用指针交换c中的字符串函数

Question

当我尝试在函数Update_student 中的字符串之间进行交换时，它没有成功。为什么？

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

#define SIZE 1

struct student {
    int id_number;
    char name[50];
    char sex[6];
    int quiz_score[2];
    int total_score;
};

void Add_Student_Records(struct student *pupil) {
    printf("ID:");
    scanf("%d", &pupil->id_number);
    printf("Name: ");
    scanf("%s", &pupil->name);
    printf("Sex :");
    scanf("%s", &pupil->sex);
    for (int i = 0; i < 2; i++) {
        printf("Quit score %d:", i + 1);
        scanf("%d", &pupil->quiz_score[i]);
    }
    pupil->total_score = pupil->quiz_score[0] + pupil->quiz_score[1];
    return; 
}

void Add_Students(struct student *students) {
   for (int i = 0; i < SIZE; i++) {
        printf("Student %d:\n", i + 1);
        Add_Student_Records(students);
    }
    return;
}

void Print_Students(struct student *students) {
    for (int i = 0; i < SIZE; i++) {
        printf("Student %d details: \n", i + 1);
        printf("ID:%d\n", students->id_number);
        printf("Name:%s\n", students->name);
        printf("Sex:%s\n", students->sex);
        for (int i = 0; i < 2; i++) {
            printf("Quit score %d:\n", students->quiz_score[i]);
        }
        printf("Total score: %d\n", students->total_score);
        students++;
    }
    return;
}

void Replace_Strings(char **old_string, char **new_string) {
        *old_string = *new_string;
        return;
}

void Update_Student(struct student *students) {
    int i = 0;
    char name[50], new_name[50], cur_name[50];
    printf("You can update name, and scores.\n");
    printf(" current Name: ");
    scanf("%s", name);
    printf("new name: ");
    scanf("%s", &new_name);
    while (i < SIZE) {
        strcpy(cur_name, students->name);
        if (strcmp(cur_name, name) == 0) {   
            char *ptr_old_stud_name = students->name;
            char *ptr_new_stud_name = new_name;
            Replace_Strings(&ptr_old_stud_name, &ptr_new_stud_name);
        }
        i++;
        students++;
    }
    return;
}

int main() {
    struct student students[SIZE];
    char ch;
    /*1.Add student, 2. Print student*/
    printf("1.Add student\n2.Print students\n3.Update student\n");
    scanf("%c", &ch);
    while (ch != 'E') {
        if (ch == '1') {
            Add_Students(&students[0]);
        }           
        else if (ch == '2') {
            Print_Students(&students[0]);
        }
        else if (ch =='3') {
            Update_Student(&students[0]);
        }
        printf("Another operation:\t");
        scanf("%c", &ch);
    }
}

Answer 1

Replace_Strings(&ptr_old_stud_name,&ptr_new_stud_name); 将ptr_old_stud_name 和ptr_new_stud_name 的地址传递给ReplaceStrings。

ptr_old_stud_name 和 ptr_new_stud_name 是局部变量。第一个是已设置为指向students->name 的指针。第二个是已经设置为指向new_name的指针。

Replace_Strings 将传递给它的指针的第一件事更改为传递给它的指针的第二件事。因此它将ptr_old_stud_name 更改为具有ptr_new_stud_name 的值。

结果是局部变量ptr_old_stud_name有了新值。这不会改变它指向的东西，students->name。

更具体地说，ptr_old_stud_name 指向students->name 的第一个字符。 students->name 是一个数组，不能通过改变指向它的指针来改变它，也不能改变它的地址。要更改其内容，您必须将新值复制到其中的字节中，您可以使用strcpy 将字节从new_name 复制到其中。

Answer 2

您的函数Update_student 令人困惑，您应该只遍历学生数组并将学生的姓名与cur_name 进行比较，并在匹配时替换姓名。

您还应该将要处理的学生人数作为参数传递。

这是修改后的版本：

void Update_Student(struct student *students, int count) {
    char cur_name[50], new_name[50];
    printf("You can update name, and scores.\n");
    printf(" current Name: ");
    scanf("%49s", cur_name);
    printf("new name: ");
    scanf("%49s", new_name);
    for (int i = 0; i < count; i++) {
        if (strcmp(cur_name, students[i].name) == 0) {
            strcpy(students[i].name, new_name);
        }
    }
}

以Update_Student(students, SIZE); 的身份从main 拨打电话

另请注意，在阅读命令时应忽略空格，方法是在 %c 之前添加一个空格：

    scanf(" %c", &ch);

这是一个针对多个学生的修改版本：

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

#define SIZE 10

struct student {
    int id_number;
    char name[50];
    char sex[6];
    int quiz_score[2];
    int total_score;
};

int Add_Student_Records(struct student *pupil) {
    printf("ID:");
    if (scanf("%d", &pupil->id_number) != 1)
        return 0;
    printf("Name: ");
    if (scanf("%49s", pupil->name) != 1)
        return 0;
    printf("Sex :");
    if (scanf("%1s", pupil->sex) != 1)
        return 0;
    for (int i = 0; i < 2; i++) {
        printf("Quiz score %d:", i + 1);
        if (scanf("%d", &pupil->quiz_score[i]) != 1)
            return 0;
    }
    pupil->total_score = pupil->quiz_score[0] + pupil->quiz_score[1];
    return 1;
}

int Add_Students(struct student *students, int count) {
    int i;
    for (int i = 0; i < count; i++) {
        printf("Student %d:\n", i + 1);
        if (Add_Student_Records(students + i) == 0)
            break;
    }
    return i;
}

void Print_Students(struct student *students, int count) {
    for (int i = 0; i < count; i++) {
        printf("Student %d details: \n", i + 1);
        printf("ID:%d\n", students->id_number);
        printf("Name:%s\n", students->name);
        printf("Sex:%s\n", students->sex);
        for (int i = 0; i < 2; i++) {
            printf("Quit score %d:\n", students->quiz_score[i]);
        }
        printf("Total score: %d\n", students->total_score);
        students++;
    }
}

void Update_Student(struct student *students, int count) {
    char cur_name[50], new_name[50];
    printf("You can update name, and scores.\n");
    printf(" current Name: ");
    if (scanf("%49s", cur_name) != 1)
        return;
    printf("new name: ");
    if (scanf("%49s", new_name) != 1)
        return;
    for (int i = 0; i < count; i++) {
        if (strcmp(cur_name, students[i].name) == 0) {
            strcpy(students[i].name, new_name);
        }
    }
}

int main() {
    struct student students[SIZE];
    int n = 0;
    char ch = 'E';
    /* print the menu */
    printf("1. Add student\n"
           "2. Print students\n"
           "3. Update student\n");
    scanf(" %c", &ch);
    while (ch != 'E') {
        if (ch == '1') {
            if (n == SIZE) {
                printf("student array is full\n");
            } else {
                /* add more students */
                n += Add_Students(&students[n], SIZE - n);
            }
        } else
        if (ch == '2') {
            Print_Students(students, n);
        } else
        if (ch =='3') {
            Update_Student(students, n);
        }
        scanf("%*[^\n]");   // consume the rest of the pending input line
        printf("Another operation:\t");
        if (scanf(" %c", &ch) != 1)
            break;
    }
    return 0;
}