指针基础 _0214

1.#验证指针存的是地址
#include <stdio.h>

int main(int argc, const char *argv[])
{
    int a,*p;
    p=&a;
    printf("%p\n",p);
    printf("%p\n",&a);
    return 0;
}
2.#没对指针赋值时，不能引用指针的值
#include <stdio.h>

int main(int argc, const char *argv[])
{
    int *p;
    printf("%d\n",*p);
    return 0;
}
3.#指针做参数
#include <stdio.h>
void swap(int *a,int *b)
{
    int tmp;
    tmp=*a;
    *a=*b;
    *b=tmp;
}

int main(int argc, const char *argv[])
{
    int a=3,b=4;
    swap(&a,&b);
    printf("a=%d    b=%d\n",a,b);
    return 0;
}
4.#返回值为指针型
#include <stdio.h>

int *max(int *a,int *b)
{
    return *a>*b? a:b;
}

int main(int argc, const char *argv[])
{
    int a=5,b=3;
    printf("max=%d\n",*max(&a,&b));
    return 0;
}
5.#永远不要返回指向局部变量的地址
#include <stdio.h>

int *func(void)
{
    int i=10;
    return &i;
}

int main(int argc, const char *argv[])
{
    int *p;
    p=func();
    printf("hello!\n");
    printf("%d\n",*p);
    return 0;
}
6.#用指针输出数组的值
#include <stdio.h>

int main(int argc, const char *argv[])
{
    int a[10]={1,2,3,4,5,6,7,8,9,10};
    int i,*p;
    p=a;
    for (i = 0; i < 10; i++)
    {
        printf("%4d",*(p+i));
    }
    printf("\n");
    return 0;
}
7.#指针的算术运算，数组名为常量，不能做加减运算
#include <stdio.h>

int main(int argc, const char *argv[])
{
    int a[10]={1,2,3,4,5,6,7,8,9,10};
    int *p=a;
    printf("%d\n",*p++);
    printf("%d\n",*++p);
    //printf("%d\n",*a++);
    int *q;
    p=&a[5];
    q=&a[1];
    printf("%d\n",p-q);
    printf("%d\n",&a[5]-&a[1]);
    printf("%d\n",(int)&a[5]-(int)&a[1]);
    return 0;
}
8.#*p++与(*p)++,一个是地址加，一个是数值加
  #++的优先级等于*的优先级,由左向右运算，先是左结合，再是右结合。
#include <stdio.h>

int main(int argc, const char *argv[])
{
    int a[10]={1,20,30};
    int *p=a;
    printf("%d\n",*p++);
    printf("%d\n",(*p)++);
    printf("%d\n",*p);
    return 0;
}
9.#用指针实现puts函数，"hello"指的是'h'的地址(只读的)，str[]="hello"可读可写
#include <stdio.h>

void myputs(char *s)
{
    while(*s)
        putchar(*s++);
    printf("\n");
}

int main(int argc, const char *argv[])
{
    char *s="hello world!";
    char str[20]="hello world!";
    myputs(s);
    return 0;
}
10.#所有的指针都为4个字节
#include <stdio.h>

int main(int argc, const char *argv[])
{
    char *arrc[3];
    int arri[3];
    printf("%4d%4d\n",sizeof(arrc),sizeof(arri));
    return 0;
}
11.#指针数组，数组的元素均为指针
   #int *arr[5];
#include <stdio.h>

int main(int argc, const char *argv[])
{
    int i;
    char *str[3]={"hello","world","akaedu"};
    puts(str[0]);
    puts(str[1]);
    puts(str[2]);
    for (i = 0; i < 3; i++)
    {
        printf("%s\n",str[i]);
    }

    return 0;
}
12.#输出该函数的各参数
#include <stdio.h>

int main(int argc, const char *argv[])
{
    int i;
    for (i = 0; i < argc; i++)
    {
        puts(argv[i]);
    }
    return 0;
}
13.#数组指针，指向数组的指针
   #int (*arr)[5];
#include <stdio.h>

int main(int argc, const char *argv[])
{
  int a[4][5]={{1,2,3,4,5},{6,7,8,9,10},{11,12,13,14,15},{16,17,18,19,20}};
  int (*p)[5];
  p=a;
  p[0]
    return 0;
}
14.#函数指针，指向函数名(函数入口地址)的指针
   #int (*p)(int,int);
#include <stdio.h>

void print(void)
{
    printf("hello!\n");
}

int add(int a,int b)
{
    return a+b;
}

int main(int argc, const char *argv[])
{

    void (*p)(void);
    int  (*q)(int,int);
    p=print;
    q=add;
    p();
    printf("%d\n",q(3,4));
    return 0;
}
15.#指针数组函数
#include <stdio.h>
#include <string.h>

