# Sort a 2D vector in C++

C++ STL | Sorting a 2D Vector: In this article, we are going to discuss how to sort a 2D vector-based on many use cases with examples?
Submitted by Radib Kar, on July 09, 2020

As per as a 2D vector is concerned it's a vector of a 1D vector. But what we do in sorting a 1D vector like,

```sort(vector.begin(),vector.end());
```

We can't do the same for a 2D vector without any user-defined comparator function, as it will merely sort based on the first element of each column.

But we can sort 2D vector based on use cases:

## 1) Sort based on a particular row

The below example sorts a particular row in the 2D vector.

Say for example, the 2D matrix is:

```[[3, 5, 4],
[6, 4, 2],
[1, 7, 3]]
```

So if we sort the first row in ascending order the output will be:

```[[3, 4, 5],
[6, 4, 2],
[1, 7, 3]]
```
```#include <bits/stdc++.h>
using namespace std;

void print(vector<vector<int> > two_D_vector)
{
for (auto it : two_D_vector) {
//it is now an 1D vector
for (auto ij : it) {
cout << ij << " ";
}
cout << endl;
}
}

int main()
{
//2D vector initialized with
//user-defined elements only
vector<vector<int> > two_D_vector{
{ 3, 5, 4 },
{ 6, 4, 2 },
{ 1, 7, 3 }
};

//printing the 2D vector
cout << "printing the 2D vector before sorting\n";
print(two_D_vector);

//sorting the 2D array based on a particular row
//here we sort the first row of the 2D vector
//so, basically we sort the 1D array(the first row)
sort(two_D_vector.begin(), two_D_vector.end());

//print the 2D vector
cout << "printing the 2D vector after sorting\n";

print(two_D_vector);

return 0;
}
```

Output:

```printing the 2D vector before sorting
3 5 4
6 4 2
1 7 3
printing the 2D vector after sorting
3 4 5
6 4 2
1 7 3
```

So if we sort the first row in descending order the output will be:

```[[3, 5, 4],
[6, 4, 2],
[7, 3, 1]]
```
```#include <bits/stdc++.h>
using namespace std;

void print(vector<vector<int> > two_D_vector)
{
for (auto it : two_D_vector) {
//it is now an 1D vector
for (auto ij : it) {
cout << ij << " ";
}
cout << endl;
}
}

int main()
{
//2D vector initialized with
//user-defined elements only
vector<vector<int> > two_D_vector{
{ 3, 5, 4 },
{ 6, 4, 2 },
{ 1, 7, 3 }
};

//printing the 2D vector
cout << "printing the 2D vector before sorting\n";
print(two_D_vector);

//sorting the 2D array based on a particular row
//here we sort the last row of the 2D vector
//in descending order
//so, basically we sort the 1D array in
//descending order(the last row)
sort(two_D_vector.begin(), two_D_vector.end(), greater<int>());

//print the 2D vector
cout << "printing the 2D vector after sorting\n";

print(two_D_vector);

return 0;
}
```

Output:

```printing the 2D vector before sorting
3 5 4
6 4 2
1 7 3
printing the 2D vector after sorting
3 5 4
6 4 2
7 3 1
```

## 2) Sort based on a particular column

The below example sorts a particular column in the 2D vector.

Say for example, the 2D matrix is:

```[[3, 5, 4],
[6, 4, 2],
[1, 7, 3]]
```

So if we sort the first column in ascending order the output will be:

```[[1, 4, 5],
[3, 4, 2],
[6, 7, 3]]
```

