How to update values in C++ std::pair

Question

I have this function that returns the location of a robot (which is just a [row][col] pair of indices from a matrix):

std::pair<int, int> World::getRobotLocation(char robot_name){
    auto const & location = robots.find(robot_name);
    if (location == robots.end()) {
        std::cout << "Robot " << robot_name << " does not exist." << std::endl;
    }
    return location->second;
}

Below, I am trying to implement the move() function, which takes in the robot name, location and which direction to move and updates the position accordingly:

std::string move(char robot, char direction) {
    // Get robot and its location
    std::pair<int, int> robot_location = std::pair<int, int> World::getRobotLocation(robot);

    // Get direction to move from user
        // if L, map_[row+1][col]
        // if D, map_[row][col+1]
        // if R, map_[row-1][col]
        // if U, map_[row][col+1]
    // According to user input, update the robot's location

    if (direction == 'L') { 
        robot_location = robot_location[+1][]
    }
    else if (direction == 'D') { 
        robot_location = robot_location[][-1]
    }
    else if (direction == 'R') { 
        robot_location = robot_location[-1][]
    }
    else { 
        robot_location = robot_location[][+1]
    }

}

In my variable robot_location, I am saving the location of that particular robot. How can I access the values of this std::pair<int, int> to be able to update them?

You either need a reference to the pair or a pair of reference_wrapper — Goswin von Brederlow, Jun 06 '22 at 05:30
@GoswinvonBrederlow Or a pair of plain references, `std::pair` — Ted Lyngmo, Jun 06 '22 at 05:32
Note that `return location->second` has undefined behaviour when `location == robots.end()` — Alan Birtles, Jun 06 '22 at 05:41
Also note this code has a design flaw (probably). Your find function returns a **copy** of the robot location, modifying that copy will do nothing to update the location in `robots`. Probably your find function should return an iterator or reference to the `robots` data structure, not a pair. — john, Jun 06 '22 at 06:32

score 3 · Accepted Answer · edited Jun 06 '22 at 06:47

Your first function has a bug. It reports when a robot is not found, but still dereferences the end iterator, which causes undefined behavior. Instead, you should return a pointer, which is conditionally null:

// Returns null if the robot is not found:
std::pair<int, int>*
World::getRobotLocation(char robot_name){
    auto const location = robots.find(robot_name);
    if (location == robots.end()) {
        return nullptr;
    }
    return &location->second;
}

And in your other function, you check to see, if the pointer is not null, you update the value:

// Returns true if move happens, 
// false otherwise.
bool
move(char robot, char direction) {
    auto const robot_location = World::getRobotLocation(robot);

    if (!robot_location) return false;

    switch (direction) {
        case 'L': {
            ++robot_location->first;
        } break;
        case 'D': {
            --robot_location->second;
        } break;
        case 'R': {
            --robot_location->first;
        } break;
        default: {
            ++robot_location->second;
        } break;
    }
    return true;
}

score 1 · Answer 2 · answered Jun 06 '22 at 05:32

I think you want something like this:

    if (direction == 'L') { 
        robot_location.first += 1;
    }
    else if (direction == 'D') {
        robot_location.second -= 1;
    }

    // ...etc (you get the idea)

first and second are the names of the two elements in a std::pair (that's also why the return value of robots.find(...) dereferences to something that has first and second - that's a std::pair of the key and value types of the map).

Bear in mind that getRobotLocation, as it's currently written, will return a copy of the std::pair of coordinates, not a reference to the original coordinates inside the map. Therefore, just updating that std::pair won't be enough on its own. You'll need to either save the value back into the robots map, or change what getRobotLocation returns (see @Goswin and @Ted's comments).

How to update values in C++ std::pair

2 Answers2