C++ 字符串相关函数

<转>自：http://zhidao.baidu.com/question/173202165.html

首先就是memcpy
表头文件: #include <string.h>
定义函数: void *memcpy(void *dest, const void *src, size_t n)
函数说明: memcpy()用来拷贝src所指的内存内容前n个字节到dest所指的内存地址上。与strcpy()不同的是,memcpy()会完整的复制n个字节,不会因为遇到字符串结束'\0'而结束
返回值:   返回指向dest的指针
附加说明: 指针src和dest所指的内存区域不可重叠
例如：
你需要复制串str=“wangyucao1989”中的“yucao”，那么可以这么写：
memcpy(newstr,str+4,5);

除了memcpy之外，string还提供了strncpy函数：
函数名称: strncpy
函数原型: char *strncpy(char *dest, const char *src，int count)
函数功能: 将字符串src中的count个字符拷贝到字符串dest中去
函数返回: 指向dest的指针
参数说明: dest-目的字符串，src-源字符串，count-拷贝的字符个数
所属文档: <string.h>

还是上面的例子，这个程序可以这样写：
#include<stdio.h>
#include<string.h>
int main()
{
    char str[] = "wangyucao1989";

    char newstr[6];
    //memcpy(newstr,str+4,5);
    strncpy(newstr,str+4,5);
    newstr[5] = '\0';
    printf("%s\n",newstr);
    return 0;
}

========================================================
位运算：
运算方法有六种：

& 与运算 
| 或运算 
^ 异或运算 
~ 非运算(求补) 
>> 右移运算 
<< 左移运算 

运用这些基本的运算，我们可以解决acm所需的各种运算,给Bit赋1，赋0，给他的值取反，还有好多段操作。如下：

功能             |      示例           | 位运算
-----------------+---------------------+--------------------
去掉最后一位     | (101101->10110)   | x >> 1
在最后加一个0    | (101101->1011010) | x < < 1
在最后加一个1    | (101101->1011011) | x < < 1+1
把最后一位变成1  | (101100->101101)  | x | 1
把最后一位变成0  | (101101->101100)  | x | 1-1
最后一位取反     | (101101->101100)  | x ^ 1
把右数第k位变成1 | (101001->101101,k=3) | x | (1 < < (k-1))
把右数第k位变成0 | (101101->101001,k=3) | x & ~ (1 < < (k-1))
右数第k位取反    | (101001->101101,k=3) | x ^ (1 < < (k-1))
取末三位         | (1101101->101)       | x & 7
取末k位          | (1101101->1101,k=5)  | x & ((1 < < k)-1)
取右数第k位      | (1101101->1,k=4)     | x >> (k-1) & 1
把末k位变成1     | (101001->101111,k=4) | x | (1 < < k-1)
末k位取反        | (101001->100110,k=4) | x ^ (1 < < k-1)
把右边连续的1变成0 | (100101111->100100000) | x & (x+1)
把右起第一个0变成1 | (100101111->100111111) | x | (x+1)
把右边连续的0变成1 | (11011000->11011111)   | x | (x-1)
取右边连续的1      | (100101111->1111)      | (x ^ (x+1)) >> 1
去掉右起第一个1的左边 | (100101000->1000)   | x & (x ^ (x-1))
判断奇数 (x&1)==1
判断偶数 (x&1)==0 
取右边第一个1所在位置 x&-x

================================================================
类型转换：
函数名: abs 功  能: 求整数的绝对值
用  法: int abs(int i);
程序例:
#include <stdio.h>
#include <math.h>

int main(void)
{
  int number = -1234;

  printf("number: %d  absolute value: %d\n", number, abs(number));
  return 0;
}
函数名: atof
功  能: 把字符串转换成浮点数
用  法: double atof(const char *nptr);
程序例:
#include <stdlib.h>
#include <stdio.h>

int main(void)
{
   float f;
   char *str = "12345.67";

   f = atof(str);
   printf("string = %s float = %f\n", str, f);
   return 0;
}

函数名: atoi
功  能: 把字符串转换成长整型数
用  法: int atoi(const char *nptr);
程序例:
#include <stdlib.h>
#include <stdio.h>

int main(void)
{
   int n;
   char *str = "12345.67";

   n = atoi(str);
   printf("string = %s integer = %d\n", str, n);
   return 0;
}

函数名: atol
功  能: 把字符串转换成长整型数
用  法: long atol(const char *nptr);
程序例:

#include <stdlib.h>
#include <stdio.h>

int main(void)
{
   long l;
   char *str = "98765432";

   l = atol(lstr);
   printf("string = %s integer = %ld\n", str, l);
   return(0);
}

===========================================================
其他函数：
函数名: bsearch
功  能: 二分法搜索
用  法: void *bsearch(const void *key, const void *base, size_t *nelem,  size_t width, int(*fcmp)(const void *, const *));
程序例:

#include <stdlib.h>
#include <stdio.h>

#define NELEMS(arr) (sizeof(arr) / sizeof(arr[0]))

int numarray[] = {123, 145, 512, 627, 800, 933};

int numeric (const int *p1, const int *p2)
{
   return(*p1 - *p2);
}

int lookup(int key)
{
   int *itemptr;

   /* The cast of (int(*)(const void *,const void*))
      is needed to avoid a type mismatch error at
      compile time */
   itemptr = bsearch (&key, numarray, NELEMS(numarray),
      sizeof(int), (int(*)(const void *,const void *))numeric);
   return (itemptr != NULL);
}

int main(void)
{
   if (lookup(512))
      printf("512 is in the table.\n");
   else
      printf("512 isn't in the table.\n");

   return 0;
}
函数名: fabs
功  能: 返回浮点数的绝对值
用  法: double fabs(double x);
程序例:

#include <stdio.h>
#include <math.h>

int main(void)
{
   float  number = -1234.0;

   printf("number: %f  absolute value: %f\n",
   number, fabs(number));
   return 0;
}

函数名: fcvt
功  能: 把一个浮点数转换为字符串
用  法: char *fcvt(double value, int ndigit, int *decpt, int *sign);
程序例:

#include <stdlib.h>
#include <stdio.h>
#include <conio.h>

int main(void)
{
   char *string;
   double value;
   int dec, sign;
   int ndig = 10;

   clrscr();
   value = 9.876;
   string = ecvt(value, ndig, &dec, &sign);
   printf("string = %s      dec = %d \
          sign = %d\n", string, dec, sign);

   value = -123.45;
   ndig= 15;
   string = ecvt(value,ndig,&dec,&sign);
   printf("string = %s dec = %d sign = %d\n",
          string, dec, sign);

value = 0.6789e5; /* scientific
                        notation */
   ndig = 5;
   string = ecvt(value,ndig,&dec,&sign);
   printf("string = %s           dec = %d\
          sign = %d\n", string, dec, sign);

   return 0;
}

函数名: gcvt
功  能: 把浮点数转换成字符串
用  法: char *gcvt(double value, int ndigit, char *buf);
程序例:

#include <stdlib.h>
#include <stdio.h>

int main(void)
{
   char str[25];
   double num;
   int sig = 5; /* significant digits */

   /* a regular number */
   num = 9.876;
   gcvt(num, sig, str);
   printf("string = %s\n", str);

   /* a negative number */
   num = -123.4567;
   gcvt(num, sig, str);
   printf("string = %s\n", str);

   /* scientific notation */
   num = 0.678e5;
   gcvt(num, sig, str);
   printf("string = %s\n", str);

   return(0);
}
函数名: itoa
功  能: 把一整数转换为字符串
用  法: char *itoa(int value, char *string, int radix);
程序例:

#include <stdlib.h>
#include <stdio.h>

int main(void)
{
   int number = 12345;
   char string[25];

   itoa(number, string, 10);
   printf("integer = %d string = %s\n", number, string);
   return 0;
}

函数名: labs
功  能: 取长整型绝对值
用  法: long labs(long n);
程序例:

#include <stdio.h>
#include <math.h>

int main(void)
{
   long result;
   long x = -12345678L;

   result= labs(x);
   printf("number: %ld abs value: %ld\n",
      x, result);

   return 0;
}
函数名: memcpy
功  能: 从源source中拷贝n个字节到目标destin中
用  法: void *memcpy(void *destin, void *source, unsigned n);
程序例:

#include <stdio.h>
#include <string.h>
int main(void)
{
   char src[] = "******************************";
   char dest[] = "abcdefghijlkmnopqrstuvwxyz0123456709";
   char *ptr;
   printf("destination before memcpy: %s\n", dest);
   ptr = memcpy(dest, src, strlen(src));
   if (ptr)
      printf("destination after memcpy:  %s\n", dest);
   else
      printf("memcpy failed\n");
   return 0;
}
函数名: memset
功  能: 设置s中的所有字节为ch, s数组的大小由n给定
用  法: void *memset(void *s, char ch, unsigned n);
程序例:

