今日做题，遇一难关，恳请指教

#include<bits/stdc++.h>
#include<cstdio>
#include<cstring>
using namespace std;
int main(){
    int n,a,b,c;
    char last,s[20],ans[20];
    scanf("%d\n",&n);
    while(n--){
        fgets(s,sizeof(s),stdin);
        if(s[0]=='a'||s[0]=='b'||s[0]=='c'){
            last=s[0];
            s[0]==' ';
        }
        sscanf(s,"%d %d",&a,&b);
        switch(last){
        case'a': c=a+b; sprintf(ans,"%d+%d=%d",a,b,c); break;
        case'b': c=a-b; sprintf(ans,"%d-%d=%d",a,b,c); break;
        case'c': c=a*b; sprintf(ans,"%d*%d=%d",a,b,c); break;
        }
        printf("%s\n%d\n",ans,strlen(ans));
    }
    return 0;
}