PTA乙级 （*1034 有理数四则运算 (20分)（string.append()））

1034 有理数四则运算 (20分)

https://pintia.cn/problem-sets/994805260223102976/problems/994805287624491008

本题代码：

#include <iostream>
#include <cmath>
#include <algorithm>
#include <cstdio>
#include <string>
#include <cstring>
typedef long long ll;
using namespace std;
ll gcd(ll a,ll b)
{
	return b?gcd(b,a%b):a;
}
string print(ll a,ll b)
{
	ll k=gcd(a,b);
	a=a/k;b=b/k;
	if(a==0) return "0";
	string s;
	int flag=0;
	if(b<0) {a=-a;b=-b;}
	if(a<0) {s.append("(-");a=-a;flag=1;}
	if(a>=b) s.append(to_string(a/b));
	if(a>=b&&a%b) s.insert(s.end(),' ');
	a=a%b;
	if(a)
	{
		s.append(to_string(a));
		s.insert(s.end(),'/');
		s.append(to_string(b));
	}
	if(flag) s.insert(s.end(),')');
	return s;
}
int main()
{
	ll a1,a2,b1,b2;
	char chr='/';
	cin>>a1>>chr>>b1;
	cin>>a2>>chr>>b2;
	cout<<print(a1,b1)<<" + "<<print(a2,b2)<<" = "<<print(a1*b2+a2*b1,b1*b2)<<endl;
	cout<<print(a1,b1)<<" - "<<print(a2,b2)<<" = "<<print(a1*b2-a2*b1,b1*b2)<<endl;
	cout<<print(a1,b1)<<" * "<<print(a2,b2)<<" = "<<print(a1*a2,b1*b2)<<endl;
	if(b1*a2) cout<<print(a1,b1)<<" / "<<print(a2,b2)<<" = "<<print(a1*b2,b1*a2)<<endl;
	else cout<<print(a1,b1)<<" / "<<print(a2,b2)<<" = Inf\n";
	return 0;
}

　　

知识点：string::append  http://www.cplusplus.com/reference/string/string/append/

// appending to string
#include <iostream>
#include <string>

int main ()
{
  std::string str;
  std::string str2="Writing ";
  std::string str3="print 10 and then 5 more";

  // used in the same order as described above:
  str.append(str2);                       // "Writing "
  str.append(str3,6,3);                   // "10 "
  str.append("dots are cool",5);          // "dots "
  str.append("here: ");                   // "here: "
  str.append(10u,'.');                    // ".........."
  str.append(str3.begin()+8,str3.end());  // " and then 5 more"
  str.append<int>(5,0x2E);                // "....."

  std::cout << str << '\n';
  return 0;
}

//Output:
//Writing 10 dots here: .......... and then 5 more.....