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

int main(void)
{
   char buffer[] = "Hello world\n";

   printf("Buffer before memset: %s\n", buffer);
   memset(buffer, '*', strlen(buffer) - 1);
   printf("Buffer after memset:  %s\n", buffer);
   return 0;
}
函数名: pow 功  能: 指数函数(x的y次方)
用  法: double pow(double x, double y);
程序例:

#include <math.h>
#include <stdio.h>

int main(void)
{
   double x = 2.0, y = 3.0;

   printf("%lf raised to %lf is %lf\n", x, y, pow(x, y));
   return 0;
}

函数名: qsort
功  能: 使用快速排序例程进行排序
用  法: void qsort(void *base, int nelem, int width, int (*fcmp)());
程序例:

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

int sort_function( const void *a, const void *b);

char list[5][4] = { "cat", "car", "cab", "cap", "can" };

int main(void)
{
   int  x;

   qsort((void *)list, 5, sizeof(list[0]), sort_function);
   for (x = 0; x < 5; x++)
      printf("%s\n", list[x]);
   return 0;
}

int sort_function( const void *a, const void *b)
{
   return( strcmp(a,b) );
}

函数名: sqrt
功  能: 计算平方根
用  法: double sqrt(double x);
程序例:

#include <math.h>
#include <stdio.h>

int main(void)
{
    double x = 4.0, result;

    result = sqrt(x);
    printf("The square root of %lf is %lf\n", x, result);
    return 0;
}
========================================================

字符串函数：
函数名: sscanf
功  能: 执行从字符串中的格式化输入
用  法: int sscanf(char *string, char *format[,argument,...]);
程序例:

#include <stdio.h>
#include <conio.h>

int main(void)
{
   char label[20];
   char name[20];
   int entries = 0;
   int loop, age;
   double salary;

   struct Entry_struct
   {
      char  name[20];
      int   age;
      float salary;
   } entry[20];

/* Input a label as a string of characters restricting to 20 characters */
   printf("\n\nPlease enter a label for the chart: ");
   scanf("%20s", label);
   fflush(stdin);  /* flush the input stream in case of bad input */

/* Input number of entries as an integer */
   printf("How many entries will there be? (less than 20) ");
   scanf("%d", &entries);
   fflush(stdin);   /* flush the input stream in case of bad input */

/* input a name restricting input to only letters upper or lower case */
   for (loop=0;loop<entries;++loop)
   {
      printf("Entry %d\n", loop);
      printf("  Name   : ");
      scanf("%[A-Za-z]", entry[loop].name);
      fflush(stdin);  /* flush the input stream in case of bad input */

/* input an age as an integer */
      printf("  Age    : ");
      scanf("%d", &entry[loop].age);
      fflush(stdin);  /* flush the input stream in case of bad input */

/* input a salary as a float */
      printf("  Salary : ");
      scanf("%f", &entry[loop].salary);
      fflush(stdin); /* flush the input stream in case of bad input */
   }

/* Input a name, age and salary as a string, integer, and double */
   printf("\nPlease enter your name, age and salary\n");
   scanf("%20s %d %lf", name, &age, &salary);

/* Print out the data that was input */
   printf("\n\nTable %s\n",label);
   printf("Compiled by %s  age %d  $%15.2lf\n", name, age, salary);
   printf("-----------------------------------------------------\n");
   for (loop=0;loop<entries;++loop)
      printf("%4d | %-20s | %5d | %15.2lf\n",
         loop + 1,
  entry[loop].name,
  entry[loop].age,
  entry[loop].salary);
   printf("-----------------------------------------------------\n");
   return 0;
}
函数名: stpcpy
功  能: 拷贝一个字符串到另一个
用  法: char *stpcpy(char *destin, char *source);
程序例:

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

int main(void)
{
   char string[10];
   char *str1 = "abcdefghi";

   stpcpy(string, str1);
   printf("%s\n", string);
   return 0;
}

函数名: strcat
功  能: 字符串拼接函数
用  法: char *strcat(char *destin, char *source);
程序例:

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

int main(void)
{
   char destination[25];
   char *blank = " ", *c = "C++", *Borland = "Borland";

   strcpy(destination, Borland);
   strcat(destination, blank);
   strcat(destination, c);

   printf("%s\n", destination);
   return 0;
}

函数名: strchr
功  能: 在一个串中查找给定字符的第一个匹配之处\
用  法: char *strchr(char *str, char c);
程序例:

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

int main(void)
{
    char string[15];
    char *ptr, c = 'r';

    strcpy(string, "This is a string");
    ptr = strchr(string, c);
    if (ptr)
       printf("The character %c is at position: %d\n", c, ptr-string);
    else
       printf("The character was not found\n");
    return 0;
}

函数名: strcmp
功  能: 串比较
用  法: int strcmp(char *str1, char *str2);
程序例:

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

int main(void)
{
    char *buf1 = "aaa", *buf2 = "bbb", *buf3 = "ccc";
    int ptr;

    ptr = strcmp(buf2, buf1);
    if (ptr > 0)
       printf("buffer 2 is greater than buffer 1\n");
    else
       printf("buffer 2 is less than buffer 1\n");

    ptr = strcmp(buf2, buf3);
    if (ptr > 0)
       printf("buffer 2 is greater than buffer 3\n");
    else
       printf("buffer 2 is less than buffer 3\n");

    return 0;
}
函数名: strcpy
功  能: 串拷贝用 法: char *strcpy(char *str1, char *str2);
程序例:

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

int main(void)
{
    char string[10];
    char *str1 = "abcdefghi";

    strcpy(string, str1);
    printf("%s\n", string);
    return 0;
}

函数名: strrev
功  能: 串倒转用 法: char *strrev(char *str);
程序例:

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

int main(void)
{
   char *forward = "string";

   printf("Before strrev(): %s\n", forward);
   strrev(forward);
   printf("After strrev():  %s\n", forward);
   return 0;
}
函数名: strset
功  能: 将一个串中的所有字符都设为指定字符
用  法: char *strset(char *str, char c);
程序例:

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

int main(void)
{
   char string[10] = "123456789";
   char symbol = 'c';

   printf("Before strset(): %s\n", string);
   strset(string, symbol);
   printf("After strset():  %s\n", string);
   return 0;
}

函数名: strstr
功  能: 在串中查找指定字符串的第一次出现
用  法: char *strstr(char *str1, char *str2);
程序例:

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

int main(void)
{
   char *str1 = "Borland International", *str2 = "nation", *ptr;

   ptr = strstr(str1, str2);
   printf("The substring is: %s\n", ptr);
   return 0;
}

函数名: strtod
功  能: 将字符串转换为double型值
用  法: double strtod(char *str, char **endptr);
程序例:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
   char input[80], *endptr;
   double value;

   printf("Enter a floating point number:");
   gets(input);
   value = strtod(input, &endptr);
   printf("The string is %s the number is %lf\n", input, value);
   return 0;
}
函数名: strtol
功  能: 将串转换为长整数
用  法: long strtol(char *str, char **endptr, int base);
程序例:

#include <stdlib.h>
#include <stdio.h>

int main(void)
{
   char *string = "87654321", *endptr;
   long lnumber;

   /* strtol converts string to long integer  */
   lnumber = strtol(string, &endptr, 10);
   printf("string = %s  long = %ld\n", string, lnumber);

   return 0;
}

函数名: strupr
功  能: 将串中的小写字母转换为大写字母
用  法: char *strupr(char *str);
程序例:

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

int main(void)
{
   char *string = "abcdefghijklmnopqrstuvwxyz", *ptr;

   /* converts string to upper case characters */ ptr = strupr(string);
   printf("%s\n", ptr);
   return 0;
}

函数名: tolower
功  能: 把字符转换成小写字母
用  法: int tolower(int c);
程序例:

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

int main(void)
{
   int length, i;
   char *string = "THIS IS A STRING";

   length = strlen(string);
   for (i=0; i<length; i++)
   {
       string[i] = tolower(string[i]);
   }
   printf("%s\n",string);

   return 0;
}

函数名: toupper
功  能: 把字符转换成大写字母
用  法: int toupper(int c);
程序例:

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

int main(void)
{
   int length, i;
   char *string = "this is a string";

   length = strlen(string);
   for (i=0; i<length; i++)
   {
      string[i] = toupper(string[i]);
   }

   printf("%s\n",string);

   return 0;
}