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Question about predicates and error in code

Dinot (9)
So this is my first post and i hope i am not doing anything wrong..Recently i picked up the book Accelerated C++:Practical Programming by Example. I am now on chapter 4 of the book and have been introduced to predicates which are used in the sample code. The code is as follows:
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#include <algorithm>
#include <iomanip>
#include <iostream>
#include <stdexcept>
#include <string>
#include <vector>

#ifdef _MSC_VER
#include "../minmax.h"
#else
using std::max;
#endif

using std::cin;
using std::cout;
using std::domain_error;
using std::endl;
using std::istream;
using std::ostream;
using std::setprecision;
using std::setw;
using std::sort;
using std::streamsize;
using std::string;
using std::vector;

struct Student_info {
	string name;
	double midterm, final;
	vector<double> homework;
};	// note the semicolon--it's required

// compute the median of a `vector<double>'
// note that calling this function copies the entire argument `vector'
double median(vector<double> vec)
{
#ifdef _MSC_VER
	typedef std::vector<double>::size_type vec_sz;
#else
	typedef vector<double>::size_type vec_sz;
#endif

	vec_sz size = vec.size();
	if (size == 0)
		throw domain_error("median of an empty vector");

	sort(vec.begin(), vec.end());

	vec_sz mid = size/2;

	return size % 2 == 0 ? (vec[mid] + vec[mid-1]) / 2 : vec[mid];
}

// compute a student's overall grade from midterm and final exam grades and homework grade
double grade(double midterm, double final, double homework)
{
	return 0.2 * midterm + 0.4 * final + 0.4 * homework;
}

// compute a student's overall grade from midterm and final exam grades
// and vector of homework grades.
// this function does not copy its argument, because `median' does so for us.
double grade(double midterm, double final, const vector<double>& hw)
{
	if (hw.size() == 0)
		throw domain_error("student has done no homework");
	return grade(midterm, final, median(hw));
}

double grade(const Student_info& s)
{
	return grade(s.midterm, s.final, s.homework);
}

// read homework grades from an input stream into a `vector<double>'
istream& read_hw(istream& in, vector<double>& hw)
{
	if (in) {
		// get rid of previous contents
		hw.clear();

		// read homework grades
		double x;
		while (in >> x)
			hw.push_back(x);

		// clear the stream so that input will work for the next student
		in.clear();
	}
	return in;
}

istream& read(istream& is, Student_info& s)
{
	// read and store the student's name and midterm and final exam grades
	is >> s.name >> s.midterm >> s.final;

	read_hw(is, s.homework);  // read and store all the student's homework grades
	return is;
}

bool compare(const Student_info& x, const Student_info& y)
{
	return x.name < y.name;
}

int main()
{
	vector<Student_info> students;
	Student_info record;
	string::size_type maxlen = 0;

	// read and store all the records, and find the length of the longest name
	while (read(cin, record)) {
		maxlen = max(maxlen, record.name.size());
		students.push_back(record);
	}

	// alphabetize the records
	sort(students.begin(), students.end(), compare);

#ifdef _MSC_VER
	for (std::vector<Student_info>::size_type i = 0;
#else
	for (vector<Student_info>::size_type i = 0;
#endif
	     i != students.size(); ++i) {

		// write the name, padded on the right to `maxlen' `+' `1' characters
		cout << students[i].name
		     << string(maxlen + 1 - students[i].name.size(), ' ');

		// compute and write the grade
		try {
			double final_grade = grade(students[i]);
			streamsize prec = cout.precision();
			cout << setprecision(3) << final_grade
			     << setprecision(prec);
		} catch (domain_error e) {
			cout << e.what();
		}

		cout << endl;
	}

	return 0;
}

When I ran the code to test it I got 
A 20 20
20 20 20
^Z
B 20 20
20 20 20^Z
C 20 20
20 20 20 ^Z
D 20 20
20 20 20 ^Z
^Z
→C 20
→D 20
A  20
B  20

Process returned 0 (0x0)   execution time : 139.446 s
Press any key to continue.

I have now looked over the code for 30 minutes and cant explain why:
1. the sorting algorithm did not work
2. where the arrows in the output come from(next to the C and the D)
Any help would be appreciated...
Thanks
Dinot
JLBorges (13770)
Give the ^Z on a separate line, all for itself.

