45

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.

I just need to sort them alphanumerically, nothing special. Indeed, the string::compare function would work.

After that, how can I iterate through them to verify that they're sorted?

Here's what I have so far:

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

for(std::vector<std::string>::iterator i = data.begin(); i != data.end(); ++i)
{
    printf("%s\n", i.c_str);
}
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1

7 Answers 7

Reset to default
80

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).

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3 Comments

Just for fun (and untested): Checking that the vector is sorted could be simplified to std::adjacent_find(names.begin(), names.end(), std::greater<std::string>()) == names.end()
This doesn't sort correctly according to case. Uppercase characters will be sorted before lowercase. I.e., it's basically useless.
@James according to the problem statement "std::string::compare would work".
5

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"));
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2

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.

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2

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.

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2 Comments

You shouldn't rely on that sgi site for info on the STL. It predates the standard. is_sorted isn't standard.
correct me if i'm wrong : is_sorted is added in c++11 en.cppreference.com/w/cpp/algorithm/is_sorted
1

Sorting the string:

using namespace std; // to avoid using std everywhere 
std::sort(data.begin(), data.end()); // this will sort the strings

Checking whether vector is sorted:

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

C++11 Method to check sorted vector: std::is_sorted(vec.begin(),vec.end())

Now printing the sorted vector:

   for(std::vector< std::string>::iterator i = vec.begin(); i != vec.end(); ++i)
{
    std::cout<< *i <<std::endl;
}
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5 Comments

I've checked on codeblocks and ideone and this WORKS just fine.
Since C++11, checking if the vector is sorted is simply std::is_sorted(vec.begin(),vec.end())
You shouldn't rely on that sgi site for info on the STL. It predates the standard. is_sorted isn't standard
is_sorted isn't standard
AFAIK, is_sorted was added in C++11
1

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

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2 Comments

Limitation: std::set doesn't support random access like std::vector.
std::set doesn't accept duplicates.
0

Try using comparer:

// 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(), mFfunction);

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

    return 0;
 }
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