Comparing characters at the same index of two strings

Question

I am trying to compare the characters of two strings at a given index. However, I am getting the following error:

Line 9: Char 26: error: invalid operands to binary expression ('const __gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' (aka 'const char') and 'const std::__cxx11::basic_string<char>')
                if (s[i] != letter) { return s.substr(0, i); }
                    ~~~~ ^  ~~~~~~

Here is my code:

string longestCommonPrefix(vector<string>& strs) {
    auto comp = [](const string& a, const string& b) { return a.size() < b.size(); };
    const auto smallest = min_element(strs.begin(), strs.end(), comp);
    for (int i = 0; i < smallest->size(); ++i) {
        const auto letter = smallest[i];
        for (const auto& s : strs) {
            if (s[i] != letter) { return s.substr(0, i); }  // <- error occurs here
        }
    }
    return smallest;
}

Could someone please explain why this does not work?

Answer 1

In your code, smallest is a

std::vector<std::string>::iterator const

which is a random access iterator, hence provides operator[].

However,

smallest[i]

is equivalent to

*(smallest + i)

so it will return a reference to the i-th string after the one pointed to by smallest instead of the i-th character of the string pointed to by smallest (which is what you want).

To get the i-th character of the string pointed to by smallest, you should first deference it:

(*smallest)[i]

And you should deference it as well in the return statement.

---

Also, I recommend you pass strs as a reference to const and check that smallest is not the end iterator (which may occurr if the input vector is empty):

string longestCommonPrefix(vector<string> const& strs) {
    auto comp = [](const string& a, const string& b) {
        return a.size() < b.size();
    };

    const auto smallest = std::min_element(strs.begin(), strs.end(), comp);
    if (smallest == strs.end()) return "";

    for (int i = 0; i < smallest->size(); ++i) {
        const auto letter = (*smallest)[i];
        for (const auto& s : strs) {
            if (s[i] != letter) return s.substr(0, i);
        }
    }

    return *smallest;
}