啥也不会的人求助P1957代码报错

#include<bits/stdc++.h>
#include <cctype>
using namespace std;
int main()
{
	int m,n,w;
	char a,j;
	string s;
	cin>>w;
	while(w--)
	{
		cin>>a;
		if (a=='a')
		{
			j=a;
			cin>>m>>n;
			sprintf(s,"%d+%d=%d",m,n,m+n);
		}
		else if (a=='b')
		{
			j=b;
			cin>>m>>n;
			sprintf(s,"%d-%d=%d",m,n,m-n);
		}
		else if (a=='c')
		{
			j=c;
			cin>>m>>n;
			sprintf(s,"%d*%d=%d",m,n,m*n);
		}
		else
		{
			cin>>m;
			if (j=='a')
			{
				cin>>m;
			    sprintf(s,"%d+%d=%d",m,a,m+a);
			}
			else if (j=='b')
		   {   
			   cin>>m;
			   sprintf(s,"%d-%d=%d",m,a,m-a);
		   }
		    else if (j=='c')
		   {
			    cin>>m;
			    sprintf(s,"%d*%d=%d",m,a,m*a);
		   }
			
		}
		cout<<s.size()<<endl;
	}
	
	return 0;
}```