字符串与数字之间的相互转换、判断大小端（用的是union）

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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;
}

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