3.2.3 处理string对象中的字符-使用下标执行随机访问

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书中页数：P85
代码名称：hexify.cc

#include <iostream>
using std::cin; using std::cout; using std::endl;

#include <string>
using std::string;

#include <cstddef>
using std::size_t;

int main()
{
	const string hexdigits = "0123456789ABCDEF";  // possible hex digits

	cout << "Enter a series of numbers between 0 and 15"
	     << " separated by spaces.  Hit ENTER when finished: " 
	     << endl;
	string result;        // will hold the resulting hexify'd string

	string::size_type n;  // hold numbers from the input
	while (cin >> n)
		if (n < hexdigits.size())    // ignore invalid input
			result += hexdigits[n];  // fetch the indicated hex digit

	cout << "Your hex number is: " << result << endl;

	return 0;
}