CODEWITHSUNDEEP
mathfloating-pointprecisionangle
polygenelubricants
polygenelubricants

Reputation: 383906

Find angle between hour and minute hands in an analog clock

I was given this interview question recently:

Given a 12-hour analog clock, compute in degree the smaller angle between the hour and minute hands. Be as precise as you can.

I'm wondering what's the simplest, most readable, most precise algorithm is. Solution in any language is welcome (but do explain it a bit if you think it's necessary).

Upvotes: 34

Views: 69974

Answers (10)

Mikey
Mikey

Reputation: 430

The java code that polygenlubricants is similar than mine. Let's assume that the clock is 12 hour instead of 24.

If it's 24 hours, then that's a different story. Also, another assumption, assume if the clock is stopped while we calculate this.

One clock cycle is 360 degree.

  1. How many degree can the minute hand run per minute? 360 / 60 = 6 degree per minute.

  2. How many degree can the hour hand run per hour? 360/12 = 30 degree per hour (since hour hand run slower than minute)

Since it's easier to calculate in the unit, "minute", let's get

"how many degree can the hour hand run per minute"?

30 / 60 = 0.5 degree per minute.

So, if you know how to get those numbers, the problem is pretty much solved with this partial mathematical solution.

Upvotes: 6

user4722851
user4722851

Reputation:

The problem is known as a “Clock Angle Problem” where we need to find the angle between the hands (hour & minute) of an analog clock at a particular time.

Solution in C Programming Language.

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>
#include<inttypes.h>
#include<assert.h>

#define STRING_LENGTH 6

double angle_between_hour_min_hand(char[]);

int main(void) {
    uint8_t test;
    printf("Enter the number of test cases\n");
    scanf("%"SCNu8,&test);
    assert(test>0);
    while(test--) {
        char time_digital[STRING_LENGTH];
        printf("Enter the time\n");
        scanf("%s",time_digital);
        double angle_between_hands_deg = angle_between_hour_min_hand(time_digital);
        abs(angle_between_hands_deg) < angle_between_hands_deg ? printf("%0.1f\n",angle_between_hands_deg) : printf("%d\n",abs(angle_between_hands_deg));
    }
    return 0;
}

double angle_between_hour_min_hand(char time_digital[]) {
    uint8_t hr,min;
    double hr_angle_deg,min_angle_deg,angle_between_hands_deg;
    char*buffer = calloc(sizeof(char),STRING_LENGTH);
    if(buffer) {
        snprintf(buffer,STRING_LENGTH,"%s",time_digital);
        hr = atoi(__strtok_r(buffer,":",&buffer));
        min = atoi(__strtok_r(NULL,":",&buffer));
        buffer -= strlen(time_digital);
        free(buffer);
        hr_angle_deg = (double)(30*hr) + (double) (0.5*min);
        // printf("hr-angle: %f\n", hr_angle_deg);
        min_angle_deg = 6*min;
        // printf("min-angle: %f\n", min_angle_deg);
        angle_between_hands_deg = (hr_angle_deg > min_angle_deg) ? hr_angle_deg - min_angle_deg : min_angle_deg - hr_angle_deg;
        if(angle_between_hands_deg > 180) {
            angle_between_hands_deg = 360 - angle_between_hands_deg;
        }
    }
    else fprintf(stderr,"Memory not allocated to the buffer pointer!\n");
    return angle_between_hands_deg;
}

Compile the above program in your system, I used Ubuntu 18.04 LTS Bionic Beaver, you can use any system which has a C compiler installed. 

gcc -Wall -g clock_angle_sol.c -o clock_angle_sol
./clock_angle_sol
Enter the time in 12-hour or 24 hour i.e (hr:min) format: 12:45
Angle: 112.00 degrees.

Notes:
1. The equation $\theta_{hr} = (30^\circ \times hour) + (0.5^\circ \times minute)$ will give you the angle made by hour hand in 12-hour clock.
2. If you want to calculate the angle made by hour hand in a 24-hour clock, then use the following equation: $\theta_{hr} = (15^\circ \times hour) + (0.25^\circ \times minute)$
3. Second hand also contribute to the rotation of the minute hand, but we ignored it because the contribution is insignificant i.e. 1/10 = 0.1.

