练习 15：指针，可怕的指针

#include <stdio.h>

int main(int argc, char *argv[])
{
    // create two arrays we care about
    int ages[] = {23, 43, 12, 89, 2};
    char *names[] = {
        "Alan", "Frank",
        "Mary", "John", "Lisa"
    };

    // safely get the size of ages
    int count = sizeof(ages) / sizeof(int);
    int i = 0;

    // first way using indexing
    for(i = 0; i < count; i++) {
        printf("%s has %d years alive.\n",
                names[i], ages[i]);
    }

    printf("---\n");

    // setup the pointers to the start of the arrays
    int *cur_age = ages;
    char **cur_name = names;

    // second way using pointers
    for(i = 0; i < count; i++) {
        printf("%s is %d years old.\n",
                *(cur_name+i), *(cur_age+i));
    }

    printf("---\n");

    // third way, pointers are just arrays
    for(i = 0; i < count; i++) {
        printf("%s is %d years old again.\n",
                cur_name[i], cur_age[i]);
    }

    printf("---\n");

    // fourth way with pointers in a stupid complex way
    for(cur_name = names, cur_age = ages;
            (cur_age - ages) < count;
            cur_name++, cur_age++)
    {
        printf("%s lived %d years so far.\n",
                *cur_name, *cur_age);
    }

    return 0;
}

assert 宏用于在程序中添加断言。它检查表达式 expression 是否为真。如果表达式为假，程序将打印错误信息并终止执行。

试着将cur_age指向names。可以需要C风格转换来强制执行，试着查阅相关资料把它弄明白。

#include <stdio.h>

int main(int argc, char *argv[]) {
    // create two arrays we care about
    int ages[] = {23, 43, 12, 89, 2};
    char *names[] = {
        "Alan", "Frank",
        "Mary", "John", "Lisa"
    };

    // safely get the size of ages
    int count = sizeof(ages) / sizeof(int);

    // setup the pointers to the start of the arrays
    int *cur_age = ages;
    char **cur_name = names;

    // forcing cur_age to point to names (note: this is just for demonstration)
    cur_age = (int *)names;

    // output using the modified cur_age pointer
    for(int i = 0; i < count; i++) {
        printf("%s has %d years alive.\n", *(cur_name + i), *(cur_age + i));
    }

    return 0;
}

⮞ ./ex15_0
Alan has -1933959164 years alive.
Frank has 22046 years alive.
Mary has -1933959159 years alive.
John has 22046 years alive.
Lisa has -1933959153 years alive.

附加题 1

使用访问指针的方式重写所有使用数组的地方。

#include <stdio.h>

int main(int argc, char *argv[]) {
    // create two arrays we care about
    int ages[] = {23, 43, 12, 89, 2};
    char *names[] = {
        "Alan", "Frank",
        "Mary", "John", "Lisa"
    };

    // safely get the size of ages
    int count = sizeof(ages) / sizeof(int);
    
    // setup the pointers to the start of the arrays
    int *cur_age = ages;
    char **cur_name = names;

    // first way using pointers
    for(int i = 0; i < count; i++) {
        printf("%s has %d years alive.\n", *(cur_name + i), *(cur_age + i));
    }

    printf("---\n");

    // second way using pointers
    for(int i = 0; i < count; i++) {
        printf("%s is %d years old.\n", *(cur_name + i), *(cur_age + i));
    }

    printf("---\n");

    // third way using pointers
    for(int i = 0; i < count; i++) {
        printf("%s is %d years old again.\n", *(cur_name + i), *(cur_age + i));
    }

    printf("---\n");

    // fourth way with pointers in a slightly complex way
    for(; cur_age < ages + count; cur_name++, cur_age++) {
        printf("%s lived %d years so far.\n", *cur_name, *cur_age);
    }

    return 0;
}

附加题 2

使用访问数组的方式重写所有使用指针的地方。

#include <stdio.h>

int main(int argc, char *argv[]) {
    // create two arrays we care about
    int ages[] = {23, 43, 12, 89, 2};
    char *names[] = {
        "Alan", "Frank",
        "Mary", "John", "Lisa"
    };

    // safely get the size of ages
    int count = sizeof(ages) / sizeof(int);
    int i = 0;

    // first way using indexing
    for(i = 0; i < count; i++) {
        printf("%s has %d years alive.\n",
                names[i], ages[i]);
    }

    printf("---\n");

    // second way using pointers (converted to array indexing)
    for(i = 0; i < count; i++) {
        printf("%s is %d years old.\n",
                names[i], ages[i]);
    }

    printf("---\n");

    // third way, pointers are just arrays (already using array indexing)
    for(i = 0; i < count; i++) {
        printf("%s is %d years old again.\n",
                names[i], ages[i]);
    }

    printf("---\n");

    // fourth way with pointers in a stupid complex way (converted to array indexing)
    for(i = 0; i < count; i++) {
        printf("%s lived %d years so far.\n",
                names[i], ages[i]);
    }

    return 0;
}

附加题 3

在其它程序中使用指针来代替数组访问。

附加题 4

使用指针来处理命令行参数，就像处理names那样。

附加题 5

将获取值和获取地址组合到一起。

#include <stdio.h>

int main(int argc, char *argv[])
{
    int i = 0;

    // Output addresses and values of argv
    for(i = 0; i < argc; i++) {
        printf("argv[%d]: %p: %s\n", i, (void *)&argv[i], argv[i]);
    }

    printf("---\n");

    // let's make our own array of strings
    char *states[] = {
        "California", "Oregon",
        "Washington", "Texas"
    };
    int num_states = 4;

