指针与数组

 一个变量的地址称作该变量的“指针”

 指针就是通过地址体现的

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#include<stdio.h>

int main(){
	int a,b;
	int * p1,* p2;
	a = 10;
	b = 20;
	p1 = &a;//变量a的地址赋值给指针p1
	p2 = &b;
        //输出指针变量所指向的整型变量的值
        //p1代表指针变量本身，*p1代表指针所指向内存的内容
	printf("*p1 = %d,*p2 = %d",*p1,*p2);
	return 0;
}

交换两个指针的地址

#include<stdio.h>

int main(){
	int a=5,b=9;
	int * p1,* p2, * temp;
	p1 = &a;
	p2 = &b;
	if(a<b){
                //交换的指针所指向的地址
		temp = p1;
		p1 = p2;
		p2 = temp;	
	}
	printf("a = %d,b = %d\n",a,b);
	printf("*p1 = %d,*p2 = %d\n",*p1,*p2);
}
/*
原值a,b并没有发生改变
output:
a = 5,b = 9
*p1 = 9,*p2 = 5
*/

 交换两个指针所指向变量的值

#include<stdio.h>

int main(){
	int a=5,b=9,temp;
	int * p1,* p2;
	p1 = &a;
	p2 = &b;
	if(a<b){
                //交换指针变量
		temp = *p1;
		*p1 = *p2;
		*p2 = temp;	
	}
	printf("a = %d,b = %d\n",a,b);
	printf("*p1 = %d,*p2 = %d\n",*p1,*p2);
}
/*
原值发生改变
output:
a = 9,b = 5
*p1 = 9,*p2 = 5
*/

 野指针

int *p;//避免没有权限的地址操作，指针需要先有地址
*p=1;

 指针与地址

int main(void){	
	int x=1,y=2,arr[10];
	int *pp;//指向int类型的指针
	
	pp=&x;//pp指向变量x的地址
	printf("x = %d,y = %d,pp = %d\n",x,y,*pp);

	*pp = *pp + 1;
	printf("x = %d,y = %d,pp = %d\n",x,y,*pp);
	y = *pp + 1;
	++*pp;//*pp++错误写法，运算符遵循从右往左顺序
	(*pp)++;
	printf("x = %d,y = %d,pp = %d\n",x,y,*pp);
	return 0;
}
/* output:
x = 1,y = 2,pp = 1
x = 2,y = 2,pp = 2
x = 4,y = 3,pp = 4
*/
int main25(void){
	
	int x=1,y=2,arr[10];
	int *pp;//指向int类型的指针
	
	pp=&x;//pp指向变量x的地址
	*pp = 10;//x的值为10
	printf("x = %d,y = %d\n",x,y);
	y = *pp;//指针pp所指向变量的值赋给y
	printf("x = %d,y = %d\n",x,y);
	*pp = 100;
	printf("x = %d,y = %d\n",x,y);
	arr[0] = 1;
	pp = &arr[0];
	printf("x = %d,y = %d,pp = %d\n",x,y,*pp);
	return 0;
}
/*output:
x = 10,y = 2
x = 10,y = 10
x = 100,y = 10
x = 100,y = 10,pp = 1
*/

 指针与数组

int main(void){
	int arr[10] = {0,1,2,3,4,5,6,7,8,9};
	int *pa = arr;
	int i;
	for(i = 0;i<sizeof(arr)/sizeof(int);i++){
		//pa+i=arr[i] their address is equals
		printf("*pa = %d\n",*(pa+i));
	}
	for(i = 0;i<sizeof(arr)/sizeof(int);i++){
		//arrys is pointer
		printf("arr[%d] = %d\n",i,*(arr+i));
	}
	for(i = 0;i<sizeof(arr)/sizeof(int);i++){
		//bath method,their address is equals
		printf("%p,%p\n",&arr[i],arr+i);//arr不是变量，不能arr++/a=pp操作
	}
	return 0;
}
/*output:
*pa = 0
*pa = 1
*pa = 2
*pa = 3
*pa = 4
*pa = 5
*pa = 6
*pa = 7
*pa = 8
*pa = 9
arr[0] = 0
arr[1] = 1
arr[2] = 2
arr[3] = 3
arr[4] = 4
arr[5] = 5
arr[6] = 6
arr[7] = 7
arr[8] = 8
arr[9] = 9
0x7ffcf91e2b00,0x7ffcf91e2b00
0x7ffcf91e2b04,0x7ffcf91e2b04
0x7ffcf91e2b08,0x7ffcf91e2b08
0x7ffcf91e2b0c,0x7ffcf91e2b0c
0x7ffcf91e2b10,0x7ffcf91e2b10
0x7ffcf91e2b14,0x7ffcf91e2b14
0x7ffcf91e2b18,0x7ffcf91e2b18
0x7ffcf91e2b1c,0x7ffcf91e2b1c
0x7ffcf91e2b20,0x7ffcf91e2b20
0x7ffcf91e2b24,0x7ffcf91e2b24
*/

使用指针计算字符串长度

int mstrlen(char *pc);
int main(void){
	printf("%d\n",mstrlen("hello"));
	return 0;
}
int mstrlen(char *pc){
	int len;
	for(len=0;*pc!='\0';pc++){
		len++;
	}
	return len;
}