c - 使用 qsort 按字母顺序组织结构体

Question

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define ARRAY_MAX 100
#define FNAME 31
#define LNAME 31

typedef int (*compfn)(const void*, const void*);

struct student {

    char firstName[FNAME];
    char lastName[LNAME];
    int quiz1, quiz2 , quiz3, quiz4, mid1, mid2, final;
    char letterGrade;
    int finalGradePercentage;
};

// Function for Qsort
int compare(struct student *element1, struct student *element2)
{
    if ( element1->firstName[0] < element2->firstName[0])
        return -1;

    else if (element1->firstName[0] > element2->firstName[0])
        return 1;

    else
        return 0;
    }


int main(int argc, char *argv[]){


    FILE *inputFile, *outputFile;
    char buf[100];
    char *token;
    int count = 0;

    inputFile = fopen("/Users/Home/Desktop/input_data.txt", "r");


    struct student studentData[ARRAY_MAX];
    while (fgets(buf, sizeof(buf), inputFile) != NULL){ //scan through the input file
        token = strtok(buf, " ");
        strcpy(studentData[count].firstName, token);
        token = strtok(NULL, ",");
        strcpy(studentData[count].lastName, token);
        token = strtok(NULL, ",");
        studentData[count].quiz1 = atoi(token);
        token = strtok(NULL, ",");
        studentData[count].quiz2 = atoi(token);
        token = strtok(NULL, ",");
        studentData[count].quiz3 = atoi(token);
        token = strtok(NULL, ",");
        studentData[count].quiz4 = atoi(token);
        token = strtok(NULL, ",");
        studentData[count].mid1 = atoi(token);
        token = strtok(NULL, ",");
        studentData[count].mid2 = atoi(token);
        token = strtok(NULL, ",");
        studentData[count].final= atoi(token);


        studentData[count].finalGradePercentage = studentData[count].quiz1 * .10 +studentData[count].quiz2 * .10 + studentData[count].quiz3 * .10 + studentData[count].quiz4 * .10 + studentData[count].mid1 * .20 + studentData[count].mid2 * .15 + studentData[count].final * .25;

        if (studentData[count].finalGradePercentage <=100 &&  studentData[count].finalGradePercentage>=90 )
        {
            studentData[count].letterGrade = 'A';
        }
        else if(studentData[count].finalGradePercentage <= 89 && studentData[count].finalGradePercentage >=80)
        {
            studentData[count].letterGrade = 'B';
        }
        else if(studentData[count].finalGradePercentage <= 79 && studentData[count].finalGradePercentage >=70)
        {
            studentData[count].letterGrade = 'C';
        }
        else if(studentData[count].finalGradePercentage <= 69 && studentData[count].finalGradePercentage >=60)
        {
            studentData[count].letterGrade = 'D';
        }
        else if(studentData[count].finalGradePercentage <= 59){
            studentData[count].letterGrade = 'F';
        }


        count++;
    }


    qsort((void *) &studentData,count,sizeof(struct student),(compfn)compare);           


    for (int i = 0; i< count; i++) {
        char name[62];

        sprintf(name, "%s %s", studentData[i].firstName, studentData[i].lastName);
        printf("%-20s %c\n", name, studentData[i].letterGrade);

    }

    fclose(inputFile);
    fclose(outputFile);

    return 0;
}

大家好我很难按字母顺序组织我的结构。如有任何帮助，我们将不胜感激。这是我到目前为止所拥有的。我认为问题出在比较函数中。但我不确定我是否真的理解 qsort 。任何人都知道一些很好的引用资料来阅读它。

Answer 1

使用现有结构，您仅比较名字的第一个字符。您需要使用 strcmp() 比较字符串，并且考虑到类(class)中可能有两个名为 John 或 Jane 的学生，您可能还想比较姓氏。

int compare(struct student *element1, struct student *element2)
{
    if ((rc = strcmp(element1->firstName, element2->firstName)) != 0)
        return rc;
    return strcmp(element1->lastName, element2->lastName);
}

总的来说，我更喜欢将比较函数编写为:

int compare(const void *v1, const void *v2)
{
    const struct student *p1 = (struct student *)v1;
    const struct student *p2 = (struct student *)v2;

    if ((rc = strcmp(p1->firstName, p2->firstName)) != 0)
        return rc;
    return strcmp(p1->lastName, p2->lastName);
}

然后，我不必将指针强制转换为 qsort()，因为指向函数的指针与 qsort() 期望的类型相同。实际上，没有太大区别。

另请参阅How to sort an array of structs in C?