    // Output addresses and values of states
    for(i = 0; i < num_states; i++) {
        printf("states[%d]: %p: %s\n", i, (void *)&states[i], states[i]);
    }

    return 0;
}

⮞ ./ex15_5 aaaa bbbb cccc
argv[0]: 0x7ffddf1cce58: ./ex15_5
argv[1]: 0x7ffddf1cce60: aaaa
argv[2]: 0x7ffddf1cce68: bbbb
argv[3]: 0x7ffddf1cce70: cccc
---
states[0]: 0x7ffddf1ccd10: California
states[1]: 0x7ffddf1ccd18: Oregon
states[2]: 0x7ffddf1ccd20: Washington
states[3]: 0x7ffddf1ccd28: Texas

附加题 6

在程序末尾添加一个for循环，打印出这些指针所指向的地址。你需要在printf中使用%p。

    // print the addresses of the pointers
    for(i = 0; i < count; i++) {
        printf("Address of names[%d]: %p, Address of ages[%d]: %p\n", 
                i, (void*)&names[i], i, (void*)&ages[i]);
    }

Address of names[0]: 0x7fff7758f590, Address of ages[0]: 0x7fff7758f570
Address of names[1]: 0x7fff7758f598, Address of ages[1]: 0x7fff7758f574
Address of names[2]: 0x7fff7758f5a0, Address of ages[2]: 0x7fff7758f578
Address of names[3]: 0x7fff7758f5a8, Address of ages[3]: 0x7fff7758f57c
Address of names[4]: 0x7fff7758f5b0, Address of ages[4]: 0x7fff7758f580

附加题 7

对于每一种打印数组的方法，使用函数来重写程序。试着向函数传递指针来处理数据。记住你可以声明接受指针的函数，但是可以像数组那样用它。

#include <stdio.h>

// Function declarations
void print_by_indexing(char **names, int *ages, int count);
void print_by_pointer_arithmetic(char **names, int *ages, int count);
void print_by_pointer_as_array(char **names, int *ages, int count);
void print_by_complex_pointer_arithmetic(char **names, int *ages, int count);
void print_addresses(char **names, int *ages, int count);

int main(int argc, char *argv[]) {
    // create two arrays we care about
    int ages[] = {23, 43, 12, 89, 2};
    char *names[] = {
        "Alan", "Frank",
        "Mary", "John", "Lisa"
    };

    // safely get the size of ages
    int count = sizeof(ages) / sizeof(int);

    // Using different methods to print arrays
    print_by_indexing(names, ages, count);
    printf("---\n");
    print_by_pointer_arithmetic(names, ages, count);
    printf("---\n");
    print_by_pointer_as_array(names, ages, count);
    printf("---\n");
    print_by_complex_pointer_arithmetic(names, ages, count);
    printf("---\n");

    // Print the addresses of the pointers
    print_addresses(names, ages, count);

    return 0;
}

// Function definitions
void print_by_indexing(char **names, int *ages, int count) {
    for(int i = 0; i < count; i++) {
        printf("%s has %d years alive.\n", names[i], ages[i]);
    }
}

void print_by_pointer_arithmetic(char **names, int *ages, int count) {
    for(int i = 0; i < count; i++) {
        printf("%s is %d years old.\n", *(names + i), *(ages + i));
    }
}

void print_by_pointer_as_array(char **names, int *ages, int count) {
    for(int i = 0; i < count; i++) {
        printf("%s is %d years old again.\n", names[i], ages[i]);
    }
}

void print_by_complex_pointer_arithmetic(char **names, int *ages, int count) {
    char **cur_name = names;
    int *cur_age = ages;

    for(int i = 0; i < count; i++, cur_name++, cur_age++) {
        printf("%s lived %d years so far.\n", *cur_name, *cur_age);
    }
}

void print_addresses(char **names, int *ages, int count) {
    for(int i = 0; i < count; i++) {
        printf("Address of names[%d]: %p, Address of ages[%d]: %p\n", 
                i, (void*)&names[i], i, (void*)&ages[i]);
    }
}

附加题 8

将for循环改为while循环，并且观察对于每种指针用法哪种循环更方便。

#include <stdio.h>

int main(int argc, char *argv[]) {
    // create two arrays we care about
    int ages[] = {23, 43, 12, 89, 2};
    char *names[] = {
        "Alan", "Frank",
        "Mary", "John", "Lisa"
    };

    // safely get the size of ages
    int count = sizeof(ages) / sizeof(int);
    int i = 0;

    // first way using indexing
    i = 0;
    while(i < count) {
        printf("%s has %d years alive.\n",
                names[i], ages[i]);
        i++;
    }

    printf("---\n");

    // setup the pointers to the start of the arrays
    int *cur_age = ages;
    char **cur_name = names;

    // second way using pointers
    i = 0;
    while(i < count) {
        printf("%s is %d years old.\n",
                *(cur_name+i), *(cur_age+i));
        i++;
    }

    printf("---\n");

    // third way, pointers are just arrays
    i = 0;
    while(i < count) {
        printf("%s is %d years old again.\n",
                cur_name[i], cur_age[i]);
        i++;
    }

    printf("---\n");

    // fourth way with pointers in a stupid complex way
    cur_name = names;
    cur_age = ages;
    i = 0;
    while((cur_age - ages) < count) {
        printf("%s lived %d years so far.\n",
                *cur_name, *cur_age);
        cur_name++;
        cur_age++;
        i++;
    }

    return 0;
}

看起来还是 for 更加简单呢。