Prefixes

Questions 1 to 5 of 6

1. 1. What does this program print? ``cpp #include <iostream> #include <string> #include <vector> std::string shorten_prefix(const std::vector<std::string>& words) { if (words.empty()) return ""; std::string prefix = words.front(); for (const std::string& w : words) { while (!w.starts_with(prefix)) { prefix.pop_back(); } } return prefix; } int main() { std::cout << "[" << shorten_prefix({"carpet", "cart", "cartoon"}) << "]\n"; } ``

   [carp][car][cart][]
2. 2. What does this program print? ``cpp #include <cstddef> #include <iostream> #include <string> #include <vector> std::string column_scan(const std::vector<std::string>& words) { if (words.empty()) return ""; const std::string& first = words.front(); for (std::size_t col = 0; col < first.size(); ++col) { for (const std::string& w : words) { if (col >= w.size() || w[col] != first[col]) { return first.substr(0, col); } } } return first; } int main() { std::cout << "[" << column_scan({"same", "sample"}) << "]\n"; } ``

   [sa][same][sample][sam]
3. 3. This function should return the longest common prefix of a list of words. What is the bug? ``cpp std::string shorten_prefix(const std::vector<std::string>& words) { std::string prefix = words.front(); for (const std::string& w : words) { while (!w.starts_with(prefix)) { prefix.pop_back(); } } return prefix; } ``

   The loop never ends, because an empty string does not start with itself.It calls words.front() without checking for an empty list, and front() on an empty vector reads a value that does not exist.pop_back() removes the first letter instead of the last, so the answer is always empty.starts_with is reversed, so the function returns the longest word instead of the shared start.
4. 4. This function should return the longest common prefix of a list of words. What is the bug? ``cpp std::string shorten_prefix(const std::vector<std::string>& words) { if (words.empty()) return ""; std::string prefix = words.front(); for (const std::string& w : words) { while (!w.starts_with(prefix)) { prefix.erase(0, 1); // remove the first letter } } return prefix; } ``

   The prefix must be a const std::string, so the function cannot shorten it.It returns the first word instead of the shared start.erase(0, 1) removes the first letter, so flower, flow and flight shrink to an empty string, though they share fl.The while loop should use < where it uses !.
5. 5. The C++20 member function that checks whether a std::string begins with a given piece is named ____. Write the name only.

   Type your answer.