[C++知识库]C++第8天

1.求非空字符串元素个数

#include <stdio.h>

int str_no_space(char* str)
{
	int count = 0;
	char* p = str;

	while (*p)
	{
		if (*p != ' ')
		{
			count++;
		}
		p++;
	}

	return count;
}

int main(void)
{
	char* str = "ni chou sha";
	int ret = str_no_space(str);

	printf("%d\n", ret);
	return 0;
}

2.字符串逆置

#include <stdio.h>

// hello
void str_inverse(char* str)
{
	char* start = str;
	char* end = str + strlen(str) - 1;

	while (start < end)
	{
		char temp = *start;
		*start = *end;
		*end = temp;
		start++;
		end--;
	}
}

int main(void)
{
	char str[] = "this is a test";
	str_inverse(str);

	printf("str = %s\n", str);

	return 0;
}

3.判断字符串是回文

int abcba(char* str)
{
	char* start = str;
	char* end = str + strlen(str) - 1;

	while (start < end)
	{
		if (*start != *end)
		{
			return 0;//不是回文
		}
		start++;
		end--;
	}
	return 1;//是回文
}

4.字符串处理函数

a.字符串拷贝 strcpy 和 strncpy

#include <stdio.h>

int main0301(void)
{
	char src[] = "ni chou sha";
	char dest[20] = { 0 };

	char* p = strcpy(dest, src);

	printf("p=%s\n", p);
	printf("dest=%s\n", dest);

	return 0;
}

int main(void)
{
	char src[] = "hello world";
	char dest[100] = { 0 };

	char* p = strncpy(dest, src,100);
	for (size_t i = 0; i < 10; i++)
	{
		printf("%c\n", p[i]);
	}

	printf("p=%s\n", p);
	printf("dest=%s\n", dest);

	return 0;
}

b.字符串拼接 strcat 和 strncat

#include <stdio.h>

int main(void)
{
	char src[] = "world";
	char dest[100] = "hello";

	char* p = strncat(dest, src, 3);

	printf("p=%s\n", p);//p=hellowor
	printf("dest=%s\n", dest);//dest=hellowor

	return 0;
}

c.字符串比较 strcmp 和 strncmp

#include <stdio.h>

int main0501(void)
{
	char* str1 = "helloworld";
	char* str2 = "helloz";

	printf("ret = %d\n", strcmp(str1, str2));

	return 0;
}

int main(void)
{
	char* str1 = "helloworld";
	char* str2 = "helloz";

	printf("ret = %d\n", strncmp(str1, str2, 8));

	return 0;
}

d.字符串格式化输入，输出 scanf 和 sprintf

#define _CRT_SECURE_NO_WARNINGS

#include <stdio.h>

//sprintf
int main(void)
{
	char buf[100] = { 0 };
	
	sprintf(buf, "%d%c%d=%d\n", 10, '+', 34, 10 + 34);
	puts(buf);//10+34=44

	return 0;
}

//sscanf
int main(void)
{
	int a, b, c;
	char str[] = "13+56=69";

	sscanf(str, "%d+%d=%d\n", &a, &b, &c);

	printf("a = %d\n", a);
	printf("b = %d\n", b);
	printf("c = %d\n", c);

	return 0;
}

e.字符串查找字符，字串 strchr strrchr strstr

f.字符串分割 strtok

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>

int main(void)
{
	char str[] = "www.itcast.cn.com.net";
	char* p = strtok(str, ".");//第1次拆分，传待拆分的原串

	//printf("%s\n", p);//www
	while (p != NULL)
	{
		p = strtok(NULL, ".");//第1+次拆分，传NULL
		printf("%s\n", p);//itcast cn com net (null)
	}

	return 0;
}

int main(void)
{
	char str[] = "www.itcast.cn$This is a strtok$test";
	char* p = strtok(str, "$ .");//第1次拆分，传待拆分的原串

	//printf("%s\n", p);//www
	while (p != NULL)
	{
		p = strtok(NULL, " .$");//第1+次拆分，传NULL
		printf("%s\n", p);//itcast cn com net (null)
	}

	return 0;
}

g.atoi/atof/atol

#include <stdio.h>
#include <string.h>

int main(void)
{
	char str1[] = "10";
	int num1 = atoi(str1);
	printf("num1 = %d\n", num1);//num1 = 10

	char str2[] = "0.123";
	double num2 = atof(str2);
	printf("num2 = %lf\n", num2);//num2 = 0.000000

	char str3[] = "123L";
	long num3 = atol(str3);
	printf("num3 = %ld\n", num3);//num3 = 123

	return 0;
}

5.局部变量，全局变量，static全局变量，static局部变量

6.全局函数，static函数

7.生命周期

8.内存4区模型

9.开辟释放 heap 空间

#include <stdio.h>

int main(void)
{
	//int arr[1000000] = { 10,20,40 };

	int* p = (int*)malloc(sizeof(int) * 1000000);

	if (p == NULL)
	{
		printf("malloc error\n");
		return -1;
	}

	//写数据到 malloc 空间
	for (size_t i = 0; i < 10; i++)
	{
		p[i] = i + 10;
	}

	//从 malloc 空间读数据
	for (size_t i = 0; i < 10; i++)
	{
		printf("%d ", p[i]);
	}

	//释放申请的内存
	free(p);
	p = NULL;

	return 0;
}

10.使用 heap 空间

11.二级指针对应的 heap 空间

#include <stdio.h>

int main(void)
{
	int** p = malloc(sizeof(int*) * 3);

	for (size_t i = 0; i < 3; i++)
	{
		p[i] = malloc(sizeof(int) * 5);
	}

	//使用空间--写
	for (size_t i = 0; i < 3; i++)
	{
		for (size_t j = 0; j < 5; j++)
		{
			p[i][j] = i + j;
		}
	}

	//使用空间--读
	for (size_t i = 0; i < 3; i++)
	{
		for (size_t j = 0; j < 5; j++)
		{
			printf("%d ", p[i][j]);
		}
		printf("\n");
	}

	//先释放内层空间
	for (size_t i = 0; i < 3; i++)
	{
		free(p[i]);
		p[i] = NULL;
	}

	//再释放外层空间
	free(p);
	p = NULL;

	return 0;
}