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C语言进阶教程之字符函数和字符串函数

CGod 人气:0

前言

一、函数介绍

1.1 strlen

size_t strlen ( const char * str );

注:

#include <stdio.h>
int main()
{
	const char* str1 = "abcdef";
	const char* str2 = "bbb";
	if (strlen(str2) - strlen(str1) > 0)
	{
		printf("str2>str1\n");
	}
	else
	{
		printf("srt1>str2\n");
	}
	return 0;
}

输出结果:

1.2 strcpy

char* strcpy(char * destination, const char* source);

1.3 strcat

char * strcat ( char * destination, const char * source );

1.4 strcmp

int strcmp ( const char * str1, const char * str2 );

1.5 strncpy

char * strncpy ( char * destination, const char * source, size_t num );

1.6 strncat

char * strncat ( char * destination, const char * source, size_t num );
#include <stdio.h>
#include <string.h>
int main()
{
	char str1[20];
	char str2[20];
	strcpy(str1, "To be ");
	strcpy(str2, "or not to be");
	strncat(str1, str2, 6);
	puts(str1);
	return 0;
}

输出结果:

1.7 strncmp

int strncmp ( const char * str1, const char * str2, size_t num );
#include <stdio.h>
#include <string.h>
int main()
{
	char str[][5] = { "R2D2" , "C3PO" , "R2A6" };
	int n;
	puts("Looking for R2 astromech droids...");
	for (n = 0; n < 3; n++)
		if (strncmp(str[n], "R2xx", 2) == 0)
		{
			printf("found %s\n", str[n]);
		}
	return 0;
}

输出结果:

1.8 strstr

char * strstr ( const char *str1, const char * str2);
#include <stdio.h>
#include <string.h>
int main()
{
	char str[] = "This is a simple string";
	char* pch;
	pch = strstr(str, "simple");
	puts(pch);
	return 0;
}

1.9 strtok

char * strtok ( char * str, const char * sep );
#include <stdio.h>
#include <string.h>
int main()
{
	char str[] = "- This, a sample string.";
	char* pch;
	printf("Splitting string \"%s\" into tokens:\n", str);
	pch = strtok(str, " ,.-");
	while (pch != NULL)
	{
		printf("%s\n", pch);
		pch = strtok(NULL, " ,.-");
	}
	return 0;
}

输出结果:

1.10 strerror

#include <stdio.h>
#include <string.h>
#include <errno.h>//必须包含的头文件
int main()
{
	FILE* pFile;
	pFile = fopen("unexist.ent", "r");
	if (pFile == NULL)
		printf("Error opening file unexist.ent: %s\n", strerror(errno));
	//errno: Last error number
	return 0;
}

输出结果:

1.11 字符分类函数

字符转换:

int tolower ( int c );
int toupper ( int c );
#include <stdio.h>
#include <ctype.h>
int main()
{
	int i = 0;
	char str[] = "Test String.\n";
	char c;
	while (str[i])
	{
		c = str[i];
		if (isupper(c))
			c = tolower(c);
		putchar(c);
		i++;
	}
	return 0;
}

输出结果:

1.12 memcpy

void * memcpy ( void * destination, const void * source, size_t num );
#include <stdio.h>
#include <string.h>
struct {
	char name[40];
	int age;
} person, person_copy;
int main()
{
	char myname[] = "Pierre de Fermat";
	/* using memcpy to copy string: */
	memcpy(person.name, myname, strlen(myname) + 1);
	person.age = 46;
	/* using memcpy to copy structure: */
	memcpy(&person_copy, &person, sizeof(person));
	printf("person_copy: %s, %d \n", person_copy.name, person_copy.age);
	return 0;
}

输出结果:

1.13 memmove

void * memmove ( void * destination, const void * source, size_t num );
#include <stdio.h>
#include <string.h>
int main()
{
	char str[] = "memmove can be very useful......";
	memmove(str + 20, str + 15, 11);
	puts(str);
	return 0;
}

输出结果:

1.14 memcmp

int memcmp ( const void * ptr1, 
 const void * ptr2, 
 size_t num );