Upvotes: 0

Dileep kurahe
Dileep kurahe

Reputation: 259

    **php code for find angle via time (minutes and hour's)**

    echo calcAngle(3,70);

function calcAngle($h, $m)
{
    // validate the input
    if ($h <0 || $m < 0 || $h >12 || $m > 60)
      {
       return "Wrong input";
      }
      else {

    if ($h == 12) $h = 0;
    if ($m == 60) $m = 0;

    $hour_angle = 0.5 * ($h*60 + $m);
    $minute_angle = 6*$m;
    $angle = abs($hour_angle - $minute_angle);
    $angle = min(360-$angle, $angle);

    return $angle;
}
}

Upvotes: 1

Santhosh
Santhosh

Reputation: 729

This is one solution (C#). This is a very simple solution and ignores precision. Hope the solution is self explanatory. 

public static double GetAngle(int hourHand, int minuteHand)
    {
        double oneMinuteAngle = (360 / 60);
        double oneHourAngle = (360 / 12);

        double hourAngle = oneHourAngle * hourHand;
        double minuteAngle = oneMinuteAngle * minuteHand;

        return (Math.Abs(hourAngle - minuteAngle));
    }

Upvotes: -1

polygenelubricants
polygenelubricants

Reputation: 383906

It turns out that Wikipedia does have the best answer:

// h = 1..12, m = 0..59
static double angle(int h, int m) {
    double hAngle = 0.5D * (h * 60 + m);
    double mAngle = 6 * m;
    double angle = Math.abs(hAngle - mAngle);
    angle = Math.min(angle, 360 - angle);
    return angle;
}

Basically:

  • The hour hand moves at the rate of 0.5 degrees per minute
  • The minute hand moves at the rate of of 6 degrees per minute

Problem solved.

And precision isn't a concern because the fractional part is either .0 or .5, and in the range of 0..360, all of these values are exactly representable in double.

Upvotes: 44

Dhiraj Himani
Dhiraj Himani

Reputation: 1102

Minute angle (from 12 o’clock): 360 * minutes / 60

Hour angle (from 12 o’clock): 360 * (hour % 12) / 12 + 360 * (minutes / 60) * (1 / 12)

Angle between hour and minute: (hour angle - minute angle) % 360 By simple arithmetic, this reduces to 30 * hours - 5.5 * minutes.

Upvotes: 1

akhilesh
akhilesh

Reputation: 1

for finding the angle between the hour hand and the minute hand is 

angle=(hour*5-min)*6

Upvotes: -3

user2688705
user2688705

Reputation:

Try this code :

import java.util.Scanner;

class Clock{

    public static void main(String args[]){
        int hours,mins;

    System.out.println("Enter the Time(hours) : ");
        Scanner dx = new Scanner(System.in);
        hours = dx.nextInt();

    System.out.println("Enter the time(mins) : ");
        Scanner fx = new Scanner(System.in);
        mins = fx.nextInt();

    if(hours>=0 && hours<=12){

        if(mins>=0 && mins<=59){
            double hDegrees = (hours * 30) + (mins * 0.5);
                    double mDegrees = mins * 6;
                    double diff  = Math.abs(hDegrees - mDegrees);

        System.out.println("The angle between sticks is (degrees) : "+diff);
                if (diff > 180){ 

                diff = 360 - diff;
        System.out.println("The angle between sticks is (degrees) : "+diff);
                }

        }

    }

    else{
        System.out.println("Wrong input ");
    }


}

}

Upvotes: 1

Suwaru Ram
Suwaru Ram

Reputation: 81

For finding the angle between the hands of a clock is ,

30 * [HRS - (MIN/5)] + (MIN/2)

Upvotes: 8

Anycorn
Anycorn

Reputation: 51535

I do not know if it's right, .something like this?

//m*360/60 - (h*360/24)+(m*360/(24*60)) ->
t = abs(25*m - 60*h)/4
t = min(t,360-t)

Upvotes: -3

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