Thread: Vector checking

You can check the vector's size. If the size is n, then there are n elements stored from [0, n-1].
Other than that, you can also try using the at method. If the index you are trying to access doesn't exist, it will throw an exception.

It sounds like you could make your life easier by using something like this.

Code:

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>  // for std::find

class Data {
private:
    std::vector<std::string> data;
public:
    void add(std::string s);
    bool contains(std::string s);
};

void Data::add(std::string s) {
    data.push_back(s);
}

bool Data::contains(std::string s) {
    std::vector<std::string>::iterator found
        = std::find(data.begin(), data.end(), s);
    
    // If s does not exist within the range [begin(), end()) then
    // end() will be returned, meaning that false should be returned
    // from this function. Likewise, if s does exist then some value
    // other than end() will be stored in found and true is thus
    // returned.
    
    return found != data.end();
}

int main() {
    Data data;
    
    data.add("one");
    data.add("two");
    
    if(data.contains("two")) std::cout << "contains \"two\"\n";
    else std::cout << "does not contain \"two\"\n";
    
    if(data.contains("four")) std::cout << "contains \"four\"\n";
    else std::cout << "does not contain \"four\"\n";
    
    return 0;
}

The output is

Code:

contains "two"
does not contain "four"

I'd rather use a map. Searching a vector is a O(n) operation.
Unless it's sorted, which would mean you could use a binary search... but I don't think there IS such a search in the standard library.

The primary part of dwks' suggestion is really about using the std::find generic algorithm. You do not have to define another class, though you could define a convenience function, e.g.,

Code:

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>  // for std::find

bool contains(const std::vector<std::string>& data, const std::string& s) {
    return std::find(data.begin(), data.end(), s) != data.end();
}

int main() {
    std::vector<std::string> data;

    data.push_back("one");
    data.push_back("two");

    if(contains(data, "two")) std::cout << "contains \"two\"\n";
    else std::cout << "does not contain \"two\"\n";

    if(contains(data, "four")) std::cout << "contains \"four\"\n";
    else std::cout << "does not contain \"four\"\n";

    return 0;
}

EDIT:

Originally Posted by Elysia

I'd rather use a map. Searching a vector is a O(n) operation.

It depends on the bigger picture too, of course.

Originally Posted by Elysia

Unless it's sorted, which would mean you could use a binary search... but I don't think there IS such a search in the standard library.

There is std::binary_search, std::lower_bound and std::upper_bound. std::binary_search would be the most appropriate in this proposed case.

Find appropriate objects to model and implement them correctly.

To me it sounds like cuo741 is just trying to add something to the end of the vector.
If that's the case, just use the push_back() function.

Code:

vector<string> items;
...
items.push_back( "some item" );