12.15

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


int main()
{
FILE *fp1;
FILE *fp2;
FILE *fp3;
    char ch1;
char ch2;
char ch3;
int sum=0;
int temp1=0,temp2=0;


if((fp1=fopen("a.c","r"))==NULL)
{
printf("fp1 open error!");
exit(0);
}
if((fp2=fopen("b.c","r"))==NULL)
{
printf("fp2 open error!");
exit(0);
}
if((fp1=fopen("c.c","w+"))==NULL)
{
printf("fp3 open error!");
exit(0);
}


while(((ch1=fgetc(fp1))!=EOF)&&(ch2=fgetc(fp2))!=EOF)
{
if(ch1<'0'||ch1>'9')
{
fputc(ch1,fp3);
}
else
{
   temp1=ch1-'0';
temp2=ch2-'0';
sum=temp1+temp2;
ch3=sum+'0';
fputc(ch3,fp3);
}
}


fclose(fp1);
fclose(fp2);
fclose(fp3);
}