尝试显示指针值和指针地址时代码未运行

Question

我正在学习 C 类(class)，刚刚开始学习指针。我刚刚去完成本部分的实验，但无法让我的代码正常运行。于是我打开了实验室的正确答案代码，然后我逐行浏览，并将实验室的答案代码与我自己的进行比较。我的一些标签是不同的，但不会导致任何错误。

我不明白为什么我的代码无法运行，但实验室的代码可以。

下面是我尝试运行代码时 xcode 中发生的情况的图片:

非常感谢任何帮助。

这是我的代码不起作用:

#include <stdio.h>

int main()
{


    int age = 40;
    float gpa = 3.25;
    char grade = 'A';
    double x = 0.000009;
    char companyName[20];

    printf("The address of age is: %d\n", &age);
    printf("The size of age is: %lu\n", sizeof(age));

    printf("The address of gpa is: %d\n", &gpa);
    printf("The size of gpa is: %lu\n", sizeof(gpa));

    printf("The address of grade is: %d\n", &grade);
    printf("The size of grade is: %lu\n", sizeof(grade));

    printf("The address of x is: %d\n", &x);
    printf("The size of x is: %lu\n", sizeof(x));

    printf("The address of companyName is: %d\n", &companyName);
    printf("The size of companyName is: %lu\n", sizeof(companyName));


    int *pointerIntAge;

    pointerIntAge = &age;

    float *pointerFloatGpa;

    pointerFloatGpa = &gpa;

    char *pointerCharGrade;
    pointerCharGrade = &grade;

    double *pointerDoubleX;

    pointerDoubleX = &x;

    char *pointerCharCompanyName;

    pointerCharCompanyName = &companyName;


    printf("The value of pointerIntAge is: %d\n", *pointerIntAge);
    printf("The value of pointerFloatGpa is: %f\n", *pointerFloatGpa);
    printf("The value of pointerCharGrade is: %c\n", *pointerCharGrade);
    printf("The value of pointerDoubleX is: %f\n", *pointerDoubleX);
    printf("The valie of pointerCharCompanyName is: %s\n", *pointerCharCompanyName); 


    printf("The address of pointerIntAge is: %p\n", pointerIntAge);
    printf("The address of pointerFloatGpa is: %p\n", pointerFloatGpa);
    printf("The address of pointerCharGrade is: %p\n", pointerCharGrade);
    printf("The address of pointer DoubleX is: %p\n", pointerDoubleX);
    printf("The address of pointerCharCompanyName is: %p\n", pointerCharCompanyName);

    *pointerIntAge+=5;
    printf("Just added 5 to pointer age\n");
    printf("The new value of age is: %d\n", age);

    printf("The value of age through pointer is: %d\n", *pointerIntAge);

    return 0;
}

这是实验室正在运行的代码:

#include <stdio.h>

int main ()
{
    int age = 40;
    float gpa = 3.25;
    char grade = 'A';
    double x = 0.000009;
    char companyName[20];

    printf("Address of age: %d\n", &age);
    printf("Size of age: %lu\n", sizeof(age));
    printf("Address of GPA: %d\n", &gpa);
    printf("Size of age: %lu\n", sizeof(gpa));
    printf("Address of grade: %d\n", &grade);
    printf("Size of grade: %lu\n", sizeof(grade));
    printf("Address of x: %d\n", &x);
    printf("Size of x: %lu\n", sizeof(x));
    printf("Address of companyName: %d\n", &companyName);
    printf("Size of companyName: %lu\n", sizeof(companyName));

    int *pAge;
    pAge = &age;
    float *pGpa;
    pGpa = &gpa;
    char *pGrade;
    pGrade = &grade;
    double *pX;
    pX = &x;
    char *pCompanyName;
    pCompanyName = &companyName;

    printf("\nValue of Age though pointer: %d", *pAge);
    printf("\nValue of GPA though pointer: %0.2f", *pGpa);
    printf("\nValue of Grade though pointer: %c", *pGrade);
    printf("\nValue of X though pointer: %f", *pX);
    printf("\nValue of Company Name though pointer: %s\n", *pCompanyName);

    printf("\nThe address from Age pointer: %p", pAge);
    printf("\nThe address from Gpa pointer: %p", pGpa);
    printf("\nThe address from Grade pointer: %p", pGrade);
    printf("\nThe address from X pointer: %p", pX);
    printf("\nThe address from companyName pointer: %p\n", pCompanyName);

    *pAge += 5; //Added 5 to Age through pointer.
    printf("\nValue of Age: %d", age);
    printf("\nValue of Age through pointer: %d", *pAge);

    return 0;
}

Answer 1

companyName 未初始化，因此 printf("The valie ofpointerCharCompanyName is: %s\n", *pointerCharCompanyName); 将打印垃圾信息(如果存在NULL 某处)或超出范围(如果不存在)。

这是因为 C 中的字符串(实际上只是字符数组)应该以 null 结尾。这意味着它们应该在末尾有一个 '\0' 字符 - 这是由对字符串进行操作的方法用来了解字符串结束的位置。

尝试 char companyName[20] = { 0 }; 或 char companyName[20] = { "some string"};

不过，还有一些其他问题需要修复。

1. 使用 %p 打印地址，而不是 %d。
2. 如果您收到编译器警告，您会注意到 pointerCharCompanyName = &companyName; 和 printf("The valie ofpointerCharCompanyName is: %s\n", *pointerCharCompanyName); code> 都存在由于对指针理解不足而导致的问题。
3. 如果假定有效的示例代码确实是示例代码，那么它也会出现这些问题，您应该找到一个新类。