How to sort an array inside a structure in c?

Question

I want to sort elements of the marks[] array inside a structure student using a sort function, so that marks/elements print in ascending order. But marks/elements are printing in same order.

Problem:

*Write a ‘C’ program to accept n student details as roll number, name, marks of three subjects. Calculate sum, percentage and sort student details based on percentage using following function prototypes.

void accept(struct student s[10], int n);
void display(struct student s[10], int n);
void sort(struct student s[10], int n);

Here is my code:

#include <stdio.h>
#include <stdlib.h>
 
struct student{
    char name[10];
    int rollno;
    int marks[3];  //array of 3 subjects marks
    int totalMarks;
    float percent;
};

void calculate(struct student s[10], int n) {
    //calculate total marks, &percent using loop
    int i, m;
    for (i = 0; i < n; i++) {
         s[i].totalMarks = 0;
         s[i].percent = 0.0;
         for (m = 0; m < 3; m++) {  
              s[i].totalMarks += s[i].marks[m];
         }
         s[i].percent = s[i].totalMarks / 3.0;
    }
}

void display(struct student s[10], int n) {
    int i, m;
    for (i = 0; i < n; i++) {
        printf("Student's Name: %s\n", s[i].name);
        printf("Student's Rollno: %d\n", s[i].rollno);
        for (m = 0; m < 3; m++) {
            printf("Subject %d Marks: %d\n", m+1, s[i].marks[m]);
        }
        printf("Total Marks: %d\n", s[i].totalMarks);
        printf("Total Percent: %.2f\n", s[i].percent);
        printf("\n");
    }  
}

void sort(struct student s[10], int n) {
    int i, j, m;

    //Sort elements of Array(marks[] ) inside Structure (student) in C
    for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {
            if (s[i].marks[m] > s[j].marks[m]) {
                struct student temp = s[i];
                s[i] = s[j];
                s[j] = temp;
            }
        }
    }

    //DISPLAY
    for (i = 0; i < n; i++) {
        printf("Student's Name: %s\n", s[i].name);
        printf("Student's Rollno: %d\n", s[i].rollno);
        for (m = 0; m < 3; m++) {
            printf("%d ", s[i].marks[m]);
        }
        printf("Total Marks: %d\n", s[i].totalMarks);
        printf("Total Percent: %.2f\n", s[i].percent);
        printf("\n");
    } 
}

int main() {
    struct student s[10]; //minimum of 10 students

    int i, n, m;
    printf("Enter the number of students: ");
    scanf("%d", &n);

    for (i = 0; i < n; i++) {
        printf("\nEnter student's name: ");
        scanf("%s", s[i].name);
        printf("Enter student's rollno: ");
        scanf("%d", &s[i].rollno);
        for (m = 0; m < 3; m++) {
            printf("Enter the student's marks: ", m + 1);
            scanf("%d", &s[i].marks[m]);
        }
        printf("\n");
    }

    calculate(s, n);  //structure,number of students

    printf("\nstudent's Information-------------\n");
    display(s, n);
    printf("\n");

    printf("\nSort the Array in ascending order of marks:");
    sort(s, n);
    printf("\n");
}

The marks don't get sorted.

Answer 1

There is a confusion in your code between the index i into the student array and the index m into the marks array of each student, which is not even initialized, giving the code undefined behavior. Compiling with warnings enabled (eg: gcc -Wall -Wextra -Werror) would have caught this problem.

You should iterate on the student array, and for each student, sort its mark array using 2 nested loops.

Here is a modified version:

#include <stdio.h>
 
struct student {
    char name[10];
    int rollno;
    int marks[3];  //array of 3 subjects marks
    int totalMarks;
    float percent;
};

void calculate(struct student s[10], int n) {
    //calculate total marks and percentage using loop
    for (int i = 0; i < n; i++) {
         s[i].totalMarks = 0;
         s[i].percent = 0.0;
         for (int m = 0; m < 3; m++) {  
             s[i].totalMarks += s[i].marks[m];
         }
         s[i].percent = s[i].totalMarks / 3.0;
    }
}

void display(struct student s[10], int n) {
    for (int i = 0; i < n; i++) {
        printf("Student's Name: %s\n", s[i].name);
        printf("Student's Rollno: %d\n", s[i].rollno);
        for (int m = 0; m < 3; m++) {
            printf("Subject %d Marks: %d\n", m + 1, s[i].marks[m]);
        }
        printf("Total Marks: %d\n", s[i].totalMarks);
        printf("Total Percent: %.2f\n", s[i].percent);
        printf("\n");
    }
}

/* sort an array of marks */
void sort_marks(int *marks, int n) {
    for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < i; j++) {
            if (marks[i] > marks[j]) {
                int temp = marks[i];
                marks[i] = marks[j];
                marks[j] = temp;
            }
        }
    }
}

void sort(struct student s[10], int n) {
    //Sort elements of Array(marks[]) inside Structure (student) in C
    for (int i = 0; i < n; i++) {
        sort_marks(s[i].marks, 3);
    }
}

int main() {
    struct student s[10] = { 0 }; //maximum of 10 students
    int n;

    printf("Enter the number of students: ");
    if (scanf("%d", &n) != 1 || n <= 0 || n > 10)
        return 1;

    for (int i = 0; i < n; i++) {
        printf("\nEnter student's name: ");
        scanf("%9s", s[i].name);
        printf("Enter student's rollno: ");
        scanf("%d", &s[i].rollno);
        for (int m = 0; m < 3; m++) {
            printf("Enter the student's marks: ", m + 1);
            scanf("%d", &s[i].marks[m]);
        }
        printf("\n");
    }

    calculate(s, n);  //structure,number of students

    printf("\nstudent's Information-------------\n");
    display(s, n);
    printf("\n");

    printf("\nSort the Array in ascending order of marks:");
    sort(s, n);
    display(s, n);
    printf("\n");
    return 0;
}