int add(int a,int b)
{
    return a+b;
}

int sub(int a,int b)
{
    return a-b;
}

int mul(int a,int b)
{
    return a*b;
}

int div(int a,int b)
{
    if(b==0)
        return 0;
    return a/b;
}

int (*fun[])(int,int)={add,sub,mul,div};

int main(int argc, const char *argv[])
{
    int a,b,result,i;
    char ch;
    char *s="+-*/";
    scanf("%d%c%d",&a,&ch,&b);

    for (i = 0; i < strlen(s); i++)
    {
        if(ch==s[i])
            result=fun[i](a,b);
    }
    printf("%d %c %d = %d\n",a,ch,b,result);
    return 0;
}
16.#使用指针改写求一个数组中的最大值
   #使用指针改写将一数列反向
#include <stdio.h>

int *max(int *p,int n)
{
    int i,*max;
    max=p;
    for (i = 0; i < n; i++)
    {
        if(*max<*(p+i))
        {
            max=p+i;
        }
    }
    return max;

}
void reverse(int *a,int n)
{
    int *p=a;
    int *q=a+n-1;
    int tmp;
    while(p<q)
    {
        tmp=*p;
        *p=*q;
        *q=tmp;
    }
}

int main(int argc, const char *argv[])
{
    int i,a[10]={1,2,3,4,5,6,7,8,9,10};
    printf("*a[%d]=%d\n",max(a,10)-a,*max(a,10));
    printf("Before:");
    for (i = 0; i < 10; i++)
    {
        printf("%4d",a[i]);
    }
    printf("\nAfter :");
    reverse(a,10);
    for (i = 0; i < 10; i++)
    {
        printf("%4d",a[i]);
    }
    printf("\n");
    return 0;
}
17.#用指针实现strlen函数
#include <stdio.h>
int mystrlen(char *s)
{
    int n=0;
    while(*s)
    {
        s++;
        n++;
    }
    return n;
}

int main(int argc, const char *argv[])
{
    char str[20];
    printf("str=");
    scanf("%s",str);
    printf("strlen=%d\n",mystrlen(str));
    return 0;
}
18.编写一程序，输入月份号，输出该月的英文名字，例如：输入“3”，则输出“March”，要求用指针数组实现
#include <stdio.h>

char *p[12]={"Janury","February","March","April","May","June","July",
                 "August","Septemble","October","November","Decemble"};
int main(int argc, const char *argv[])
{
    int m;
    printf("month=");
    scanf("%d",&m);
    printf("%s\n",p+m-1);
    return 0;
}
19.#学习使用并实现atoi函数
#include <stdio.h>

int myatoi(char *s)
{
    int i,flag=0,num=0,cnt=0;
    char *p;
    p=s;
    while(*s)
    {
        cnt++;
        s++;
    }
    s=p;
    if(s[0]=='-')
        flag=1;
    for (i = 0+flag; i < cnt; i++)
    {
        num=num*10+(s[i]-'0');
    }
    if(flag==1)
        num*=-1;
    return num;

}

int main(int argc, const char *argv[])
{
    char str[10];
    printf("str=");
    scanf("%s",str);
    printf("toi=%d\n",myatoi(str));
    return 0;
}
20.#学习使用strcmp函数
#include <stdio.h>

int mystrcmp(char *s1,char *s2)
{
    while(*s1==*s2)
    {
        if(*s1='\0')
            return 0;
        s1++;
        s2++;
    }
    return *s1>*s2?1:-1;
}

int main(int argc, const char *argv[])
{
    char str1[20],str2[20];
    printf("str1=");
    scanf("%s",str1);
    printf("str2=");
    scanf("%s",str2);
    printf("%d\n",mystrcmp(str1,str2));
    return 0;
}
21.#学习并实现strcpy
#include <stdio.h>

char *mystrcpy(char *dd,char *ss)
{
    int i=0;
    while(*(ss+i))
    {
        *(dd+i)=*(ss+i);
        i++;
    }
    *(dd+i)='\0';
    return dd;
}

int main(int argc, const char *argv[])
{
    char d[100],s[32];
    printf("d=");
    scanf("%s",d);
    printf("s=");
    scanf("%s",s);
    printf("d=%s\n",mystrcpy(d,s));
    return 0;
}
22.#学习并实现strchr和strrchr
#include <stdio.h>

char *mystrchr(char *s,char c)
{
    while(*s)
    {
        if(c==*s)
            return s;
        s++;
    }
    return NULL;
}

char *mystrrchr(char *s,char c)
{
    char *bak=s;
    while(*s)
        s++;
    while(--s>=bak)
    {
        if(*s==c)
            return s;
    }
    return NULL;
}

int main(int argc, const char *argv[])
{
    char str[32]="hello world";
    char ch='o';
    printf("strchr=%s\n",mystrchr(str,ch));
    printf("strrchr=%s\n",mystrrchr(str,ch));
    return 0;
}