Here we need to define our user-defined comparator function to do the above thing. Like we will take each element of the 2D vector (which is a 1D vector, to be specific each row) and compare based on the first element (or any particular element) only. That's why we need a user-defined comparator.

```#include <bits/stdc++.h>
using namespace std;

void print(vector<vector<int> > two_D_vector)
{
for (auto it : two_D_vector) {
//it is now an 1D vector
for (auto ij : it) {
cout << ij << " ";
}
cout << endl;
}
}

bool mycomp(vector<int>& A, vector<int>& B)
{
//if first element of first
//row<first element of second row
if (A < B)
return true; //no swap
//other wise swap the rows
return false;
}

int main()
{
//2D vector initialized with
//user-defined elements only
vector<vector<int> > two_D_vector{
{ 3, 5, 4 },
{ 6, 4, 2 },
{ 1, 7, 3 }
};

//printing the 2D vector
cout << "printing the 2D vector before sorting\n";
print(two_D_vector);

//sorting the 2D array based on a particular row
//here we sort the last row of the 2D vector
//in descending order
//so, basically we sort the 1D array in
//descending order(the last row)
sort(two_D_vector.begin(), two_D_vector.end(), mycomp);

//print the 2D vector
cout << "printing the 2D vector after sorting\n";

print(two_D_vector);

return 0;
}
```

Output:

```printing the 2D vector before sorting
3 5 4
6 4 2
1 7 3
printing the 2D vector after sorting
1 7 3
3 5 4
6 4 2
```

To sort in descending order, we need to just change the comparator function.

```#include <bits/stdc++.h>
using namespace std;

void print(vector<vector<int> > two_D_vector)
{
for (auto it : two_D_vector) {
//it is now an 1D vector
for (auto ij : it) {
cout << ij << " ";
}
cout << endl;
}
}
//to sort based on column in descending order
bool mycomp(vector<int>& A, vector<int>& B)
{
//if first element of first
//row<first element of second row
if (A < B)
return false; //swap the rows
//other wise  no swap
return true;
}

int main()
{
//2D vector initialized with
//user-defined elements only
vector<vector<int> > two_D_vector{
{ 3, 5, 4 },
{ 6, 4, 2 },
{ 1, 7, 3 }
};

//printing the 2D vector
cout << "printing the 2D vector before sorting\n";
print(two_D_vector);

//sorting the 2D array based on a particular row
//here we sort the last row of the 2D vector
//in descending order
//so, basically we sort the 1D array in
//descending order(the last row)
sort(two_D_vector.begin(), two_D_vector.end(), mycomp);

//print the 2D vector
cout << "printing the 2D vector after sorting\n";
print(two_D_vector);

return 0;
}
```

Output:

```printing the 2D vector before sorting
3 5 4
6 4 2
1 7 3
printing the 2D vector after sorting
6 4 2
3 5 4
1 7 3
```

There can be various use cases to sort a 2D vector and we need to write our comparator functions.

Exercises

(a) Sort based on row sizes in ascending order

```Say the 2D vector is
{
{2, 3, 4, 5},
{3, 4, 1},
{1}}

After sorting the 2D vector based on
row size in ascending order:
{
{1},
{3, 4, 1},
{2, 3, 4, 5}
}
```

Here we need to use our user define a function which will swap the rows as per their sizes.

```#include <bits/stdc++.h>
using namespace std;

void print(vector<vector<int> > two_D_vector)
{
for (auto it : two_D_vector) {
//it is now an 1D vector
for (auto ij : it) {
cout << ij << " ";
}
cout << endl;
}
}

//to sort based on column in descending order
bool mycomp(vector<int>& A, vector<int>& B)
{
//if first row size>second row size
if (A.size() > B.size())
return false; //swap the rows
//other wise  no swap
return true;
}

int main()
{
//2D vector initialized with
//use- defined elements only
vector<vector<int> > two_D_vector{
{ 2, 3, 4, 5 },
{ 3, 4, 1 },
{ 1 }
};

//printing the 2D vector
cout << "printing the 2D vector before sorting\n";
print(two_D_vector);

//sorting the 2D array based on a particular row
//here we sort the last row of the 2D vector
//in descending order
//so, basically we sort the 1D array in
//descending order(the last row)
sort(two_D_vector.begin(), two_D_vector.end(), mycomp);

//print the 2D vector
cout << "printing the 2D vector after sorting\n";

print(two_D_vector);

return 0;
}
```

Output:

```printing the 2D vector before sorting
2 3 4 5
3 4 1
1
printing the 2D vector after sorting
1
3 4 1
2 3 4 5
```

(b) Can you sort based on column size anyway?

If you can't sort in that way, then do comment why you cant?