#include <stdio.h>
#include <string.h>
int main()
{
	char buffer1[] = "DWgaOtP12df0";
	char buffer2[] = "DWGAOTP12DF0";
	int n;
	n = memcmp(buffer1, buffer2, sizeof(buffer1));
	if (n > 0) printf("'%s' is greater than '%s'.\n", buffer1, buffer2);
	else if (n < 0) printf("'%s' is less than '%s'.\n", buffer1, buffer2);
	else printf("'%s' is the same as '%s'.\n", buffer1, buffer2);
	return 0;
}

输出结果:

二、库函数的模拟实现

2.1 模拟实现strlen

三种方式:

方式1:

//计数器方式
int my_strlen(const char* str) 
{
	int count = 0;
	while (*str)
	{
		count++;
		str++;
	}
	return count;
}

方式2:

//不能创建临时变量计数器
int my_strlen(const char * str) 
{
	if(*str == '\0')
		return 0;
	else
		return 1+my_strlen(str+1);
}

方式3:

//指针-指针的方式
int my_strlen(char *s) 
{
    char *p = s;
    while(*p != ‘\0' )
        p++;
    return p-s; 
}

2.2 模拟实现strcpy

参考代码:

//1.参数顺序
//2.函数的功能,停止条件
//3.assert
//4.const修饰指针
//5.函数返回值
//6.题目出自《高质量C/C++编程》书籍最后的试题部分
char* my_strcpy(char* dest, const char* src)
{
	char* ret = dest;
	assert(dest != NULL);
	assert(src != NULL);
	while ((*dest++ = *src++))
	{
		;
	}
	return ret;
}

2.3 模拟实现strcat

参考代码:

char* my_strcat(char* dest, const char* src) 
{
	char* ret = dest;
	assert(dest != NULL);
	assert(src != NULL);
	while (*dest)
	{
		dest++;
	}
	while ((*dest++ = *src++))
	{
		;
	}
	return ret;
}

2.4 模拟实现strstr

参考代码:

char* strstr(const char* str1, const char* str2) 
{
	char* cp = (char*)str1;
	char* s1, * s2;
	if (!*str2)
		return((char*)str1);
	while (*cp)
	{
		s1 = cp;
		s2 = (char*)str2;
		while (*s1 && *s2 && !(*s1 - *s2))
			s1++, s2++;
		if (!*s2)
			return(cp);
		cp++;
	}
	return(NULL);
}

2.5 模拟实现strcmp

参考代码:

int my_strcmp(const char* src, const char* dst) 
{
	int ret = 0;
	assert(src != NULL);
	assert(dest != NULL);
	while (!(ret = *(unsigned char*)src - *(unsigned char*)dst) && *dst)
		++src, ++dst;
	if (ret < 0)
		ret = -1;
	else if (ret > 0)
		ret = 1;
	return(ret);
}

2.6 模拟实现memcpy

参考代码:

void* memcpy(void* dst, const void* src, size_t count) 
{
	void* ret = dst;
	assert(dst);
	assert(src);
	/*
	 * copy from lower addresses to higher addresses
	 */
	while (count--) {
		*(char*)dst = *(char*)src;
		dst = (char*)dst + 1;
		src = (char*)src + 1;
	}
	return(ret);
}

2.7 模拟实现memmove

参考代码:

void* memmove(void* dst, const void* src, size_t count) 
{
	void* ret = dst;
	if (dst <= src || (char*)dst >= ((char*)src + count)) 
	{
		/*
		 * Non-Overlapping Buffers
         * copy from lower addresses to higher addresses
         */
		while (count--) {
			*(char*)dst = *(char*)src;
			dst = (char*)dst + 1;
			src = (char*)src + 1;
		}
	}
	else {
		/*
		 * Overlapping Buffers
		 * copy from higher addresses to lower addresses
		 */
		dst = (char*)dst + count - 1;
		src = (char*)src + count - 1;
		while (count--) {
			*(char*)dst = *(char*)src;
			dst = (char*)dst - 1;
			src = (char*)src - 1;
		}
	}
	return(ret);
}

总结

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