How to print array which I use in a map?

Question

Here is my code:

typedef array<int,6> Array;
Array dayHours{0,0,0,0,0,0};

I am using this data in here:

void Schedule::studentSchedule()
{
    string c, s;
    int courseNum;
    list<string>::iterator studentLoc;
    map<pair<string, int>, pair<string, Array> >::iterator location;

    cout << "Enter the student name" << endl;
    cin >> s;
    cout << "Enter how many course you want?" << endl;
    cin >> courseNum;
wrongCourse:
    cout << "Enter the course names you want" << endl;
    for (int i = 0; i < courseNum; ++i)
    {
        cin >> c;
        auto predicate = [&](auto& course) { return compareName(course, c); };
        studentLoc = find(getStudentList().begin(), getStudentList().end(), s);
        location = find_if(getMatchMap().begin(), getMatchMap().end(), predicate);
        map<pair<string, int>, pair<string, Array> >::iterator it;
        cout << "Student:\t\t" << "Course:\t\t" << "Course Day:\t\t" << "Course Hours:" << endl;
        if (studentLoc != getStudentList().end() && location != getMatchMap().end())
        {
            getCourseScList().insert({ make_pair(s,c),make_pair(getDay1()[i],getDayHours()) });
        }
        else
        {
            cout << "The course you're writing isn't available.Please enter existed courses!" << endl;
            goto wrongCourse;
        }
    }
}

I am sending the array to the map here:

if (studentLoc != getStudentList().end() && location != getMatchMap().end())
{
    getCourseScList().insert({ make_pair(s,c),make_pair(getDay1()[i],getDayHours())});
}

The question is how can I reach the array element:

map< pair<string, string>, pair<string, Array> >::iterator last;
for (last = getCourseScList().begin(); last != getCourseScList().end(); ++last)
{
    cout << (last->first).first << "\t\t\t\t"
         << (last->first).second
         << "\t\t" << (last->second).first
         << (last->second).second << endl;
}

The (last->second).second is representing my array but I can not print this to screen. What can I do?

"How to print array which I use in a map?" The same way that one which is not in map. I meant, you might reduce your sample to array<int,6> Array; Array dayHours{0,0,0,0,0,0}; std::cout << dayHours; // Error: How to print dayHours? — Jarod42
– Jarod42, Commented Dec 29, 2019 at 8:52

JeJo · Accepted Answer · 2019-12-29 11:10:27Z

There is no operator<< defined for std::array<T>, Therefore, you need to iterate through the elements in the array to print it.

Let say your map is

using Array = std::array<int, 6> ;
using MyMap = std::map<std::pair<std::string, std::string>, std::pair<std::string, Array>>;

Then using iterators MyMap::iterator. (Assuming you have MyMap& getCourseScList(); getter overload in your class. In case of MyMap::const_iterator, you should be having const MyMap& getCourseScList() const; overload)

#include <array>
#include <map>

for (MyMap::iterator last = getCourseScList().begin(); // preferably cbegin() and cend(), as the entries will not be modified
                     last != getCourseScList().end(); ++last)
{ 
    // print the key-pair (i.e. std::pair<std::string, std::string>)
    std::cout << last->first.first << "\t\t\t\t" << last->first.second << "\t\t";
    // print the first of value-pair (i.e. string of std::pair<std::string, Array>)
    std::cout << last->second.first;
    // now to print the array `Array`
    for (const int element : last->second.second)
        std::cout << element << " ";
}

In c++11, you could use range-based for-loop, instead of the iterator based one.

for (const auto& entry : getCourseScList())
{
    std::cout << entry.first.first << " " << entry.first.second << "\n";
    std::cout << entry.second.first << " ";
    for (const int element : entry.second.second)
       std::cout << element << " ";
}

However, if you have access to the c++17 or later compiler use structured binding for key-value pair, along with a range-based for-loop to make it more intuitive.

for (auto const&[key, value]: getCourseScList())
//              ^^^^^^^^^^^^             -->structured binding
{
    std::cout << key.first << " " << key.second << "\n";
    std::cout << value.first << " ";
    for (const int element : value.second)  
       std::cout << element << " ";
}

As a side note, please keep in mind the followings:

@sonelektrikci Your welcome. Check out the update for the alternative in C++17.

jignatius · Accepted Answer · 2020-01-05 21:16:18Z

2

You can also create your own stream insertion operator (<<) for your type Array:

#include <algorithm>
#include <array>
#include <iterator>
#include <iostream>

typedef std::array<int, 6> Array;

std::ostream& operator<<(std::ostream& o, const Array& arr)
{
    std::copy(arr.cbegin(), arr.cend(), std::ostream_iterator<int>(o, " "));
    return o;
}

Then you should be able to do the following without the loop:

std::cout << last->second.second;

which will call the operator above automatically.

Example:

int main()
{
    Array dayHours{ 0,1,0,0,0,0 };
    std::cout << dayHours;
    //prints 0 1 0 0 0 0
}

edited Jan 5, 2020 at 21:16

answered Dec 29, 2019 at 9:32

jignatius

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2 Comments

themmfa Over a year ago

I tried overloaded operator but I can't use it in my class.

jignatius Over a year ago

@sonelektrikci You should define the operator << outside your class.

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