C语言（指针与..）test_3_7

C语言（指针与..）test_3_7

#define _CRT_SECURE_NO_WARNINGS 1


假设环境是x86环境，程序输出的结果是啥？
#include <stdio.h>

int main()
{
    int a[5][5];

    int (*p)[4];//p是一个数组指针，p指向的数组是4个整型元素的

    p = a;

    printf("%p,%d\n", &p[4][2] - &a[4][2], &p[4][2] - &a[4][2]);
    
    return 0;
}



指针-指针 绝对值 得到的是指针和指针之间的元素个数


#include <stdio.h>
int main()
{
    int aa[2][5] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

    int* ptr1 = (int*)(&aa + 1);

    int* ptr2 = (int*)(*(aa + 1));

    printf("%d,%d", *(ptr1 - 1), *(ptr2 - 1));

    return 0;
}




#include <stdio.h>

int main()
{
    char* a[] = { "work","at","alibaba" };
    char** pa = a;
    pa++;
    printf("%s\n", *pa);
    return 0;
}



#include <stdio.h>
int main()
{
    char* c[] = { "ENTER","NEW","POINT","FIRST" };
    char** cp[] = { c + 3,c + 2,c + 1,c };
    char*** cpp = cp;

    printf("%s\n", **++cpp);
    printf("%s\n", *-- * ++cpp + 3);
    printf("%s\n", *cpp[-2] + 3);
    printf("%s\n", cpp[-1][-1] + 1);
    return 0;
}

#include <ctype.h>

int main()
{
	int ret = islower('A');
	int ret = isspace('	');
	printf("%d\n", ret);

	return 0;
}


写一个代码，将字符串中的小写字母转大写，其他字符不变。

a - 97 A -65 =32
b - 98 B -66 =32
int main()
{
	char arr[] = "I Am a Student";
	            I Am a Student\0
	            0123 
	int i = 0;
	while (arr[i] != '\0')
	{
		if (arr[i] >= 'a' && arr[i] <= 'z')
		if(islower(arr[i]))
		{
			arr[i] = toupper(arr[i]);
		}
		i++;
	}
	printf("%s\n", arr);

	return 0;
}



int main()
{
	printf("%c\n", toupper('a'));
	printf("%c\n", toupper('B'));
	printf("%c\n", tolower('B'));
	return 0;
}


#include <string.h>

int main()
{
	    size_t        size_t
	       3        -         6
	if ((int)strlen("abc") - (int)strlen("abcdef")>0)
	{
		printf(">\n");
	}
	else
	{
		printf("<=\n");
	}

	return 0;
}


#include <assert.h>

计数器的版本
size_t my_strlen(const char* s)
{
	assert(s);
	int count = 0;//计数器
	while (*s != '\0')
	{
		count++;
		s++;
	}
	return count;
}

不创建临时变量，求字符串的长度 -- 递归
size_t my_strlen(const char* s)
{
	if (*s == '\0')
		return 0;
	else
		return 1 + my_strlen(s+1);
}

int main()
{
	char arr[] = "abcdef";
	size_t len = my_strlen(arr);
	printf("%zd\n", len);

	return 0;
}

#include <string.h>

void my_strcpy(char* dest, const char* src)
{
	assert(src != NULL);
	assert(dest != NULL);

	拷贝\0前面的内容
	while (*src != '\0')
	{
		*dest = *src;
		dest++;
		src++;
	}
	*dest = *src;//拷贝\0
}

void my_strcpy(char* dest, const char* src)
{
	assert(src != NULL);
	assert(dest != NULL);

	拷贝\0前面的内容
	while (*dest++ = *src++)
	{
		;
	}
}

char* my_strcpy(char* dest, const char* src)
{
	assert(src != NULL);
	assert(dest != NULL);
	char* ret = dest;
	拷贝\0前面的内容
	while (*dest++ = *src++)
	{
		;
	}
	return ret;//
}

int main()
{
	char arr1[] = "hello bit";
	char arr2[20] = "xxxxxxxxxxxxxx";
	
	char* ret = my_strcpy(arr2, arr1);

	printf("%s\n", arr2);
	printf("%s\n", ret);

	return 0;
}

#include <string.h>

strcat 函数用来连接字符串
函数返回的是目标空间的起始地址

char* my_strcat(char* dest, const char* src)
{
	assert(dest && src);
	char* ret = dest;
	1. 找打目标空间的\0
	while (*dest != '\0')
		dest++;
	2. 拷贝
	while (*dest++ = *src++)
		;//空语句
	return ret;
}

int main()
{
	char arr1[20] = "hello ";//hello world
	char arr2[] = "world";

	char*s = my_strcat(arr1, arr2);
	printf("%s\n", arr1);
	printf("%s\n", s);
	printf("%s\n", my_strcat(arr1, arr2));

	return 0;
}