c - Returning array address from function did not work -
i have c program in user enters sets of grades. works correctly. gpa calculated correctly, etc. however, when numbers printed out, both pointers in student structs point same address reason, leading both students display grades of second when information printed out. rest of information correct, it's grades identical.
the thing can think of second initialization of grades array overwrites first. don't understand why happens or how fix it.
the following sample io of program:
enter number of students:> 2 enter number of grades track:> 3 there 2 students. there 3 grades. enter information student: enter sid:> 101 enter last name:> enright enter first name:> reed enter grades (separated space):> 70.1 60 92 enter information student: enter sid:> 123 enter last name:> claire enter first name:> heidi enter grades (separated space):> 82.5 96.1 89.0 student id #101: name: reed enright grades: 82.5 96.1 89.0 gpa: 74.03 student id #123: name: heidi claire grades: 82.5 96.1 89.0 gpa: 89.20 and complete code:
#include <stdio.h> #define name_size 25 typedef struct { int sid; char last_name[name_size]; char first_name[name_size]; float *grades; float gpa; } student; // function prototypes // student information student prompt_student(int number_of_grades); // calculate gpa based on grades float calculate_gpa(student student, int number_of_grades); // prints of students void print_all_students(student students[], int number_of_students, int number_of_grades); int main(){ // initialise variables int number_of_students; int number_of_grades; // prompt number of students printf("\nenter number of students:> "); scanf("%d", &number_of_students); // prompt number of grades printf("enter number of grades track:> "); scanf("%d", &number_of_grades); // confirm above printf("\nthere %d students. \nthere %d grades.\n", number_of_students, number_of_grades); // initialise student list student students[number_of_students]; // , store student information for(int = 0; < number_of_students; i++){ students[i] = prompt_student(number_of_grades); } // confirm above print_all_students(students, number_of_students, number_of_grades); return 0; } student prompt_student(int number_of_grades){ // initialise student variable student student; float grades[number_of_grades]; printf("\nenter information student: \n"); // prompt student info printf("\tenter sid:> "); scanf("%d", &(student.sid)); printf("\tenter last name:> "); scanf("%s", student.last_name); printf("\tenter first name:> "); scanf("%s", student.first_name); printf("\tenter grades (separated space):> "); for(int = 0; < number_of_grades; i++){ scanf("%f", &grades[i]); } student.grades = grades; student.gpa = calculate_gpa(student, number_of_grades); return student; } float calculate_gpa(student student, int number_of_grades){ float total = 0; // initialise variable sum of grades // add grades for(int = 0; < number_of_grades; i++){ total += student.grades[i]; } // return average return total / number_of_grades; } void print_all_students(student students[], int number_of_students, int number_of_grades){ // loop through students for(int = 0; < number_of_students; i++){ // print student info printf("\nstudent id #%d:", students[i].sid); printf("\n\tname:\t%s %s", students[i].first_name, students[i].last_name); printf("\n\tgrades:\t"); for(int n = 0; n < number_of_grades; n++){ printf("%.1f ", students[i].grades[n]); } printf("\n\tgpa:\t%.2f", students[i].gpa); } printf("\n"); }
the problem inside function prompt_student declared local array
float grades[number_of_grades]; and address of first element of local array assigned data member grades of structure student
student.grades = grades; so data member have same address each call of function. , program has undefined behaviour because after exiting function local array not alive. in general case destroyed.
you must dynamically allocate array , assign address of allocated array data member grades.
for example
float *grades = malloc( number_of_grades * sizeof( float ) ); it obvious in main should free allocated memory when corresponding object of structure not used more.