Sort function in c++ with objects?

Question

I have the following program to compare time in the form of Hours:minutes:seconds.

class time
{
public:

    string Hours;
    string Minutes;
    string Seconds;
};

bool CompareTimes(time A, time B)
{
    if (A.Hours < B.Hours)
    {
        return true;
    }

    if (A.Minutes < B.Minutes)
    {
        return true;
    }

    if (A.Seconds < B.Seconds)
    {
        return true;
    }

    return false;
}

And in the main...

sort(TimeArray, TimeArray + NumberOfTimes, CompareTimes);

However, this does not seem to sort it properly. On the other hand, if I change the CompareTimes method to the following:

bool CompareTimes(time A, time B)
    {
        if (A.Hours > B.Hours)
        {
            return false;
        }

        if (A.Minutes > B.Minutes)
        {
            return false;
        }

        if (A.Seconds > B.Seconds)
        {
            return false;
        }

        return true;
    }

Then everything works properly. I thought the sort function needs to return true if the second input is greater than the first input. Why did it not work in the first case, but work in the second case?

Possible duplicate of Operator< and strict weak ordering or How to map a bool to a 3d point struct with std::map? (I prefer the accepted answer of the latter) — Oktalist
– Oktalist, Commented Sep 26, 2013 at 13:27

memo1288 · Accepted Answer · 2013-09-26 02:23:21Z

2

if (A.Hours < B.Hours)
{
        return true;
}

After that, you have two options: the hours are equal, or A.hours>B.hours. If they are equal, then it makes sense to compare the minutes. If A has more hours, then it makes no sense to compare the minutes. Your second condition should be:

if (A.Hours == B.Hours && A.Minutes < B.Minutes)

Similarly, your third condition should be:

if (A.Hours == B.Hours && A.Minutes == B.Minute && A.Seconds < B.Seconds)

The last return should remain the same. Also, be aware that storing these as strings will cause them to be ordered alphabetically. (or in order of their ASCII code).

answered Sep 26, 2013 at 2:23

memo1288

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Comments

Balamurugan A · Accepted Answer · 2013-09-26 10:17:06Z

I think to return true, you need to check all three conditions in one shot.

return (A.Hours*60*60 +A.Minutes*60+A.Seconds > B.Hours*60*60 + B.Minutes*60 + B.Seconds);

Just wrote the sample as below.

#include<iostream>
#include<stdlib.h>
#include<string.h>
#include<stdio.h>
using namespace std;
class time_
{
public:
    string Hours;
    string Minutes;
    string Seconds;
    time_(int h,int m,int s){ 
        char buf[6]={0,};
        memset(buf,0,sizeof(buf));sprintf(buf,"%d",h);Hours+=buf;
        memset(buf,0,sizeof(buf));sprintf(buf,"%d",m);Minutes+=buf;
        memset(buf,0,sizeof(buf));sprintf(buf,"%d",s);Seconds+=buf;
    }   
};

bool CompareTimes(time_ A, time_ B){ 
    return (\
        ((atoi(A.Hours.c_str())*60*60)+(atoi(A.Minutes.c_str())*60)+atoi(A.Seconds.c_str())) > \
        ((atoi(B.Hours.c_str())*60*60)+(atoi(B.Minutes.c_str())*60)+atoi(B.Seconds.c_str())));
}
int main(){
    time_ A(10,10,10);
    time_ B(10,10,11);
    std::cout<<(CompareTimes( A, B)?"greater":"smaller")<<endl;
    time_ A1(10,11,10);
    time_ B1(10,10,10);
    std::cout<<(CompareTimes( A1, B1)?"greater":"smaller")<<endl;
}

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