Tudor Gălățan - 5 months ago 40

C++ Question

I have a list of vectors from STL. They have different lengths and I want to sort them by the first element of each vector. For example, I have the vectors: 4 **4** 5 6 10, 1 **8**, 2 **2** 3 and 3 **1** 7 9. The first number is the size of each vector and it is followed by the vector's elements. I want to display the vectors in a file, sorted by the first element. In this example, my vectors should display in this order: 3 **1** 7 9, 2 **2** 3, 4 **4** 5 6 10 and 1 **8**.

Here is my code:

`ofstream fout ("retele.out");`

fout << T << '\n';

for (i=1; i<=T; i++)

{

fout << sol[i].size() << ' ';

sort(sol[i].begin(),sol[i].end());

for (j=0; j<sol[i].size(); j++)

fout << sol[i][j] << ' ';

fout << '\n';

}

fout.close();

"T" is the number of vectors. The "sort" function is for sorting each vector, and I also need an another sort for sorting the vectors as I just described.

Answer

A small clarification for other answers: the `std::vector<T>`

already has `operator<`

with exactly the same behavior as you described (cplusplus description).

`template < class T, class Alloc > bool operator < (const vector<T, Alloc>& lhs, const vector<T, Alloc>& rhs);`

Description:

The less-than comparison (operator<) behaves as if using algorithm lexicographical_compare, which compares the elements sequentially using operator< in a reciprocal manner (i.e., checking both a < b and b < a) and stopping at the first occurrence.

So, you can just write

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

*UP*: It's written in the comments that you have an array of `std::vector`

, not `std::list<std::vector>`

. The array has no methods `begin()`

and `end()`

, so this exact code will not work. The solution is

```
sort(sol, sol + SIZE);
```

where `SIZE`

is size of the part of the array you want to sort.