描述
Given a set of constraints like 0<N<=M<=100 and values for all the variables, write a checker program to determine if the constraints are satisfied.
More precisely, the format of constraints is:
token op token op ... op token
where each token is either a constant integer or a variable represented by a capital letter and each op is either less-than ( < ) or less-than-or-equal-to ( <= ).
输入
The first line contains an integer N, the number of constraints. (1 ≤ N ≤ 20)
Each of the following N lines contains a constraint in the previous mentioned format.
Then follows an integer T, the number of assignments to check. (1 ≤ T ≤ 50)
Each assignment occupies K lines where K is the number of variables in the constraints.
Each line contains a capital letter and an integer, representing a variable and its value.
It is guaranteed that:
1. Every token in the constraints is either an integer from 0 to 1000000 or an variable represented by a capital letter from 'A' to 'Z'.
2. There is no space in the constraints.
3. In each assignment every variable appears exactly once and its value is from 0 to 1000000.
输出
For each assignment output Yes or No indicating if the constraints are satisfied.
- 样例输入
2 A<B<=E 3<=E<5 2 A 1 B 2 E 3 A 3 B 5 E 10
样例输出
Yes No
比较简单的字符串处理题。写这篇是要记录下C++的istream类的用法。
朴素实现：
#include <bits/stdc++.h> using namespace std; map<char,int> mp; string c[20]; bool used[26]; bool ok(int a, string &op, int b){ if(op=="<") return a<b; return a<=b; } int get_val(string &s, int &i){ int res=0; for(; s[i] && isdigit(s[i]) && !isalpha(s[i]); res*=10, res+=s[i++]-'0'); if(isalpha(s[i])) res=mp[s[i++]]; return res; } string get_op(string &s, int &i){ string res; for(; s[i] && ispunct(s[i]); res+=s[i++]); return res; } bool check(int n){ int a, b; string op; for(int i=0; i<n; i++){ int j=0; a=get_val(c[i], j); for(;;){ op=get_op(c[i], j); if(op=="") break; b=get_val(c[i], j); if(!ok(a, op, b)) return false; a=b; } } return true; } int main(){ int n, T; cin>>n; for(int i=0; i<n; i++){ cin>>c[i]; for(auto x:c[i]) if(isalpha(x)) used[x-'A']=true; } int nv=0; for(int i=0; i<26; i++) nv+=used[i]; for(cin>>T; T--; ){ for(int i=0; i<nv; i++){ char x; int v; cin>>x>>v; mp[x]=v; } puts(check(n)?"Yes":"No"); } }
借助stringstream类的实现：
#include <bits/stdc++.h> using namespace std; map<char,int> mp; string c[20]; bool used[26]; int get_val(stringstream &x){ int res; char v; x>>res; if(x.fail()){ //check whether badbit or failbit is set //When an istream object turns fail, it stops working until flags reset x.clear(); //new x.get(v), res=mp[v]; } return res; } string get_op(stringstream &x){ char v; string res; if(x.str().empty()) return res; for(;;){ x.get(v); if(ispunct(v)) res+=v; else{ x.putback(v); return res; } } } bool ok(int a, string &op, int b){ if(op=="<") return a<b; return a<=b; } bool check(int n){ int a, b; string op; stringstream x; for(int i=0; i<n; i++){ x.str(c[i]); x.clear(); //error-prone, new a=get_val(x); for(;;){ op=get_op(x); if(op=="") break; b=get_val(x); if(!ok(a, op, b)) return false; a=b; } } return true; } // Input stream objects can read and interpret input from sequences of characters.
// Specific members are provided to perform // these input operations. The standard object cin is an object of this type. int main(){ int n, T; cin>>n; for(int i=0; i<n; i++){ cin>>c[i]; for(auto x:c[i]) if(isalpha(x)) used[x-'A']=true; } int nv=0; for(int i=0; i<26; i++) nv+=used[i]; for(cin>>T; T--; ){ for(int i=0; i<nv; i++){ char x; int v; cin>>x>>v; mp[x]=v; } puts(check(n)?"Yes":"No"); } }
这里系统介绍一下C++的Stream I/O. （以下内容来自 The C++ Programming Language 4th Ed. by Bjarne Stroustrup, 38 I/O Streams）.
- The I/O stream library provides formatted and unformatted buffered I/O of text and numeric values. The definitions for I/O stream facilities are found in <istream>, <ostream>, etc.;
- An istream converts a stream of characters (bytes) to typed objects:
Byte sequences --> stream buffer --> istream --> typed values
An iostream is a stream that can act as both an istream and an ostream. You need stream buffers (streambufs) to define a mapping from an iostream to a new kind of device, file, or provide a new locale, you need a copy of the standard, a good systems manual, and examples of working code in addition to what is presented here.
The key components of the stream I/O system can be represented graphically like this:
ios_base: |-------------------------------------->|---------------------------------->locale: format information
locale independent format state | |
^ |------> basic_streambuf<>: --------------------------------> real destination/source:
I | buffering |
I | |
basic_ios<>: ------------------------------------| |--------------------------------> character buffer
locale dependent format state
stream state
^
|
|
basic_iostream<>:
formatting (<<, >>, etc.)
setup/cleanup
The vertical arrows represent "derived from." The horizontal arrows represent "pointer to." The classes marked with <> are templates parameterized by a character type and containing a locale.
38.3 Error Handling
An iostream can be in one of four states, defined in basic_ios from <ios>:
Stream States
good() The previous iostream operations succeeded
eof() We hit end-of-input ("end-of-file")
fail() Something unexpected hapened (e.g., we looked for a digit and found 'x')
bad() Something unexpected and serious happened (e.g., disk read error)
Any operation attempted on a stream that is not in the good() state has no effect; it is a no-op. An iostream can be used as a condition. In that case, the condition is true (succeeds) if the state of the iostream is good().
38.4.4 Stream State
In <ios>, the standard library defines the base class ios_base defining most of the interface to a stream class.
The basic_ios class manages the state of a stream:
- The mapping between a stream and its buffers
- The formatting options
- The use of locales
- Error handling
- Connections to other streams and stdio
It might be the most complicated class in the standard library.
ios_base Stream State iostate Member Constants
badbit Something unexpected and serious happened (e.g., a disk read error)
failbit Something unexpected hppened (e.g., we looked for a digit and found 'x')
eofbit We hit end-of-input (e.g., end-of-file)
goodbit All is well
Functions for reading these bits (good(), fail(), etc.) in a stream are provided by basic_ios.