vector help

[cppnoob25]

So I'm trying to delete a value stored inside one of my vectors but I can't accomplish this. My attempts are down below. I've commented out one attempt since it gives me errors. How can I do what I am trying to do?

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<vector<int> >row;
    vector<int> newColumn;

    row.push_back(newColumn);
    row.push_back(newColumn);
    row.push_back(newColumn);
    row.push_back(newColumn);

    row.at(0).push_back(1);
    row.at(1).push_back(2);
    row.at(2).push_back(3);
    row.at(3).push_back(4);

    cout << row[0][0] << "\t";
    cout << row[1][0] << "\t";
    cout << row[2][0] << "\t";
    cout << row[3][0] << endl << endl;

    row.at(0).pop_back();
    //row.at[0].back().pop_back();
    //I want to delete the element in [0][0]
    cout << row[0][0] <<  endl;

}

[cire]

// http://ideone.com/V0icCs
#include <iostream>
#include <vector>

std::ostream& operator<<(std::ostream& os, const std::vector<int>& v)
{
    os << '{';

    if (v.empty())
        os << " empty";
    else
    {
        for (auto e : v)
            os << ' ' << e;
    }

    return os << " }";
}

std::ostream& operator<<(std::ostream& os, const std::vector<std::vector<int>>& v)
{
    for (auto e : v)
        os << e << '\n';

    return os;
}

int main()
{
    std::vector<std::vector<int>> row{ { 1 }, { 2 }, { 3 }, { 4 } };

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

    row[0].erase(row[0].begin());

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

[cppnoob25]

When I try to run the code provided by cire I get the error that e does not name a type.

I'm really not familiar with the ostream& operator. I know your referencing the ostream class to overload the << operator if I'm correct but I don't know else goes on after that.

Help me understand this as well someone.

 os << '{';
 
if (v.empty())
os << " empty";
else
{
for (auto e : v)
os << ' ' << e;
}
 
return os << " }";

I get that os << '{'; prints this to the console window but what does os mean exactly? Is it like saying out?

And I suspect that for(auto e : v) is an iterator that goes through the range e through v?

I still can't get my code to do what I want it to.

[cppnoob25]

Can anyone else help.

[sujitnag]

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<vector<int> >row;
    vector<int> newColumn;

    row.push_back(newColumn);
    row.push_back(newColumn);
    row.push_back(newColumn);
    row.push_back(newColumn);

    row.at(0).push_back(1);
    row.at(1).push_back(2);
    row.at(2).push_back(3);
    row.at(3).push_back(4);

    cout << row[0][0] << "\t";
    cout << row[1][0] << "\t";
    cout << row[2][0] << "\t";
    cout << row[3][0] << endl << endl;

    row.erase(row.begin());  /// do it and see the result
    //row.at[0].back().pop_back();
    //I want to delete the element in [0][0]
    cout << row[0][0] <<  endl;

}

[tipaye]

@sujitnag, that just erases the entire vector row[0], I think what he's after is " row[0].erase(row[0].begin());" as per Cire.

cppnoob, what's your compile command line? Cire's code should compile and run. Maybe you're not using the right flags to force C++11 compatibility. Most compilers still default to c++98

g++ cire.cpp won't work, but
g++ -std=c++11 cire.cpp will, if you're using a recent version of gcc

Otherwise confirm that your compiler supports c++11, and check what the necessary flag is.

Cire has used a new form of initialisation and a new form of for loop (ranged for), that were unavailable in C++98. As you can see, it makes code cleaner and life a lot easier once you get used to it.

[cppnoob25]

You're rigjt @tipaye I'm just trying to delete the first item in row[0]. There is more code to this that further fills those rows and I don't want to delete those other items.

But I am using the code blocks IDE which is using MingW. I'll look into getting it to support c++11 and then try cires code again.

[cppnoob25]

Cires code works great but I guess I should have been more clear on what I'm trying to accomplish.

I was to push numbers into rows and columns through a vector but I want to be able to do this.

Say I have this.


5 7 8 9
2 3 4 6
5 7 3

I want to be able to remove the number 3. With Cires code I can only remove the 8 as it removes the number at the begining. I couldn't figure out how to make it so that it works on the last element added to that vector. pop_back doesn't seem to work here. How can I remove [2][2] without hard coding that in case I push more numbers in there? I want to be able to remove the last number added into the vector.

[tipaye]

Instead of row[0].erase(row[0].begin()); which will remove the first element
Try row[0].erase(row[0].end() - 1); which I hope will remove the last element.

pop_back works for me though...

#include "std_lib_facilities.h"

int main()
{
	vector<vector<int>> row{ { 5, 7, 8, 9 }, { 2, 3, 4, 6 }, { 5, 7, 3 } };
	for (auto v : row) {
		for (auto x : v) {
			cout << x << " ";
		}
		
		cout << endl;
	}

	row[2].pop_back();

	cout << endl;
	for (auto v : row) {
		for (auto x : v) {
			cout << x << " ";
		}
		
		cout << endl;
	}
	
	row[2].erase(row[2].end() - 1);

	cout << endl;
	for (auto v : row) {
		for (auto x : v) {
			cout << x << " ";
		}
		
		cout << endl;
	}
}

output:

5 7 8 9
2 3 4 6
5 7 3

5 7 8 9
2 3 4 6
5 7

5 7 8 9
2 3 4 6
5

[cppnoob25]

Great it all works.

Thanks tipaye.

Just one more thing. I've never use auto before and I would like to know how it works.

for (auto v : row) {
		for (auto x : v) {
			cout << x << " ";
		}
		
		cout << endl;
	}

what exactly is going on there? I get that it's printing out the numbers from row to row but aside from that I can't understand the arguments there. I can make guesses but I'd like to know what auto and the colon do.

I did look this up but I only ran into more complicated examples.

[tipaye]

"auto" automatically figures out the type, very useful for complex situations where the type is not so obvious. In this case, we could have written it as for

for (vector<int> v : row) {
		for (int x : v) {
			cout << x << " ";
		}
}

the ranged for may be considered in the following light:

for (int x : v)
{
    ...
}

has the same effect logically, as

for (int i = 0; i < v.size(); ++i)
{
    int x = v[i];
    ...
}

You can find short but clear explanations of both auto and ranged for, as well as some other new features at: http://herbsutter.com/elements-of-modern-c-style/