#include<stdio.h>
#include<conio.h>
#include<string.h>
#define N 20
void beep();
void GetNumberStr (char s[]);
void AddNumberStr (char a[],char b[],char c[]);
char AddChar (char ch1,char ch2);
void LeftTrim (char str[]);
int tag=0;
void main ()
{
    char a[N+1]={0},b[N+1]={0},c[N+2];
    printf("a=");
    while (strlen (a)==0)
      GetNumberStr (a);
    printf("\nb=");
    while (strlen (b)==0)
      GetNumberStr (b);
     AddNumberStr (a,b,c);
     printf("\na+b=%s\n",c);
}
void GetNumberStr (char s[]);
{
    int i=0;
    char ch;
    while (1)
    {ch=getch();
    if (ch=='\r')
        break;
    if (ch=='\b')
    {if (i>0)
    {printf("%c %c",ch,ch);i--;}
    else
        beep();
    continue;}
    if (ch<'0'||ch>'9')
    {
        beep();
        continue;}
    if (i<N)
    {printf("%c",ch);
    s[i++]=ch;}
    else
        beep();}
    s[i]='\0';}   
void beep()
{printf("\07");}

void AddNumberStr (char a[],char b[],char c[])
{int i,j,k;
memset (c,' ',N+2);
i=strlen (a)-1;
j=strlen (b)-1;
k=N;
while (i>=0&&j>=0)
c[k--]=AddChar (a[i--],b[j--]);
while (i>=0)
c[k--]=AddChar (a[i--],'0');
while (j>=0)
c[k--]=AddChar (b[i--],'0');
if (tag==1)
c[k]='1';
c[N+1]='\0';
LeftTrim (c);}

char AddChar(char ch1,char ch2)
{char ch;
ch=(ch1-0x30+ch2-0x30)+tag;
if (ch>=10)
{tag=1;
return (ch-10+0x30);}
else
{tag=0;
return (ch+0x30);}

void LeftTrim (char str[])
{int i;
for (i=0;str[i]==' ';i++)
;
strcpy (str,str+i);}

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