#include <bits/stdc++.h>
using namespace std;
int main()
{
	string a,b;
	int  l=0,r=0;
	cin >> a >> b;
	l=b.size();
	for(int i=0;i<=l-1;i++){
		if(a[i]==b[i]) r++;
	}
	cout <<fixed<<setprecision(2)<<(r*1.0/l);
	return 0;
}

要初始化变量，坑了我好几次

#include<bits/stdc++.h> using namespace std; string a,b; double s; int cnt; int main(){ cin>>a>>b; for(int i=0;i<a.size();i++){ if(a[i]==b[i]){ cnt++; } } s=cnt*1.0/a.size(); printf("%.2f",s); return 0; }

WATERRRRRRRRRRRRRRRRRRRRR

#include<bits/stdc++.h>
using namespace std; 
string a,b; 
double s; 
int cnt; 
int main(){
  cin>>a>>b; 
    for(int i=0;i<a.size();i++){
      if(a[i]==b[i]){ 
        cnt++; 
      } 
    }
  s=cnt*1.0/a.size();
  printf("%.2f",s);
  return 0; 
}

#include<bits/stdc++.h>
using namespace std;
int cnt;
string s1,s2;
signed main(){
	cin>>s1>>s2;
	for(int i=0;i<s1.size();i++)if(s1[i]==s2[i])cnt++;
	cout<<fixed<<setprecision(2)<<cnt*1.0/s1.size();
}

本地过掉，但是提交神奇的获得了一个 Warning。
$13$ 行！

#include<bits/stdc++.h>
#define int long long
#define INF 0x3f3f3f
using namespace std;
string s1,s2;
double ans=0.0;
signed main(){
	cin>>s1>>s2;
	for(int i=0;i<s1.size();i++)
		if(s1[i]==s2[i])ans++;
	printf("%.2lf",ans/s2.size());
	return 0;
}

water。

#include<bits/stdc++.h>
using namespace std;
int main()
{
	string a,b;
	cin>>a>>b;
	double qwq;
	int awa=0;
	for(int i=0;i<a.size();i++)
	{
		if(a[i]==b[i])
		{
			awa++;
		}
	}
	qwq=awa*1.0/a.size();
	printf("%.2lf",qwq);
	return 0;
}

C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	//freopen("10.in","r",stdin);
	//freopen("10.out","w",stdout);
	string a,b;
	int l,m;
	
	//getline(cin,a);
	//getline(cin,b);
	cin>>a;
	cin>>b;
	l=a.size();
	m=0;
	for(int i=0;i<l;i++){
		if(a[i]==b[i]){
			m++;	
		}
	}
	
	printf("%.2lf",m/(l*1.0));
	//fclose(stdin);
	//fclose(stdout);
	return 0;
}