PAT（甲级）1001 A+B Format （20 分)

Calculate a+b and output the sum in standard format -- that is, the digits must be separated into groups of three by commas (unless there are less than four digits).

Input Specification:

Each input file contains one test case. Each case contains a pair of integers a and b where −10​6​​≤a,b≤10​6​​. The numbers are separated by a space.

Output Specification:

For each test case, you should output the sum of a and b in one line. The sum must be written in the standard format.

大概意思：a+b然后输出带逗号的那种表示方式

#include<iostream>
#include<algorithm>
#include<string>
using namespace std;

int main()
{
    int a, b;
    cin >> a >> b;
    int c = a + b;
    string s = to_string(c);  //因为有‘-’所以采用字符串来表示
    if(s[0] == '-')
    {
        cout << '-';
        s = s.substr(1, s.length() - 1);  //如果存在负号，则输出，再将字符串变成绝对值
    }
    int t = s.length() % 3;               //每三位输出一个','，但是要考虑最前面可能不足三位
    for (int i = 0; i < t;i++)
    {
        cout << s[i];
    }
    string s2 = s.substr(t, s.length() - t);
    int g = s2.length()/3;
    for (int i = 0; i < g;i++)
    {
        if(t !=0)
            cout << ',' << s2[i * 3] << s2[i * 3 + 1] << s2[i * 3 + 2];
        else
            {
                cout<< s2[i * 3] << s2[i * 3 + 1] << s2[i * 3 + 2];
                if(i != g-1)
                    cout << ',';
            }
    }
    return 0;
}