Question

我正在尝试用C语言编写成绩簿。但是，由于我在处理指针方面缺乏经验，因此在将值打印到控制台时会出现奇怪的值。我的代码如下：

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

int main(int agrc,char * argv[]) {
// Create null pointers
char * students = 0;
char * grades = 0;
int * namelen = 0;

// Variable to track class size
int classSize   = 0;


printf("Please enter the class size: ");
scanf("%d",&classSize);
printf("Class size = %d\n",classSize);

// Allocate the memory for the null pointers
students = malloc(classSize * sizeof(char)+classSize);
grades   = malloc(classSize * sizeof(int));
namelen   = malloc(classSize * sizeof(int));

int i = 0;
char * tmp;
int pos = 0;

for(;i<classSize;i++) {
    printf("Please enter student %d's name: ",i+1);

    // Read name into dummy variable
    scanf("%s",tmp);

    // Store the length of the name
    *(namelen + i) = strlen(tmp);

    // Read in the name of the student
    strcpy(students+pos,tmp);

    printf("Please enter %s's grade: ",students + pos);

    // Read in the grade of the student
    scanf("%d",grades+i);

    pos += *(namelen+i)+1;
}

printf("\n");
printf("The data that you entered is as follows:\n");
printf("----------------------------------------\n");

i = 0;
pos = 0;

for(;i<classSize;i++) {
    printf("Student %d: %s. Grade: %d\n",i+1,students+pos,*(grades+i));
    pos += *(namelen+i)+1;
}
}

问题是，有时学生的姓名和成绩不正确。我知道这与指针有关。但我似乎无法发现任何错误。有人可以帮帮我吗？感谢。

Answer 1

students = malloc(classSize * sizeof(char)+classSize); // allocates one char for each student name.

这应该替换为每个名称的预期大小。

students = malloc(classSize * sizeof(char) * 20); // assuming size of each to max of 20 bytes including null termination.

char * tmp;

这应该是

char tmp[20]; // assumed max size of each name is 20 bytes including null.

Answer 2

更新了您的代码并发布在下方。这对我来说很有用。

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

int main(int agrc,char * argv[]) {
// Create null pointers
char * students = 0;
int * grades = 0;
int * namelen = 0;

// Variable to track class size
int classSize   = 0;


printf("Please enter the class size: ");
scanf("%d",&classSize);
printf("Class size = %d\n",classSize);

// Allocate the memory for the null pointers
students = malloc(classSize * 20);
grades   = malloc(classSize * sizeof(int));
namelen   = malloc(classSize * sizeof(int));

int i = 0;
char tmp[20];
int pos = 0;

for(;i<classSize;i++) {
    printf("Please enter student %d's name: ",i+1);

    // Read name into dummy variable
    scanf("%s",tmp);

    // Store the length of the name
    *(namelen + i) = strlen(tmp);

    // Read in the name of the student
    strcpy(students+pos,tmp);

    printf("Please enter %s's grade: ",students + pos);

    // Read in the grade of the student
    scanf("%d",grades+i);

    pos += *(namelen+i)+1;
}

printf("\n");
printf("The data that you entered is as follows:\n");
printf("----------------------------------------\n");

i = 0;
pos = 0;

for(;i<classSize;i++) {
    printf("Student %d: %s. Grade: %d\n",i+1,students+pos,*(grades+i));
    pos += *(namelen+i)+1;
}
}

C成绩册中的错误

2 个答案: