Java - 12 H to 24 Hour Format (hackerrank)

Question

Note : I looked as much as I could for 2 days to check if this is a duplicate. If I missed something, I apologize. This question is to find what the issue is with my attempt at a solution, not one of the existing solutions out there.

My Question

I was trying to solve some problems on hackerrank in Java 7 and I came across the time conversion problem where the problem statement is:

Problem: "Given a time in -hour AM/PM format, convert it to military (24-hour) time."

Sample Input 07:05:45PM

Sample Output 19:05:45

I looked at solutions involving libraries (such as java.text.SimpleDateFormat, Calendar etc.) but I am trying to do it on my own without them. The issue that I am facing here is that my solution is failing on some test cases but is working on others. In short, it is not the correct solution. I see other solutions but I want to know why mine fails as the correct answer. Could you please help me by telling me where this would fail and how I can go about correcting it?

Some of the solutions that I looked at are:

Conversion from 12 hours time to 24 hours time in java

Convert 12 hours to 24 hours

My code is here:

import java.io.*;
import java.math.*;
import java.text.*;
import java.util.*;
import java.util.regex.*;

public class Solution {

    static String timeConversion(String s) {

        //get the string into an array using : as a separator
        String[] time_array = s.split(":");

        //military_time variable to be returned
        String military_time = new String();

        //final HH part
        String hh_final = new String();
        //Rest after HH to be concatenated to get military_time
        String rest = new String();


        StringBuilder REST_mil_builder = new StringBuilder();
        for (int i = 2; i < 8; i++) {
            REST_mil_builder.append(s.charAt(i));
        }
        //"rest" basically gets everything after HH excluding AM/PM, so 01:03:40PM would have a "rest" value of ":03:40"
        rest = REST_mil_builder.toString();

        int hh = Integer.parseInt(time_array[0]);
        String AMPM_contains = time_array[2];

        //converting if the last piece after the split contains "PM"
        if (AMPM_contains.contains("PM")) {
            hh = hh + 12;
            hh = hh == 24 ? 0 : hh;
        }

        //converting hh to have a 0 before it because when it is an integer 01 will be just 1 which we don't want
        StringBuilder hh_build = new StringBuilder();
        if (hh >= 0 && hh <= 9) {
            hh_build.append("0");
            hh_build.append(hh);
            hh_final = hh_build.toString();
        } else {
            hh_build.append(hh);
            hh_final = hh_build.toString();
        }

        //military time concatenation
        military_time = hh_final + rest;
        //Midnight is 12:00:00AM on a 12-hour clock, and 00:00:00 on a 24-hour clock
        military_time = s == "12:00:00AM" ? "00:00:00" : military_time;
        //Noon is 12:00:00PM on a 12-hour clock, and 12:00:00 on a 24-hour clock.
        military_time = s == "12:00:00PM" ? "12:00:00" : military_time;

        return military_time;

    }

    private static final Scanner scan = new Scanner(System.in);

    public static void main(String[] args)  {

        //tried several 12 hour time formats here
        String result = timeConversion("01:30:59PM");
        System.out.println(result);
    }
}

Time to learn how to use a debugger. Step through the program, look at the variables and how they change. — Robert, May 26 '18 at 19:27

Andreas · Accepted Answer · 2018-05-26T20:09:48.820

Your code fails because it doesn't correctly handle hour 12, i.e. 12:xx:xxAM should map to 00:xx:xx, and 12:xx:xxPM should map to 12:xx:xx, as pointed out in answer by Ole V.V.

Rather than trying to fix the overly complicated code you have, here is a different approach, without using SimpleDateFormat.

Parse the first 2 digits to a number. If the number is 12, set it to 0. A trick way to do that is to use modulo 12. If input ends with PM, add 12. Now rebuild string, replacing first 2 digits with new number, and removing AM/PM suffix.

Like this:

public static String timeConversion(String s) {
    int hour = Integer.parseInt(s.substring(0, 2)) % 12;
    if (s.endsWith("PM"))
        hour += 12;
    return String.format("%02d", hour) + s.substring(2, 8);
}

Using tests by Ole V.V.

System.out.println(timeConversion("12:30:59AM"));
System.out.println(timeConversion("11:30:59AM"));
System.out.println(timeConversion("12:30:59PM"));
System.out.println(timeConversion("11:30:59PM"));

Output

00:30:59
11:30:59
12:30:59
23:30:59

I like this solution not using `SimpleDateFormat`. – Oleg Cherednik May 26 '18 at 20:30 — Oleg Cherednik, May 26 '18 at 20:30

Ole V.V. · Answer 2 · 2018-05-26T20:44:36.727

    System.out.println(timeConversion("12:30:59AM"));
    System.out.println(timeConversion("11:30:59AM"));
    System.out.println(timeConversion("12:30:59PM"));
    System.out.println(timeConversion("11:30:59PM"));

Expected output:

00:30:59
11:30:59
12:30:59
23:30:59

Observed output:

12:30:59 (wrong)
11:30:59 (correct)
00:30:59 (wrong)
23:30:59 (correct)

Edit: In short it seems your code doesn’t take into account that hour 12 is different nn the 12 hour clock. You may say it’s used as hour 00. I don’t think it will be too hard to fix your program, so I will let you have the pleasure of the first shot. If you need a hint, please follow up in comments. We’re all happy to help.

As an aside, you are using new String() in three places in your code. I think it’s superfluous because you are never using the created empty strings for anything.

This is only an answer to *"Could you please help me by telling me where this would fail?"*, but not to *"how I can go about correcting it?"* --- Still, it is *useful*, so +1 — Andreas, May 26 '18 at 19:50

anoopknr · Answer 3 · 2018-05-26T19:21:23.770

0

Please try this static String timeConversion(String s) function.I hope this will help you.

static String timeConversion(String s) {

        String[] time = s.split(":");

        String hours = time[0];
        String minutes = time[1];
        String seconds = time[2].substring(0, 2);
        String dayEve = time[2].substring(2, 4);
        if (dayEve.equals("AM")) {
            return ((hours.equals("12") ? "00" : hours) + ":" + minutes + ":" + seconds);
        } else {
            return ((hours.equals("12") ? hours : (Integer.parseInt(hours) + 12)) + ":" + minutes + ":" + seconds);
        }

    }

The prime logic is :-

If 'AM' and hours == 12 then hours set to 00 else do not change hours.

If 'PM' and hours == 12 then do not change hours else hours is set to hours = hours+12.

edited May 26 '18 at 19:21

answered May 26 '18 at 19:14

anoopknr

3,177
2
23
33

2

But why reinvent the wheel if the standard Java libraries already have what you need? – Robert May 26 '18 at 19:26
If you're going to hardcode `substring` offsets, why not just do that instead of using `split`, i.e. `hours = s.substring(0, 2), minutes = s.substring(3, 5), seconds = s.substring(6, 8), dayEve = s.substring(8)` – Andreas May 26 '18 at 19:40
Thank you Anoop and Andreas. This helped me understand things a little more. – Rahul P May 26 '18 at 20:23

Oleg Cherednik · Answer 4 · 2018-05-26T20:18:14.810

0

Do use SimpleDateFormat:

public static String to24Time(String str) throws ParseException {
    DateFormat df12 = new SimpleDateFormat("hh:mm:ssa", Locale.US);
    DateFormat df24 = new SimpleDateFormat("HH:mm:ss", Locale.US);
    return df24.format(df12.parse(str));
}

P.S. This is task from Hackerrank: Algorithms/Warmup/Time Conversion.

There is not any notes that it is forbidden to use java internal classes for time parsing and you have to do it manually.
Provided answer gives 100% test pass for this task.

P.P.S In case you do not want to use SimpleDateTime, this is example how to use Regular Expression. Note, that I do not provide full validation of input sting; in some cases (e.g. for illegal 12h time format like 21:17:17PM) it returns illega result instead of exception:

public static String to24Time(String str) throws ParseException {
    final Pattern TIME_12 = Pattern.compile("(?<hour>\\d{2}):(?<minute>\\d{2}):(?<second>\\d{2})(?<part>am|pm)");
    Matcher matcher = TIME_12.matcher(str.toLowerCase());

    if (matcher.matches()) {
        int hour = Integer.parseInt(matcher.group("hour"));
        return ("pm".equals(matcher.group("part")) ? hour + 12 : hour) + ":" + matcher.group("minute") + ':' + matcher.group("second");
    }

