用指针对C中的结构进行排序

Question

I am just starting in C and have very little clue as to what's going on behind the scenes. I'm learning it on the fly for a data structures class which makes things a bit harder.

Update: I've stripped the program back down and am starting with memory and on up. I have the allocate and deallocate functions in there and I'm getting a malloc error: Q1(9882) malloc: * error for object 0x7fff59daec08: pointer being freed was not allocated * set a breakpoint in malloc_error_break to debug

Update2 here's my revised code, it's still missing something, a couple of my printf statements aren't showing up:

#include <stdio.h>
#include<stdlib.h>
#include<math.h>
#include<assert.h>

static int size = 10;

struct student{
    int id;
    int score;
};

struct student* allocate(){
     /*Allocate memory for ten students*/
     struct student *s = malloc(size*(sizeof(struct student)));
     assert(s != 0);
     /*return the pointer*/
     return s;
}

void generate(struct student* students){
    /*Generate random ID and scores for ten students, ID being between 1 and 10, scores between 0 and 100*/
    srand((unsigned int)time(NULL));
    int id[size];
    int y;

    for (int i = 0; i < size; i++){
        y = rand() % size + 1;
        while(dupe(id, i, y)){
            y = rand() % size + 1;
        }
        id[i] = y;
    }

    for (int j = 0; j < size; j++){
        (students + j)->id = id[j];
        (students + j)->score = rand() % 101;
        printf("ID: %d\tScore: %d\n", (students + j)->id, (students + j)->score);
    }
}

int dupe(int id[], int size1, int i){
    for (int x = 0; x < size1; x++){
        if(id[x] == i)
            return 1;
    }
    return 0;
}

void output(struct student* students){
     /*Output information about the ten students in the format:
              ID1 Score1
              ID2 score2
              ID3 score3
              ...
              ID10 score10*/
    sort(&students);
    for(int x = 0; x < size; x++){
        printf("ID: %d\tScore: %d\n", (students + x)->id, (students + x)->score); //print stmt not showing
    }
}

void sort(struct student* students){
    struct student *sd = allocate();

    struct student *stud;

    for(int i = 0; i < size; i++){
        stud = &students[i];
        sd[stud->id] = *stud;
    }
    for(int x = 0; x < size; x++){
        printf("ID: %d\tScore: %d\n", (sd + x)->id, (sd + x)->score); //print stmt not showing
    }
    students = &sd;
    deallocate(sd);
}

void summary(struct student* students){
     /*Compute and print the minimum, maximum and average scores of the ten students*/

}

void deallocate(struct student* stud){
     /*Deallocate memory from stud*/
    free(stud);
}

int main(){
    struct student* stud = NULL;
    char c[] = "------------------------------\n";
    /*call allocate*/
    stud = allocate();
    /*call generate*/
    generate(&stud);
    /*call output*/
    printf("%s", c);
    output(&stud);
    /*call summary*/

    /*call deallocate*/
    deallocate(stud);

    return 0;
}

Answer 1

students = &students[x];

This changes where students points, so the next time through the loop you will be offseting from there, not from the beginning. That is, you're getting originalstudents[0] , originalstudents[1] , originalstudents[1+2] , originalstudents[1+2+3] , etc. You have the same problem with sd .

Instead you want to use a different variable, something like

struct student* st = &students[x];
printf("id = %d\tscore = %d\n", st->id, st->score);
etc

Also, what is sd for? You seem to allocate some space and copy students to sd for no apparent reason. The allocated space isn't saved or returned ... it's a memory leak. Oh, wait, I see .. you reorder the students in sd in order of their id. So you just should free the memory when you're done. But for both students and sd, you need a different pointer to the array than to the elements of the array. There are many different naming conventions you could use, but it's good to use a consistent one. For example:

void output(struct Student* students){
    struct Student *idstudents = allocate(); /* sorted by id */
    if (!idstudents)
        /* handle allocation error */;

    for (int x = 0; x < 10; x++){
        struct Student* student = &students[x];
        printf("id = %d\tscore = %d\n", student->id, student->score);
        struct Student* idstudent = &idstudents[student->id];
        *idstudent = *student; /* copy all fields at once */
        printf("id = %d\tscore = %d\n", idstudent->id, idstudent->score);/* pointless here, since we just printed the same info via student */
    }

    for (int x = 0; x < 10; x++){
        struct Student* idstudent = &idstudents[x];
        printf("id = %d\tscore = %d\n", idstudent->id, idstudent->score);
    }
    deallocate(idstudents);
}

Answer 2

Your output() function is misusing pointers so you are trampling all over your student data. Try something like this (assuming the IDs are array indexes, because that is how you are using them):

struct student* allocate()
{
    /*Allocate memory for ten students*/ 
    struct student *s = malloc(10 * sizeof(struct student)); 
    assert(s != NULL); 
    /*return the pointer*/ 
    return s; 
} 

void deallocate(struct student* stud)
{ 
    /*Deallocate memory from stud*/ 
    free(stud); 
} 

int main()
{ 
    struct student* stud = NULL; 

