?–a Student cannot fit another entire Student inside of it!Alternativetypedef struct Student { // correct int id, year; double gpa; struct Student* sigother;} Student;CSE303 Au09 18Linked data structures•C does not include collections like Java's ArrayList, HashMap–must build any needed data structures manually–to build a linked list structure, create a chain of
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