深入了解C语言字符函数和字符串函数

目录

• 前言
• 求字符串长度
  • strlen介绍
  • strlen直接使用：
  • strlen模拟实现：
  • strlen对比sizeof
• 长度不受限制的字符串函数
  • strcpy
    • 直接使用
    • 模拟实现：
  • strcat
    • 直接使用：
    • 模拟实现：
  • strcmp
    • 运行结果：
    • 模拟实现：
• 长度受限制的字符串函数
  • strncpy
  • strncat
  • strncmp
• 字符串查找
  • strstr
  • strtok
• 字符分类函数
• 内存操作函数
  • memcpy
  • memmove
  • memcmp
• 总结

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <string.h>
int main()
{
	char a[] = "abcdefg";
	char b[] = { "a", "b", "c", "d", "e", "f", "g" };
	int len1 = strlen(a);
	int len2 = strlen(b);
	printf("%d
", len1);
	printf("%d
", len2);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <string.h>
#include <assert.h>
int my_strlen(const char* str)
{
	int count = 0;//计数器
	assert(str != NULL);
	while (*str != "")
	{
		count++;
		str++;
	}
	return count;
}
int main()
{
	char a[] = "abcdefg";
	int len  = my_strlen(a);//使用自己创建的strlen函数来计算数组长度！
	printf("%d
", len);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <string.h>
int main()
{
	char a[] = "abcdefg";
	printf("%d
", strlen(a));
	printf("%d
", sizeof(a));//一个字符所占空间为一个字节
	printf("%d
", (sizeof(a) / sizeof(a[0])));
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>

int main()
{
	char  a[10];
	char  b[] = "hello bit";
	strcpy(a, b);
	printf("%s", a);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
void my_strcpy(char *a, const char *b)
{
	int i = 0;
	while (*b != "")
	{
		*a = *b;
		*a++;
		*b++;
	}
	*a++ = "";
}
int main()
{
	char  a[10];
	char  b[] = "hello bit";
	my_strcpy(a, b);
	printf("%s", a);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>

int main()
{
	char  a[20]="hello ";
	char  b[20] = "bit ";
	char c[20] = "hello ";
	strcat(a, b);
	strcat(b, c);
	printf("%s
",a);
	printf("%s
", b);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
#include<assert.h>
void my_strcat(char *x, const char *y)
{
	assert(x != NULL);
	assert(y != NULL);
	while (*x)//找到目标字符串中的""
	{
		x++;
	}
	while ((*x++ = *y++))
	{
		;
	}

}
int main()
{
	char  a[20]="hello ";
	char  b[20] = "bit ";
	my_strcat(a, b);
	printf("%s
",a);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
#include<assert.h>

int main()
{
	char  a[20]=  "ABCD";
	char  b[20] = "ABCG";
	int i=strcmp(a, b);
	if (i > 0)
	{
		printf("a>b
");
	}
	else if (i==0)
	{
		printf("a=b
");
	}
	else
	{
		printf("a<b
");
	}
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
#include<assert.h>
int my_strcmp(const char *x, const char *y)
{
	int ret = 0;
	assert(x != NULL);
	assert(y != NULL);
	while (!(ret = *(unsigned char *)x - *(unsigned char *)y) && *y)
	{
		++x;
		++y;
	}
	if (ret < 0)
		ret = -1;
	else if (ret >0)
		ret = 1;
	return ret;
}
int main()
{
	char  a[20]=  "ABCD";
	char  b[20] = "ABCG";
	int i=my_strcmp(a, b);
	if (i > 0)
	{
		printf("a>b
");
	}
	else if (i==0)
	{
		printf("a=b
");
	}
	else
	{
		printf("a<b
");
	}
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
int main()
{
	char arr1[20]="1234567";
	char arr2[] = "abcdefg";
	strncpy(arr1, arr2, 3);//拷贝3个字符到arr1数组
	printf("%s
", arr1);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h.>
#include<string.h>
int main()
{
	char arr1[20] = "hello ";
	char arr2[] = "csdn";
	strncat(arr1, arr2, 1);
	printf("%s
", arr1);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
int main()
{
	char* a = "abcdef";
	char* b = "abchij";
	int i = strncmp(a, b, 4);
	if (i > 0)
	{
		printf("a>b
");
	}
	else if (i == 0)
	{
		printf("a=b
");
	}
	else
	{
		printf("a<b
");
	}
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
int main()
{
	char arr1[] = "hello bit";
	char arr2[] = "bit";
	char* ret = strstr(arr1, arr2);
	if (ret == NULL)
		printf("没找到
");
	else
		printf("找到了：%s
", ret);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
int main()
{
	char arr1[] = "abcdef";
	char arr2[20] = { 0 };
	memcpy(arr2, arr1, 3);//数字3的单位是字节，而一个字符占一个字节
	printf("%s
", arr2);
	return 0;
}

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <string.h>
int main()
{
	const char a[] = "abcdefg";
	const char b[] = "1234567";
	printf("a = %s,b = %s
", a, b);
	memmove(a, b, 7);
	printf("a = %s,b = %s
", a, b);
	return(0);
}

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
int main()
{
	int  a[] = { 1, 2, 3 };
	int  b[] = { 1, 5};
	int ret = memcmp(a,b, 8);//注意第3个参数的单位是字节
	if (ret > 0)
	{
		printf("a>b
");
	}
	else if (ret < 0)
	{
		printf("a<b
");
	}
	else
	{
		printf("a==b
");
	}
	return 0;
}