求大佬帮忙看一下我的代码哪里错了，谢谢！（C++）

	#include<cstdio>
	#include<iostream>
	#include<cstring>
	#include<cmath>

	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));
		}
	}