求如何解决，我尝试写但写错了

#include<stdio.h>
int main()
{
int a,b=0,i;
scanf("%d",&a);
for(i=0;i<log10(a)+1;i++){
if(a%10==1)
{b==2;b++;}
else if(a%10==0||a%10==6||a%10==9)
{b==6;b++ ; }
else if(a%10==2||a%10==3||a%10==5)
{

  b==5;b++ ;}
   else if(a%10==4)
   {b==4; b++ ; }
   else if(a%10==7)
   {b==3;b++ ;  }
   else if(a%10==8)
  {

   b==7; b++ ;}
   printf("%d",b++);
}
return 0;

}

#include <stdio.h>
int main(void) { 
    int n,i=0;
    scanf("%d",&n);
    int j,sum=0;
    while(n>0)
    {
        j=n%10;
        n/=10;
        switch(j)
        {
            case 0:
            case 6:
            case 9:
                sum+=6;
                break;
            case 1:
                sum+=2;
                break;
            case 2:
            case 3:
            case 5:
                sum+=5;
                break;
            case 4:
                sum+=4;
                break;
            case 7:
                sum+=3;
                break;
            case 8:
                sum+=7;
                break;
        }
    }
    printf("%d\n",sum);
    return 0;
}

#include <stdio.h>
#include <string.h>
int main(void)
{
    int a[10] = {6, 2, 5, 5, 4, 5, 6, 3, 7, 6}, sum = 0;
    char s[100] = {'\0'};
    scanf("%s", s);
    for (unsigned int i = 0; i < strlen(s); i++) sum += a[s[i] - '0'];
    printf("%d\n", sum);
    return 0;
}