    return null;
}

edited May 26 '18 at 20:18

answered May 26 '18 at 19:19

Oleg Cherednik

17,377
4
21
35

@OleV.V. Do not agree. It works perferctly for years for it's job and it is not deprecated. Yes, I know there're other time libraty with ZonedDateTime etc. You're free to use what you like. I use SimpleDateTime sometime - and I HAVE NEVER FACED WITH PROBLEM using it. – Oleg Cherednik May 26 '18 at 19:38
@OleV.V. Downloading a 3rd-party library, just for such a small task, is overkill. Besides, this is for hackerrank (question says so), so you can't add 3rd-party library. Your arguments are invalid. So, constrained to Java 7 and no libraries, please present a valid argument for not using `SimpleDateFormat`. – Andreas May 26 '18 at 19:41
@OleV.V. If that's your argument for not using `SimpleDateFormat`, then you should have made that argument to begin with. But, you're right, that *is* a valid argument. – Andreas May 26 '18 at 19:53
The asker was explicit about trying to solve the problem without `SimpleDateFormat`, so your answer isn’t helpful. For real-world/production code one should of course use a library class, namely `DateTimeFormatter` from [`java.time`, the modern Java date and time API](https://docs.oracle.com/javase/tutorial/datetime/), and should shun `SimpleDateFormat`. The latter is long outdated and notoriously troublesome. Yes, I too have used it in the past without problems, but such an observation subtracts nothing from the trouble that very many programmers have had with that class. – Ole V.V. May 26 '18 at 20:31

score 0 · Answer 5 · answered Sep 03 '19 at 12:00

Here is how I solved it using JavaScript:

function timeConversion(s) {


    var res;

    var pos = s.substring(-2, 8);

    var split = pos.split(':');

    if (split[0] == 12 && s.endsWith('AM')) {
        res = '00' + ':' + split[1] + ':' + split[2];

    } else if (split[0] == 12 && s.endsWith('PM')) {
        res = '12' + ':' + split[1] + ':' + split[2];

    } else if (split[0] < 12 && s.endsWith('AM')) {
        res = split[0] + ':' + split[1] + ':' + split[2];

    } else if (split[0] < 12 && s.endsWith('PM')) {
        var add = Number(split[0]) + 12;
        res = add + ':' + split[1] + ':' + split[2];
    }
    return res;
}

score 0 · Answer 6 · answered Jul 05 '20 at 18:35

static String timeConversion(String s) {
    int hrs = 12;
    String[] split = s.split(":");
    String originalAMPM = split[2].substring(2,4);
    String hours = split[0]; String minutes = split[1]; String seconds = split[2].substring(0,2);
    if(originalAMPM.equals("PM")) {
        if(!hours.equals("12"))
            hrs = Integer.parseInt(hours) + 12;
        return hrs + ":" + minutes + ":" + seconds;
    }
    else {
        if(hours.equals("12"))
            hours = "00";
        return hours + ":" + minutes + ":" + seconds;
    }
}

score 0 · Answer 7 · answered Jul 09 '20 at 08:48

#!/bin/python3

import os
import sys
import re
#
# Complete the timeConversion function below.

def timeConversion(s):

    if s[len(s)-2:]=='PM':
        
        if s[0] == '0':
            s1=s.strip('0')
            sn = s1.strip('PM')
            return sn.replace(sn[0],str(int(sn[0])+12))

        elif s[0:2]=='12':
            
            return s.strip('PM')
        else:
            sn = s.strip('PM')
            return sn.replace(sn[0:2],str(int(sn[0:2])+12)) 

    else: #s[len(s)-2:]=='AM':

        if s[0:2]=='12':
            s1=s.strip('AM')
            return s1.replace(s[0:2],'00')
        else:
            return s.strip('AM') 
                     
        

    
if __name__ == '__main__':
    f = open(os.environ['OUTPUT_PATH'], 'w')

    s = input()

    result = timeConversion(s)

    f.write(result + '\n')

    f.close()

score 0 · Answer 8 · answered Oct 02 '21 at 08:33

Here is how I solved it using JavaScript:

function timeConversion(s) {
    let time = [];
    let mid = s.slice(0,2);
    if(s.includes('PM')){        
        if( mid === '12' ){
            time = s;
        } else{
          let mid1 = Number(mid)+12;
          time = s.replace(mid, mid1.toString());   
        }             
    } else if(mid === '12'){ 
       time = s.replace(mid, '00');        
    } else time = s;    
   return time.slice(0,-2);
};

score 0 · Answer 9 · answered Nov 10 '21 at 15:09

SimpleDateFormat df = new SimpleDateFormat("hh:mm:ss aa");
StringBuilder std = new StringBuilder(s);
if(std.toString().contains("P"))
{
   std.insert(std.indexOf("P"), " ");
}else{
   std.insert(std.indexOf("A"), " ");
}
   
String.format("%tT",df.parse(std.toString())));

Java - 12 H to 24 Hour Format (hackerrank)

9 Answers9