How can I build a std::vector and then sort them?

Question

I have a bunch of strings that I need to sort. I think a std::vector would be the easiest way to do this. However, I\'ve never used vectors before and so would like some help.

Answer 1

You can just do

std::sort(data.begin(), data.end());

And it will sort your strings. Then go through them checking whether they are in order

if(names.empty())
    return true; // empty vector sorted correctly
for(std::vector<std::string>::iterator i=names.begin(), j=i+1; 
        j != names.end(); 
        ++i, ++j)
    if(*i > *j)
        return false;
return true; // sort verified

In particular, std::string::compare couldn't be used as a comparator, because it doesn't do what sort wants it to do: Return true if the first argument is less than the second, and return false otherwise. If you use sort like above, it will just use operator<, which will do exactly that (i.e std::string makes it return first.compare(second) < 0).

Answer 2

Try using comaprator:

 #include <cmath>
 #include <cstdio>
 #include <vector>
 #include <iostream>
 #include <algorithm>
 using namespace std;

//comparing function only sorts if string size is equal and keeps the larger integgers at last.
bool myfunction (string i,string j) 
{ 
int n=i.length();
int m=j.length();
if(n==m)
    return (i<j);

return n<m;   
  }


int main() {
int n;
cin>>n;
vector <string> arr(n);
for(int i=0;i<n;i++)
    cin>>arr[i];


sort(arr.begin(),arr.end(),myfunction);

for(int i=0;i<n;i++)
    cout<<arr[i]<<endl;

return 0;
 }

Answer 3

What is the question exactly? It seems everything is already there.

However, you should probably use std::cout << *i << std::endl;

1. i is an iterator == pointer to the data in the container, so * is needed
2. c_str() is a function of std::string and not a variable

The problems in your code do not relate to your question?

Some hints for you:

• std::vector also overrides [] operator, so you can instead save the iterator hassle and use it like an array (iterate from 0 to vector.size()).
• You could use std::set instead, which has automatically sorting on insertion (binary tree), so you save the extra sorting.
• Using a functor makes your output even more fun: copy(V.begin(), V.end(), ostream_iterator<std::string>(cout, "\n"));

Answer 4

For sort use:
std::sort or std::vector< std::string>::sort(..) method.
To check if it is sorted:
use std::is_sorted for check is sorted - http://www.sgi.com/tech/stl/is_sorted.html
or
std::adjacent_find( v.begin(), v.end(), std::greater< std::string >() ) == v.end()

for your case you could use default comparator

EDITED:
std::is_sorted is not standard stl function, it defined in sgi stl implementation.
Thanks @Brian Neal for this note.

Answer 5

You could use a std::set, which is naturally a sorted container.

Answer 6

litb is correct, as always.

I just wanted to point out the more general point - anything that can be compared with < can be sorted with std::sort. I'll sometimes sneak an operator< member function into a struct, just so I can do this.