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Understanding Pointers and Structures in C: A Comprehensive Guide

This guide explores the use of pointers with structures in C programming, covering key concepts such as defining a structure, manipulating strings within structures, and practical applications like shuffling a deck of cards. It discusses the significance of dynamic memory management and illustrates how to safely work with memory using examples like initializing and updating structure members. Through clear code snippets and discussions on scope and memory management, readers will gain a solid understanding of pointers and structures in C.

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Understanding Pointers and Structures in C: A Comprehensive Guide

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  1. 240-222 Computer Programming TechniquesSemester 1, 1998 11. Pointers and Structures Objective of these slides: to discuss how pointers are used with structs

  2. Overview 1. A struct can contain Strings 2. Shuffling Cards 3. Pointers to structs 4. Updating a struct

  3. 1. A struct can Contain Strings struct card{ char *face; char *suit;}; :struct card a = {"Three", "Hearts"}; :printf("%s", a.suit); /* prints Hearts */

  4. 2. Shuffling Cards Sec. 10.7 #include <stdio.h>#include <stdlib.h>#include <time.h>struct card { char *face; char *suit;};typedef struct card Card;void filldeck(Card [], char *[], char *[]);void shuffle(Card []);void deal(Card []); continued

  5. int main(){ Card deck[52]; char *face[] = {"Ace", "Deuce", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine","Ten","Jack", "Queen", "King"}; char *suit[] = {"Hearts", "Diamonds", "Clubs", "Spades"}; srand(clock()); /* set random num */ filldeck(deck, face, suit); shuffle(deck); deal(deck); return 0;} continued

  6. void filldeck(Card wdeck[], char *wface[], char *wsuit[])/* initialise the deck of cards */{ int i; for (i = 0; i <= 51; i++) { wdeck[i].face = wface[i % 13]; wdeck[i].suit = wsuit[i / 13]; }} continued

  7. void shuffle(Card wdeck[])/* randomly rearrange the deck of cards */{ int i, j; Card temp; for (i = 0; i <= 51; i++) { j = rand() % 52; /* get rand num */ if (i != j) { temp = wdeck[i]; wdeck[i] = wdeck[j]; wdeck[j] = temp; } }} continued

  8. void deal(Card wdeck[])/* display the deck */{ int i; for (i = 0; i <= 51; i++) printf("%5s of %-8s%c", wdeck[i].face, wdeck[i].suit, (i + 1) % 2 ? '\t' : '\n');}

  9. Output (see fig. 10.4): Eight of Diamonds Ace of HeartsEight of Clubs Five of Spades : :

  10. 3. Pointers to Structs struct card a, *c;a = {"Four", "Spades"};c = &a; :printf("%s", c->suit); /* prints Spades */ c->suit is equivalent to (*c).suit

  11. Example #include <stdio.h> /* fig 10.2 */struct card { char *face; char *suit;}; continued

  12. int main(){ struct card a; struct card *aptr; a.face = "Ace"; a.suit = "Spades"; aptr = &a; printf("%s of %s\n%s of %s\n%s of%s\n", a.face, a.suit, aptr->face, aptr->suit, (*aptr).face, (*aptr).suit); return 0;}

  13. Output: Ace of SpadesAce of SpadesAce of Spades

  14. 4. Updating a struct • Code fragment: struct card { char *face; char *suit;};int main(){ struct card a; a.face = “Three; printf(“%s”, a.face);}

  15. Discussion • This code works, so what is the problem? • Answer: the scope of “Three” • at the end of main(), “Three” goes out of scope and is deleted • this does not matter for this example because the scope of main() is the entire program

  16. A More Dangerous Example int main(){ struct card a; a = initialise(); printf(“%s”, a.face); : printf(“%s”, a.face);}struct card initialise(void){ struct card b; b.face = “Three”; return b;}

  17. Discussion • The scope of “Three” is initialise(), and so will be deleted after initialise() returns • return copies out the b struct, including its two pointers • a is assigned the pointers, but what do they point at? • the deletion may not happen immediately, but depend on the memory usage of the rest of the program • the first printf() may work, sometimes!

  18. First Solution struct card1( char face[10]; char suit[10];};int main(){ struct card1 a; a = initialise1(); : printf(“%s”, a.face);}struct card1 initialise1(void){ struct card1 b; strcpy(b.face, “Three”); return b;}

  19. Discussion • The general solution is to make a copy of the string • “Three” is copied into the fixed size array b.face using strcpy() • that array is copied out as part of the b struct using return

  20. Second Solution struct card( char *face; /* no fixed size */ char *suit;};int main(){ struct card a; a = initialise2(); : printf(“%s”, a.face);}struct card initialise2(void){ struct card b;b.face = (char *)malloc(6); strcpy(b.face, “Three”); return b;}

  21. Discussion • The second solution still uses copying, but now calls malloc() to make dynamic memory • return only copies out the pointers inside b • but malloc’ed memory is not deleted even though the scope of b is initialise2() • so a.face will point to the malloc’ed memory from initialise2()

  22. Third Solution struct card( char *face; /* no fixed size */ char *suit;};int main(){ struct card a; initialise3(&a); : printf(“%s”, a.face);}void initialise3(struct card *ap){ ap->face = (char *)malloc(6); strcpy(ap->face, “Three”); }

  23. Discussion • The third solution uses malloc() to make dynamic memory, but for the a struct. • pass in a pointer to a, and alter a via the pointer (this is how C implements call-by-reference). • this is the most common coding style for manipulating structs inside functions

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