Question

When using this code to remove duplicates I get invalid operands to binary expression errors. I think that this is down to using a vector of a struct but I am not sure I have Googled my question and I get this code over and over again which suggests that this code is right but it isn’t working for me.

std::sort(vec.begin(), vec.end());
vec.erase(std::unique(vec.begin(), vec.end()), vec.end());

Any help will be appreciated.

EDIT:

fileSize = textFile.size();
vector<wordFrequency> words (fileSize);
int index = 0;
for(int i = 0; i <= fileSize - 1; i++)
{
    for(int j = 0; j < fileSize - 1; j++)
    {
        if(string::npos != textFile[i].find(textFile[j]))
        {
            words[i].Word = textFile[i];
            words[i].Times = index++;
        }
    }
    index = 0;
}

sort(words.begin(), words.end());
words.erase(unique(words.begin(), words.end(), words.end()));

Answer 1

First problem.

unique used wrongly

unique(words.begin(), words.end(), words.end()));

You are calling the three operand form of unique, which takes a start, an end, and a predicate. The compiler will pass words.end() as the predicate, and the function expects that to be your comparison functor. Obviously, it isn’t one, and you enter the happy world of C++ error messages.

Second problem.

either use the predicate form or define an ordering

See the definitions of sort and unique.

You can either provide a

bool operator< (wordFrequency const &lhs, wordFrequency const &rhs)
{
    return lhs.val_ < rhs.val_;
}

, but only do this if a less-than operation makes sense for that type, i.e. if there is a natural ordering, and if it’s not just arbitrary (maybe you want other sort orders in the future?).

In the general case, use the predicate forms for sorting:

auto pred = [](wordFrequency const &lhs, wordFrequency const &rhs)
{
    return lhs.foo < rhs.foo;
};

sort (words.begin(), words.end(), pred);
words.erase (unique (words.begin(), words.end(), pred));

If you can’t C++11, write a functor:

struct FreqAscending { // should make it adaptible with std::binary_function
    bool operator() (wordFrequency const &lhs, wordFrequency const &rhs) const
    { ... };
};

I guess in your case (“frequency of words”), operator<makes sense.

Also note vector::erase: This will remove the element indicated by the passed iterator. But, see also std::unique, unique returns an iterator to the new end of the range, and I am not sure if you really want to remove the new end of the range. Is this what you mean?

words.erase (words.begin(),
             unique (words.begin(), words.end(), pred));

Third problem.

If you only need top ten, don’t sort

C++ comes with different sorting algorithms (based on this). For top 10, you can use:

• nth_element: gives you the top elements without sorting them
• partial_sort: gives you the top elements, sorted

This wastes less watts on your CPU, will contribute to overall desktop performance, and your laptop batteries last longer so can do even more sorts.