1. CSE1301Computer ProgrammingLecture 23Structures (Part 2)

2. Topics • Structs and functions • Arrays of structures

3. Passing structs as Parameters • Like any other variable, you can pass a struct as a parameter to a function • Passing by value • Pass the struct – use the values of the members, but don't change them • Passing by reference • Pass a pointer to the struct – you can change the values of the members

4. Passing a struct by Value • As always, the formal parameters are copies of the actual parameters void printRecord (Student item) { printf("Last name: %s\n",item.lastname); printf(" Mark: %.1f\n\n",item.mark); } main() { Student studentA = {“Gauss”, 99.0}; printRecord(studentA); }

5. Function Returning a struct StudentreadRecord ( void ) { Student newStudent; printf("Enter last name and mark: "); scanf("%s %f",newStudent.lastname,&(newStudent.mark)); return newStudent; } • A “package” containing several values main() { Student studentA; studentA =readRecord(); }

6. Passing structs by Reference • Pass a pointer to a struct • Change the value of some or all of the members • Changes are visible in the calling function as well as the called function

7. Passing structs by Reference (cont) void readStudent (Student* sptr) { printf(“Please enter name and ID\n”); scanf(“%s”, (*sptr).name ); /*parens needed*/ scanf(“%ld”, &((*sptr).id) ); } int main() { Student studentA; readStudent(&studentA); }

8. Passing structs by Reference (cont) • The parentheses around the (*sptr) are needed because of precedence • e.g., (*sptr).item • If you need the address of a member, you need another set of parentheses • e.g., &((*sptr).item)

9. Arrays of structs • Each element of the array is a whole struct, which includes all the members of that struct • To access a single value, you need to know which element of the array you're dealing with and which member of the struct, e.g., studentList[0].name studentList[i].id

10. id:123456789 0 name: "fred" id:123456788 1 name: "ralph" id: 123456787 2 name: "fong" id: 123456786 3 name: "rachel" Array of structs studentList

11. id:123456789 0 name: "fred" id:123456788 1 name: "ralph" id: 123456787 2 name: "fong" id: 123456786 3 name: "rachel" Array of structs studentList[0]gives you the whole struct studentList

12. id:123456789 0 name: "fred" id:123456788 1 name: "ralph" id: 123456787 2 name: "fong" id: 123456786 3 name: "rachel" Array of structs studentList studentList[3].namegives you the struct member

13. Arrays of structs #define MAXCLASS 30 typedef struct { long int id; char name[20]; } Student; ... Student sem1Class[MAXCLASS];

14. Arrays of structs Student sem1Class[MAXCLASS]; int i; for (i=0;i<MAXCLASS;i++) { printf("enter name\n"); scanf("%s",sem1Class[i].name); printf("enter id\n"); scanf("%d",&(sem1Class[i].id)); } name of array

15. Arrays of structs Student sem1Class[MAXCLASS]; int i; for (i=0;i<MAXCLASS;i++) { printf("enter name\n"); scanf("%s",sem1Class[i].name); printf("enter id\n"); scanf("%d",&(sem1Class[i].id)); } index into array

16. Arrays of structs Student sem1Class[MAXCLASS]; int i; for (i=0;i<MAXCLASS;i++) { printf("enter name\n"); scanf("%s",sem1Class[i].name); printf("enter id\n"); scanf("%d",&(sem1Class[i].id)); } the structure at this position in the array

17. Arrays of structs Student sem1Class[MAXCLASS]; int i; for (i=0;i<MAXCLASS;i++) { printf("enter name\n"); scanf("%s",sem1Class[i].name); printf("enter id\n"); scanf("%d",&(sem1Class[i].id)); } name of the member of the structure at that position in the array

18. #include <stdio.h> #include <stdlib.h> #define MAXLEN 50 #define MAXN 20 struct StudentRec { char lastname[MAXLEN]; float mark; }; typedef struct StudentRec Student; Student readRecord ( void ) { Student newStudent; printf("Enter last name and mark: "); scanf("%s %f", newStudent.lastname, &(newStudent.mark)); return newStudent; } void printRecord ( Student item ) { printf("Last name: %s\n", item.lastname); printf(" Mark: %.1f\n\n", item.mark); return; } Example: array of structs and pass-by-value-1

19. Example: array of structs and pass-by-value-2 int main() { int count = 0; Student class[MAXN]; int i; printf("How many students? "); scanf("%d", &count); if (count > MAXN) { printf("Not enough space.\n"); exit(1); } for (i=0; i < count; i++) { class[i] =readRecord(); } printf("\nClass list:\n\n"); for (i=0; i < count; i++) { printRecord(class[i]); } return 0; }

20. Using Arrays and Pointers typedef struct { int age; char name[20]; } Child; Child s[20]; • Some struct: s[13] • An item in some struct: s[4].age • A character in some name: s[7].name[3] • Address of some struct: &(s[2]) • An item in some struct pointed to: (*sptr).age • Address of above: &((*sptr).age) This is an address within a struct

21. Example: array of structs and pass-by-reference-1 #include <stdio.h> #include <stdlib.h> #define MAXLEN 50 #define MAXN 20 struct StudentRec { char lastname[MAXLEN]; float mark; }; typedef struct StudentRec Student; void readStudent ( Student* sptr ) { printf("Please enter name and mark\n"); scanf("%s", (*sptr).lastname ); /*parens needed*/ scanf("%f", &((*sptr).mark) ); return; } void printRecord ( Student item ) { printf("Last name: %s\n", item.lastname); printf(" Mark: %.1f\n\n", item.mark); return; }

22. Example: array of structs and pass-by-reference-2 int main() { int count = 0; Student class[MAXN]; int i; printf("How many students? "); scanf("%d", &count); if (count > MAXN) { printf("Not enough space.\n"); exit(1); } for (i=0; i < count; i++) { readStudent(&class[i]); } printf("\nClass list:\n\n"); for (i=0; i < count; i++) { printRecord(class[i]); } return 0; }

23. Reading • King, 16.1 – 16.3 • Deitel & Deitel,10.1 – 10.7 • Kernighan & Ritchie, 6.1, 6.2, 6.7