好用的函数

二进制是Binary，简写为B。

八进制是Octal，简写为O。

十进制为Decimal，简写为D。

十六进制为Hexadecimal，简写为H。

一、求二进制中1的个数：

int f(unsigned int n)
{ 
   unsigned int number=0;
   for(number=0;n;++number)
   {
     n&=(n-1);               //每与一次就少一个1
    }
    return number;          //number即为二进制中1的个数
}

二、求二进制中0的个数：

int f(unsigned int n)
{
    unsigned int number = 0;
	while(n != 0) {
	if((n&1)==0)
		number++;
	n>>=1;
	}
    return number;			//number即二进制中0的个数
}

三、返回数组中最大值的下标

#include <stdio.h>
int fun ( int * s,int n )			//n为指针s的长度
{ 
    int i,k;
    for ( i＝0, k ＝ i; i < n; i++)
        if ( s[i] > s[k])    
            k = i;
    return (k);
}

四、选择排序（降序）

int a[]={4,0,2,3,1},i,j,t; 
for(i=1;i<5;i++)
{
	t=a[i];
	j=i-1;
	while(j>=0&&t>a[j])
	{
		a[j+1]=a[j];
		--j;
	}
	a[j+1]=t;
}

五、将十六进制的字符串转化为二进制的字符串

    int i = 1024;

    char bs[16],b[16];

    sscanf(hs,"%x",&i);

    itoa(i,b,2);

    sprintf(bs,"%016s",b);

    printf("\nhs=%s,bs=%s\n",hs,bs);        //hs为原始的十六进制，bs为转化完成的二进制字符串

void itoa(unsigned long val,char *buf,unsigned radix) //buf为转化完成的二进制字符串
{
 char *p; /* pointer to traverse string */
 char *firstdig; /* pointer to first digit */
 char temp; /* temp char */
  unsigned digval; /* value of digit */
 p = buf;
  firstdig = p; /* save pointer to first digit */
 do{
       digval = (unsigned)(val%radix);
      val /= radix; /* get next digit */
/* convert to ascii and store */
      if(digval > 9)
      *p++ = (char)(digval - 10 + 'a'); /* a letter */
      else
      *p++ = (char)(digval + '0'); /* a digit */
  }while(val>0);/* We now have the digit of the number in the buffer, but in reverse order. Thus we reverse them now. */
  *p-- = '\0'; /* terminate string; p points to last digit */
   do{
  temp = *p;
  *p = *firstdig;
  *firstdig = temp; /* swap *p and *firstdig */
  --p;
  ++firstdig; /* advance to next two digits */
}while(firstdig < p); /* repeat until halfway */
}

六、字符串转化为十六进制    

pbDest1为输出的十六进制 pbSrc1为输入的字符串

/*

// C prototype : void StrToHex(byte *pbDest, char *pszSrc, int nLen)

// parameter(s): [OUT] pbDest - 输出缓冲区

//    [IN] pszSrc - 字符串

//    [IN] nLen - 16进制数的字节数(字符串的长度/2)

// return value:

// remarks : 将字符串转化为16进制数

*/

void StrToHex( char *pbDest1,  char *pbSrc1, int nLen)     
{
	char h1,h2;
	unsigned char s1,s2;
	int i;
	for (i=0; i<nLen; i++)
	{
		h1 = pbSrc1[2*i];
		h2 = pbSrc1[2*i+1];
		s1 = toupper(h1) - 0x30;
		if (s1 > 9)
			s1 -= 7;
		s2 = toupper(h2) - 0x30;
		if (s2 > 9)
			s2 -= 7;
		pbDest1[i] = s1*16 + s2;
	}
}

七、十六进制转化为字符串   

 pbDest2为输出的字符串  　pbSrc2为输入的十六进制

/*

// C prototype : void HexToStr(char *pszDest, byte *pbSrc, int nLen)

// parameter(s): [OUT] pszDest - 存放目标字符串

//    [IN] pbSrc - 输入16进制数的起始地址

//    [IN] nLen - 16进制数的字节数

// return value:

// remarks : 将16进制数转化为字符串

*/

void HexToStr( char  *pbDest2,  unsigned char *pbSrc2, int nLen)  
{
	char ddl,ddh;
	int i;
	for (i=0; i<nLen; i++)
	{
		ddh = 48 + pbSrc2[i] / 16;
		ddl = 48 + pbSrc2[i] % 16;
		if (ddh > 57) ddh = ddh + 7;
		if (ddl > 57) ddl = ddl + 7;
		pbDest2[i*2] = ddh;
		pbDest2[i*2+1] = ddl;
	}	
	pbDest2[nLen*2] = '\0';
}

八、十六进制字符串转化为十进制数

int tolower(int c)
{
    if (c >= 'A' && c <= 'Z')
    {
        return c + 'a' - 'A';
    }
    else
    {
        return c;
    }
}

int htoi(char s[])
{
    int i;
    int n = 0;
    if (s[0] == '0' && (s[1]=='x' || s[1]=='X'))
    {
            i = 2;
        }
    else
    {
            i = 0;
        }
    for (; (s[i] >= '0' && s[i] <= '9') || (s[i] >= 'a' && s[i] <= 'z') || (s[i] >='A' && s[i] <= 'Z');++i)
    {
            if (tolower(s[i]) > '9')
            {
                        n = 16 * n + (10 + tolower(s[i]) - 'a');
                    }
            else
            {
                        n = 16 * n + (tolower(s[i]) - '0');
                    }
        }
    return n;
}

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