    /*call allocate*/ 
    stud = allocate(); 

    /*call generate*/ 

    /*call output*/ 
    output(stud);

    /*call summary*/ 

    /*call deallocate*/ 
    deallocate(stud); 

    return 0; 
} 

void output(struct student* students)
{ 
    /*allocate array for sorting*/
    struct student *sd = allocate(); 

    struct student *stud; 

    /*make copy of students in sorted order*/
    for (int x = 0; x < 10; ++x)
    { 
        stud = &students[x]; 
        printf("id = %d\tscore = %d\n", stud->id, stud->score); 
        sd[stud->id] = *stud; 
    } 

    /*output sorted students*/
    for (int x = 0; x < 10; ++x)
    { 
        stud = &sd[x]; 
        printf("id = %d\tscore = %d\n", stud->id, stud->score); 
    } 

    /*deallocate array for sorting*/
    deallocate(sd); 
} 

Since you have hard-coded the number of students, you can eliminate the need for dynamically allocating a new array of students in output() , and just sort the pointers you already have in the original array:

void output(struct student* students)
{ 
    /*array for sorting*/
    struct student* sd[10]; 

    struct student *stud; 

    /*sort students*/
    for (int x = 0; x < 10; ++x)
    { 
        stud = &students[x]; 
        printf("id = %d\tscore = %d\n", stud->id, stud->score); 
        sd[stud->id] = stud; 
    } 

    /*output sorted students*/
    for (int x = 0; x < 10; ++x)
    { 
        stud = sd[x]; 
        printf("id = %d\tscore = %d\n", stud->id, stud->score); 
    } 
} 

Update : now that you have shown more of your code, you are still making some big mistakes with the pointers. Your code, as you have shown it, should not even compile. Try this instead:

#include <stdio.h> 
#include <stdlib.h> 
#include <math.h> 
#include <assert.h> 

static const int numStudents = 10; 

struct student
{ 
    int id; 
    int score; 
}; 

struct student* allocate()
{ 
     /*Allocate memory for ten students*/ 
     struct student *s = malloc(numStudents * sizeof(struct student)); 
     assert(s != 0); 
     /*return the pointer*/ 
     return s; 
} 

void generate(struct student* students)
{ 
    /*Generate random ID and scores for ten students, ID being between 1 and 10, scores between 0 and 100*/ 
    int id[numStudents]; 
    int y; 
    struct student* stud;

    for (int i = 0; i < numStudents; i++)
    { 
        do
        {
          y = rand() % size + 1; 
        }
        while (dupe(id, i, y) != 0);
        id[i] = y; 
    } 

    for (int j = 0; j < numStudents; j++)
    { 
        stud = &students[j];
        stud->id = id[j]; 
        stud->score = rand() % 101; 
    } 
} 

int dupe(int id[], int size, int i)
{ 
    for (int x = 0; x < size; x++)
    { 
        if (id[x] == i) 
            return 1; 
    } 
    return 0; 
} 

void output(struct student* students)
{ 
     /*Output information about the students in the format: 
              ID1 Score1 
              ID2 score2 
              ID3 score3 
              ... 
              ID10 score10*/ 

    struct student* stud;

    for(int x = 0; x < numStudents; x++)
    { 
        stud = &students[x];
        printf("ID: %d\tScore: %d\n", stud->id, stud->score);
    } 
} 

void sort(struct student* students)
{ 
    struct student *sd = allocate(); 
    struct student *stud; 

    for(int i = 0; i < numStudents; i++)
    { 
        stud = &students[i]; 
        sd[stud->id - 1] = *stud; 
    } 

    for(int x = 0; x < numStudents; x++)
    { 
        stud = &sd[x]; 
        students[x] = *stud; 
    } 

    deallocate(sd); 
} 

void summary(struct student* students)
{ 
    /*Compute and print the minimum, maximum and average scores of the ten students*/ 
} 

void deallocate(struct student* stud)
{ 
    /*Deallocate memory from stud*/ 
    free(stud); 
} 

int main()
{ 
    /*seed random number generator*/
    srand(time(NULL)); 

    struct student* stud = NULL; 
    const char* c = "------------------------------\n"; 

    /*allocate students and generate info*/ 
    stud = allocate(); 
    generate(stud); 
    output(stud); 

    printf("%s", c); 

    /*sort students*/ 
    sort(students); 
    output(stud); 

    printf("%s", c); 

    /*display summary*/ 
    summary(stud); 

    /*deallocate students*/ 
    deallocate(stud); 

    return 0; 
} 

Answer 3

This statement

students = &students[x];

modifies the incoming argument. You have lost what 'students' pointed to, which is the beginning of struct student [] .

Remove this statement and try running you program again.

There are other errors, but this should get you unstuck.

Pointers are hard.