A 20 20
20 20 20
^Z
B 20 20
20 20 20
^Z
C 20 20
20 20 20 
^Z
D 20 20
20 20 20 
^Z
^Z


And, get rid of all this stuff:
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#ifdef _MSC_VER
	typedef std::vector<double>::size_type vec_sz;
#else
	typedef vector<double>::size_type vec_sz;
#endif

Just prefix standard names with std:: - the code would be easier to read and easier to write.

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#include <algorithm>
#include <iomanip>
#include <iostream>
#include <stdexcept>
#include <string>
#include <vector>
#include <sstream>

struct Student_info {
    std::string name;
	double midterm, final;
	std::vector<double> homework;
};	// note the semicolon--it's required

// compute the median of a `vector<double>'
// note that calling this function copies the entire argument `vector'
double median( std::vector<double> vec)
{
	typedef std::vector<double>::size_type vec_sz;

	vec_sz size = vec.size();
	if (size == 0)
		throw std::domain_error("median of an empty vector");

	sort( vec.begin(), vec.end() );

	vec_sz mid = size/2;

	return size % 2 == 0 ? (vec[mid] + vec[mid-1]) / 2 : vec[mid];
}

// compute a student's overall grade from midterm and final exam grades and homework grade
double grade(double midterm, double final, double homework)
{
	return 0.2 * midterm + 0.4 * final + 0.4 * homework;
}

// compute a student's overall grade from midterm and final exam grades
// and vector of homework grades.
// this function does not copy its argument, because `median' does so for us.
double grade(double midterm, double final, const std::vector<double>& hw)
{
	if (hw.size() == 0)
		throw std::domain_error("student has done no homework");
	return grade(midterm, final, median(hw));
}

double grade(const Student_info& s)
{
	return grade(s.midterm, s.final, s.homework);
}

// read homework grades from an input stream into a `vector<double>'
std::istream& read_hw( std::istream& in, std::vector<double>& hw )
{
	if (in) {
		// get rid of previous contents
		hw.clear();

		// read homework grades
		double x;
		while (in >> x)
			hw.push_back(x);

		// clear the stream so that input will work for the next student
		in.clear();
	}
	return in;
}

std::istream& read( std::istream& is, Student_info& s)
{
	// read and store the student's name and midterm and final exam grades
	is >> s.name >> s.midterm >> s.final;

	read_hw(is, s.homework);  // read and store all the student's homework grades
	return is;
}

bool compare(const Student_info& x, const Student_info& y)
{
	return x.name < y.name;
}

int main()
{
	std::vector<Student_info> students;
	Student_info record;
	std::string::size_type maxlen = 0;

	// read and store all the records, and find the length of the longest name
       /*
	while (read( std::cin, record )) {
		maxlen = std::max(maxlen, record.name.size());
		students.push_back(record);
	}
	*/

	std::string test_data[] = { "Ccccc 80 90 5 6", "Aaaaa 10 20 1 2 3", "Ddddd 50 50  6 5 6", "Bbbbb 50 30 4 5 6 7" } ;
	for( std::string datum : test_data )
        {
            std::istringstream stm(datum) ;
            read( stm, record ) ;
	    maxlen = std::max(maxlen, record.name.size());
	    students.push_back(record);
        }

	// alphabetize the records
	std::sort(students.begin(), students.end(), compare);

	for (std::vector<Student_info>::size_type i = 0;
	     i != students.size(); ++i) {

		// write the name, padded on the right to `maxlen' `+' `1' characters
		std::cout << students[i].name
		     << std::string(maxlen + 1 - students[i].name.size(), ' ');

		// compute and write the grade
		try {
			double final_grade = grade(students[i]);
			std::streamsize prec = std::cout.precision();
			std::cout << std::setprecision(3) << final_grade
			     << std::setprecision(prec);
		} catch ( const std::domain_error& e) {
			std::cout << e.what();
	  }

		std::cout << '\n';
	}
}

http://coliru.stacked-crooked.com/a/fe2a7f79a5500677
http://rextester.com/DYACC12033
Dinot (9)
Thank you! I did what you told me and the program now does what it's supposed to do!
Just one more question: What was the significance of putting the ^Z on a new line/how did it manage to alter the output?
JLBorges (13770)
AFAIK, in Windows, Ctrl-Z at the start of a line always signifies eof.
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