字符串与数字之间的相互转换、判断大小端（用的是union）

一、Myitoa

1.初级版

void Myitoa( char str[100],int num)

{

 assert(str != NULL);

     int i = 0;

     int a = 0; 

  int b = 0;

  int tmp0 = 0;

 char tmp[100];

 if(num <0){

      num = -num;

   tmp0 = -1;

 } 

     while (num>0)

     {

         tmp[i] = (num % 10)+'0';

         num = num / 10;

         i++;

     }

  b = i-1;//数组长度              

  if(tmp0<0){

  tmp[i] = '-';

       i++;   

 }

 tmp[i] = '\0';

   int j = 0; 

  i--; 

  /*  do        //ok

    { 

        str[j] = tmp[i]; 

        j++; 

        i--;     

    }

 while(i>=0);*/ 

  while(i>=0)

  {

   str[j] = tmp[i]; 

        j++; 

        i--;

  }

  i = 0;

  /*if(num == 0)

  {

   str[j] =  '0';     

  }

  str[j] = '\0';

}*/

int main()

{

    char s[10] = {};

 char s1[10] = {};

 char s2[10] = {};

 char s3[10] = {};

    Myitoa(s,1234);    

    printf("%s\n", s);

 Myitoa(s1,-1234);    

    printf("%s\n", s1);

 Myitoa(s2,9);    

    printf("%s\n", s2);

 Myitoa(s3,0);    

    printf("%s\n", s3);

    return 0;

}

2.升级版（支持2~36进制）

char *Myitoa(int num,int len,int radix) //2<=radix<=36;(123,10)->"123";(123,16)->"7B"

{

 char *str = (char *)malloc(sizeof(char)*(len));

 char *str0 = (char *)malloc(sizeof(char)*(len+1));

 //char *str2 = str0;

 int i = 0;

 int m = 0;

 int j = 0;

 int tmp = 0; 

  if(radix>=2 && radix<= 10)

  {

      while(num>0)

      {

    tmp = num%radix;

       str[m] = tmp + '0';

       num/=radix;

       m++;

      }

  }

  

  if(radix>10 && radix<=36)

  {

   m = 0;

   while(num>0)

      {

    tmp = num%radix;

    if(tmp >=0 && tmp <=9)

    {

           str[m] = tmp + '0';

    }

    if(tmp >9)

    {

     str[m] = tmp-10 + 'a';

    }

       num/=radix;

       m++;

      }

  }

  while(m>0)//字符串逆置

   {

    str0[j] = str[m-1];

    j++;

    m--;

   }

   str0[j] = '\0';

  

 return str0;

 free(str);

 free(str0);

}

int main()

{

 printf("%s\n",Myitoa(123,3,10));

 printf("%s\n",Myitoa(123,3,16));

 printf("%s\n",Myitoa(12311,5,2));

 printf("%s\n",Myitoa(12334,5,6));

 printf("%s\n",Myitoa(123665,6,36));

 return 0;

}

二、Myatoi

4、"123"—>123

#include<stdio.h>

#include<assert.h>

#include<ctype.h>

int Myatoi(const  char *str)

{

 assert(str != NULL);

 int  tmp = 0;

 

 if(*str == '-'){

 

 while(isdigit(*(str+1))){

  

  tmp = tmp * 10+((*(str+1))-'0');//数字在ASCII表里是连续的，*str-'0':ASCII值相减结果是整型数字

  

  str++;

 }

 return (-tmp);

 }

 else{

 while(isdigit(*str)){

  

  tmp = tmp * 10+(*str-'0');//数字在ASCII表里是连续的，*str-'0':ASCII值相减结果是整型数字

  

  str++;

 }

 return tmp;

 }

 

 

return 0;

}

*    Myatoi测试用例  */

int main()

{

 /*char arr[10] = "a42";

 char brr[10] = "1a42";

 char crr[10] = "-2";

 char drr[10] = "-24";

 char err[10] = "142";*/

  char frr[100] = "1234567898765";

 /*printf("%d\n",Myatoi(arr));

 printf("%d\n",Myatoi(brr));

    printf("%d\n",Myatoi(crr));

 printf("%d\n",Myatoi(drr));

 printf("%d\n",Myatoi(err));*/

 printf("%u\n",Myatoi(frr));

 return 0;

}

三、判断大小端

bool Islittle()

{

 union A

 {

  int a;

  char b;

 }c;

 c.a = 1;

 if(c.b == 1)

  return true;

 else

  return false;

}

int main()

{

 printf("%d\n",Islittle());

 return 0;

}