Why the last element in istringstream doesn't disappear in my code? - c++

Question

I am writing a calculator program in c++ , and I met two problems .
After I calculate 1 calculation questions , I only need to press enter ,then the program will output the last number in the previous calculation .
There are some incorrect answer , like " 7 / 9 / 10 * 1 * 4 / 6 - 3 * 9 - 8 - 7 " , the output should be -41.95 , but my answer is -11.95 .And I can't find what's wrong with my code.
The cin problem has been fixed when I use if(cin.get()!='\n')break;
And At the bottom there is a correct code of others .
my code use istringstream
#include<iostream>
#include<string>
#include<sstream>
#include<stack>
#include<iomanip>
using namespace std;
int priority(char op) {
if (op == '+' || op == '-') return 1;
else return 2;
}
double calculate(double x, double y, char c)
{
if (c == '+') return x + y;
if (c == '-') return x - y;
if (c == '*')return x * y;
return x / y;
}
int main() {
int n;
char op;
stack<double> nums;
stack<char> ops;
string a;
int count = 0;
while (getline(cin, a)) {
count++;
int allNum = 0;
int allzero = 0;
allNum++;
istringstream b(a);
b >> n;
if (n == 0) allzero++;
nums.push(n);
while (b >> op >> n) {
if (op == '\n') break;
allNum++;
if (n == 0) allzero++;
if (!ops.empty() && priority(op) <= priority(ops.top())) {
double y = nums.top();
nums.pop();
double x = nums.top();
nums.pop();
char tem = ops.top();
ops.pop();
double new_num = calculate(x, y, tem);
nums.push(new_num);
}
ops.push(op);
nums.push(n);
}
while (!ops.empty()) {
double y = nums.top();
nums.pop();
double x = nums.top();
nums.pop();
char tem = ops.top();
ops.pop();
double new_num = calculate(x, y, tem);
nums.push(new_num);
}
if (allzero != allNum) cout<< fixed << setprecision(2) << nums.top() << endl;
b.clear();
nums.pop();
}
}
this is the code I find on the internet
I learned the method from here , the biggest differentenss is that he use C ,and I use C++ .But his result is right .
#include <iostream>
#include <cstdio>
#include <cstring>
#include<stack>
using namespace std;
int P(char c)
{
if (c == '+' || c == '-') return 1;
return 2;
}
double Ans(double x, double y, char c)
{
if (c == '+') return x + y;
if (c == '-') return x - y;
if (c == '*')return x*y;
return x / y;
}
int main() {
int n;
while (scanf("%d",&n)!=EOF)
{
char c = getchar();
if (c=='\n'&&n == 0)break;
stack<char> op;
stack<double>num;
num.push(n);
while (true)
{
scanf("%c %d", &c, &n);
char k = getchar();
while (!op.empty()&&P(c)<=P(op.top()))
{
char t = op.top();
op.pop();
double y = num.top();
num.pop();
double x = num.top();
num.pop();
double ans = Ans(x, y, t);
num.push(ans);
}
op.push(c);
num.push(n);
if (k == '\n')break;
}
while (!op.empty())
{
char t = op.top();
op.pop();
double y = num.top();
num.pop();
double x = num.top();
num.pop();
double ans = Ans(x, y, t);
num.push(ans);
}
printf("%.2f\n", num.top());
}
return 0;
}