C程序设计第二章

第二章代码抄写心得

优先级问题

1、赋值运算符和比较运算符的优先级问题

  int c ;
  c = getchar() != EOF
  (c = getchar()) != EOF

2、x *= y +1 问题。

#

#include 
void sizeof_type_test(void);
void strlen_test(void);

int my_strlen(char s[]);

int is_leap_year(unsigned int year);
void is_leap_year_test(void);

void relation_operate_test(void);

void my_atoi_test(void); 
int my_atoi(char s[]);

int my_lower(int c);
void my_lower_test(void);
void signed_unsigned_test(void);

int my_rand(void);
void my_srand(unsigned int seed);

void squeeze_test(void);
void squeeze(char s[],int c);

void my_strcat_test(void);
void my_strcat(char s[],char t[]);
void char_point_test(void);

void getbits_test(void); 
unsigned int getbits(unsigned int x,int p,int n);

int bitcount(unsigned x);
void bitcount_test(void);


void question_operate_test(void);
void question_operate(int a[],int n);
int main(void)
{
    //sizeof_type_test();
    //strlen_test();

    //
    //relation_operate_test();
    //my_atoi_test();
    //my_lower_test();
    //signed_unsigned_test();

    //squeeze_test();
    //char_point_test();

    //my_strcat_test();

    //getbits_test();
    //bitcount_test();

    question_operate_test();
    return 0;
}

void sizeof_type_test(void)
{
    printf("sizeof short = %d\n",sizeof(short));
    printf("sizeof int = %d\n",sizeof(int));
    printf("sizeof long long = %d\n",sizeof(long long));
    printf("sizeof long int = %d\n",sizeof(long int));
    printf("sizeof  double = %d\n",sizeof(double ));
}

void strlen_test(void)
{
    char *name = "China";
    printf("the name of length is %d",my_strlen(name)); 
}

int my_strlen(char s[])
{
    int i;

    while (s[i] != '\0')
        ++i;

    return i;
}

void is_leap_year_test(void)
{
    unsigned int year = 2016;
    int i;

    for (i = 0; i < 100; ++i)
    {
        is_leap_year(year + i); 
    }

}
int is_leap_year(unsigned int year)
{
    if ( year %4 == 0 && year % 100 !=0 || year % 400 == 0)
        printf("%d is a leap year \n",year);
    else
        printf("%d is not a leap year \n",year);
}

void relation_operate_test(void)
{
    int i =1;
    int j =2;

    if (i >= j)
        printf("i >= j\n");
    else
        printf("i < j\n");

    if (i <= j)
        printf("i <= j\n"); 
    else
        printf("i > j\n");  
}

void my_atoi_test(void)
{
    char *pdata = "124a45";

    printf("%d\n",my_atoi(pdata));
}
int my_atoi(char s[])
{
    int i,n;

    for ( i = 0; s[i] >= '0' && s[i] <= '9'; ++i)
        n = 10 * n + ( s[i] - '0');

    return n;
}
void my_lower_test(void)
{
    char c = 'A';
    int i;

    for (i = 0; i < 26; ++i)
        printf("%c ",my_lower('A' + i) );
}
int my_lower(int c)
{
    if ( c >= 'A' && c <= 'z' )
        return 'a' + c - 'A';
    else
        return c;
}

void signed_unsigned_test(void)
{
    unsigned char ui = 10;
    signed char si = -1;
//有符号和无符号之间的运算，我总是模不清楚
    unsigned int n = ui + si;

    printf("n  = %d\n", n);
}

unsigned long int next  = 1;

int my_rand(void)
{
    next = next *1102515245 + 12345;
    return (unsigned int)(next/65536) %32678;
}

void my_srand(unsigned int seed)
{
    next = seed;
}
//函数用来测试字符指针和字符数组的区别
//看字符指针是否可以修改
void char_point_test(void)
{
    char *pstr = "HAC";
    puts(pstr);
    //pstr[1] = 'a';
    //pstr[0] = pstr[1]; //这里不能修改，是指针指向的内容本身就是 不能修改的
    puts(pstr);

    while (*pstr)
    {
        putchar(*pstr);
        printf(" %p\n",pstr); //每个字节的地址，是从低到高的
        pstr++;

    }
}

void squeeze_test(void)
{
     char str[]= "bbbbbaaabbbcccbb";
    //char *pstr= "bbbbbaaabbbcccbb";//用字符串指针操作是会有段错误
    squeeze(str,'b');
    printf("%s\n",str);
}

void squeeze(char s[],int c)
{
    int i, j;

    for (i = j = 0; s[i] != '\0'; i++)
    {
        if (s[i] != c)
        {
            s[j++] = s[i];
        }

    }
    //字符串结束符，一定要有，并且必须放到for循环外面
    s[j] = '\0';
}
void my_strcat_test(void)
{
    char s[1024] ="HAC";
    puts(s);
    char t[] =" is my company";
    my_strcat(s,t);
    puts(s);

    char d[] = "and I love this  company"; 
    my_strcat(s,d);
    puts(s);
}
void my_strcat(char s[],char t[])
{
    int i , j;

    i = j = 0;

    while (s[i] != '\0')
        i++;

    while ( (s[i++] = t[j++]) != '\0')//这里抄写犯的错误是 两个数组写成一样的。
        ;
}
void getbits_test(void)
{
    //‭0001 0001 0010 0011‬
    unsigned int n = 0x1123;

    printf("%d\n", getbits(n,5,3));
}
unsigned int getbits(unsigned int x,int p,int n)
{
    //0x89
    return ( x >> (p + 1 - n) &~(~ 0<< n));
}

void bit_operate_test(void)
{

    // & | << >> ~  ^ C语言提供了6个位操作符号
    int i = 0x020;
    int tmp;
    tmp = i << 1;
    printf("%d\n",tmp);


    tmp = i << 1;
    printf("%d\n",tmp);

    tmp = i >> 1;
    printf("%d\n",tmp);

    tmp = i & 0x01;
    printf("%d\n",tmp);

        tmp = i | 0x01;
    printf("%d\n",tmp);
}

void bitcount_test(void)
{
    //unsigned char ui = 0x2f;
    //printf("%d\n",bitcount(ui));
    signed char si = -2;
    printf("%d\n",bitcount(si));
}

//此函数是为了验证 x *= y +1 等价于 x = x * (y + 1)
int bitcount(unsigned x)
{

    int b;

    for (b = 0; x != 0; x >>= 1)
        if (x & 01)
            b++;

    return b;
}
//测试的时候，为什么会11，12另外输出？
void question_operate_test(void)
{
    int a[] = {1,2,3,4,5,6,7,8,9,10,11,12,14,15};
#define N  sizeof(a) /sizeof(a[0])
    question_operate(a,N);
}

void question_operate(int a[],int n)
{
    int i;

    for (i = 0; i < n; ++i)
        printf("%d%c",a[i],(i%10 == 9 || i == '\n') ? '\n':' ');
}
//1、说明 C语言的运算符非常的丰富，这个我们可以从以上的代码中可以看出来
//2、记住C语言代码里面的每一个运算符的具体含义，是比较难的，唯一理解这些好的用法，就是去实践。
void operat_test(void)
{
    int  i = +-10;
    printf("i = %d",i);

    //if ( (x & MASK) == 0)
        //dosomthing...

    //这个例子就是为了说明，x = f() + g()计算顺序对于结果的影响。
    //printf("%d %d\n",++n,power(2,n));
}