学步园

Often, especially at the start of semesters, I get a lot of questions about how to write very simple programs. Typically, the problem to be solved is to read in a few numbers,
do something with them, and write out an answer. Here is a sample program that does that:

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

	int main()
	{
		vector<double> v;

		double d;
		while(cin>>d) v.push_back(d);	// read elements
		if (!cin.eof()) {		// check if input failed
			cerr << "format error\n";
			return 1;	// error return
		}

		cout << "read " << v.size() << " elements\n";

		reverse(v.begin(),v.end());
		cout << "elements in reverse order:\n";
		for (int i = 0; i<v.size(); ++i) cout << v[i] << '\n';

		return 0; // success return
	}

Here are a few observations about this program:

• This is a Standard ISO C++ program using the standard library. Standard library facilities are declared in namespace std in headers without a .h suffix.
• If you want to compile this on a Windows machine, you need to compile it as a "console application". Remember to give your source file the .cpp suffix or the compiler might think that it is C (not C++) source.
• Yes, main() returns an int.
• Reading into a standard vector guarantees that you don't overflow some arbitrary buffer. Reading into an array without making a "silly error" is beyond the ability of complete novices - by the
  time you get that right, you are no longer a complete novice. If you doubt this claim, I suggest you read my paper "Learning Standard C++ as a New Language", which you can download from my publications list.
• The !cin.eof() is a test of the stream's format. Specifically, it tests whether the loop ended by finding end-of-file (if not, you didn't get input of the expected type/format). For more information, look up "stream state" in your C++ textbook.
• A vector knows its size, so I don't have to count elements.
• Yes, I know that I could declare i to be a vector<double>::size_type rather than plain int to quiet warnings from some hyper-suspicious compilers, but in this case,I consider that too pedantic
  and distracting.
• This program contains no explicit memory management, and it does not leak memory. A vector keeps track of the memory it uses to store its elements. When a vector needs more memory for elements, it allocates more; when a vector goes out of scope, it frees
  that memory. Therefore, the user need not be concerned with the allocation and deallocation of memory for vector elements.
• for reading in strings, see How do I read a string from input?.
• The program ends reading input when it sees "end of file". If you run the program from the keybord on a Unix machine "end of file" is Ctrl-D. If you are on a Windows machine that because of a bug doesn't recognize an end-of-file character, you might prefer
  this slightly more complicated version of the program that terminates input with the word "end":

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

	int main()
	{
		vector<double> v;

		double d;
		while(cin>>d) v.push_back(d);	// read elements
		if (!cin.eof()) {		// check if input failed
			cin.clear();		// clear error state
			string s;
			cin >> s;		// look for terminator string
			if (s != "end") {
				cerr << "format error\n";
				return 1;	// error return
			}
		}

		cout << "read " << v.size() << " elements\n";

		reverse(v.begin(),v.end());
		cout << "elements in reverse order:\n";
		for (int i = 0; i<v.size(); ++i) cout << v[i] << '\n';

		return 0; // success return
	}