How do I compare 2 objects if they have three values to be tested against? [duplicate]-CodePudding

I'm writing a less than and equal operator function for a class object.

The three variables that are apart of the class are

string firstName;
string LastName;
int age;

To compare them, I wrote this operator function:

bool person::operator < (person b)
{
    if (firstName < b.firstName && lastName < b.lastName && age < b.age)
        return true;
    else
        return false;
}

However, my code doesn't work and I don't understand why.

When comparing the objects, priority goes to lastName, firstName, then age.

So, Bill Gates 43 would be technically less than Bill Gates 44.

I hope that is clear enough.

CodePudding user response：

Here's my version of the comparison:

bool person::operator< (const person& b)
{
    if (lastName != b.lastName)
    {
        return lastName < b.lastName;
    }
    if (firstName != b.firstName)
    {
        return firstName < b.firstName;
    }
    return age < b.age;
}

If one field is equal to the object, then the next field is compared. Otherwise the method compares the two fields for less than.

CodePudding user response：

Your comparison logic is wrong. You should write something like:

bool person::operator<(person b) {
    if (lastName < b.lastName) {
        return true;
    } else if (lastName > b.lastName) {
        return false;
    }

    if (firstName < b.firstName) {
        return true;
    } else if (firstName > b.firstName) {
        return false;
    }

    if (age < b.age) {
        return true;
    }

    return false;
}

CodePudding user response：

Here is a solution using std::tie

It makes writing comparison operators like this very simple and very hard to get wrong.

#include <algorithm>
#include <iostream>
#include <string>
#include <tuple>
#include <vector>

struct Person
{
    std::string firstName;
    std::string lastName;
    int age;

    bool operator<(const Person &other) const
    {
        return std::tie(lastName, firstName, age)
            < std::tie(other.lastName, other.firstName, other.age);
    }
};

void print(const std::string& title, const std::vector<Person> &people)
{
    std::cout << title << "\n";
    for (const Person &person : people)
    {
        std::cout << person.lastName << ", " 
            << person.firstName << ", " 
            << person.age << "\n";
    }
}

int main()
{
    std::vector<Person> people {
        { "Bill", "Gates", 44 },
        { "Gates", "Bill", 44 },
        { "Back", "Gates", 43 },
        { "Front", "Gates", 44 },
        { "Bill", "Gates", 43 },
        { "Gates", "Bill", 43 },
        { "Back", "Gates", 44 },
        { "Front", "Gates", 43 },
    };

    print("Before Sort:", people);
    std::sort(people.begin(), people.end());
    print("After Sort:", people);

    return 0;
}

Online example: https://ideone.com/YFnEdB

Output:

Before Sort:
Gates, Bill, 44
Bill, Gates, 44
Gates, Back, 43
Gates, Front, 44
Gates, Bill, 43
Bill, Gates, 43
Gates, Back, 44
Gates, Front, 43
After Sort:
Bill, Gates, 43
Bill, Gates, 44
Gates, Back, 43
Gates, Back, 44
Gates, Bill, 43
Gates, Bill, 44
Gates, Front, 43
Gates, Front, 44

CodePudding user response：

Your code doesn't work for Bill Gates 43 and Bill Gates 44, because you are only using less than operator here and Bill < Bill will return false which makes the entire if statement false.

bool person::operator < (const person& b) //Always pass your object as reference
{
    if (firstName <= b.firstName && lastName <= b.lastName && age < b.age)
        return true;
    else
        return false;
}

Now it will work for the example you gave. But a better way of doing this would be like this considering you want to compare all three attributes in given order.

bool person::operator < (const person& b)
{
     if (lastName != b.lastName)
       {
         return lastName < b.lastName;   
       }     
    else if(firstname != b.firstName)
       {
            return firstname < b.firstName;
       }
      else
         return age < b.age;
         
    return false;
}