- Problem description:
- Given n seconds
- Compute the number of hours, number of minutes and number of seconds in n seconds
- Example:
n = 15432 seconds 15432 = 4*(60*60) + 17*(60) + 12 = 4 hours + 17 minutes + 12 seconds
- Given n seconds
- Advice on developing algorithms:
- Use a concrete example to
discover the steps that need to be
performed to solve the problem.
- Determine the
steps that you must
do to
complete the task (solve the problem)
- Write down the steps
in "pseudo code"
(see below)
- Verify that the
algorithm in pseudo code is
correct,
- You can verify an algorithm by running the algorithm by hand with a small example
- After you have Verify that the algorithm in pseudo code is correct,
- Use a concrete example to
discover the steps that need to be
performed to solve the problem.
- Pseudo code:
- Pseudo code
is a compact and
informal high-level description
of an algorithm
- Pseudo code uses the
structural conventions of a
programming language, but is
intended for human reading
(rather than machine reading).
- Pseudo code
omits details
that are not essential
for human understanding of the algorithm,
such as variable declarations
In other words:
- Pseudo code are
commands
that are
easy for humans
to follow (no complex descriptions)
- Pseudo code are not Java statements !!
- Pseudo code are
commands
that are
easy for humans
to follow (no complex descriptions)
- Pseudo code is made up by yourself
There is no "standard pseudo code"
- Requirement for pseudo code:
- An algorithm in pseudo code must be easily translated into any programming language
- Purpose: rapid prototyping algorithms
- An algorithm written
in pseudo code
is very easy to change
- You can correct errors easily
in an algorithm written in
pseudo code
(Trust me, correcting errors in a large computer program is a pain - talk to someone in Microsoft....)
- An algorithm written
in pseudo code
is very easy to change
- Pseudo code
is a compact and
informal high-level description
of an algorithm
- Use a concrete example to determine the steps used to find (hours, minutes, seconds):
Let n = 15432 seconds 1. We can find the # seconds as follows: 257 <---- # minutes ------------- 60 / 15432 15420 -------- 12 <--- # seconds So: 15432 seconds = 257 minutes + 12 seconds Or: 15432 seconds = 15432 / 60 minutes + 15432 % 60 seconds
2. Next we convert the remaining 257 minutes into hours: 4 <--- # hours -------- 60 / 257 240 ---- 17 <--- # minutes Therefore: 257 minutes = 257 / 60 hours + 257 % 60 minutes
- Pseudo code:
input n; // n = total number of seconds seconds = n % 60; // computes seconds n = n / 60; // n is now = remaining minutes minutes = n % 60; // computes minutes n = n / 60; // n is now = remaining hours hours = n;
- Verify the algorithm (in pseudo code):
(use a small example, e.g.
n = 15432)
n = 15432; hours = 15432 / 3600 = 4 r = 15432 % 3600 = 1032 minutes = 1032 / 60 = 17 seconds = 1032 % 60 = 12
- Write the algorithm in Java (translate from pseudo code):
import java.util.Scanner; public class Hours { public static void main(String[] args) { int n, r, hours, minutes, seconds; Scanner in = new Scanner(System.in); System.out.print("Enter # seconds: "); n = in.nextInt(); // in.nextInt() reads in an integer value seconds = n % 60; // computes seconds n = n / 60; // n is now = remaining minutes minutes = n % 60; // computes minutes n = n / 60; // n is now = remaining hours hours = n; System.out.print(n); System.out.print(" seconds = "); System.out.print(hours); System.out.print(" hours + "); System.out.print(minutes); System.out.print(" minutes+ "); System.out.print(seconds); System.out.println(" seconds."); } }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile: javac Hours.java
- To run: java Hours
- Preliminary:
- The
method
System.currentTimeMillis()
will return the following value:
- System.currentTimeMillis() returns the number of milli seconds (1/1000 second) since midnight, Jan 1 1970 in GMT
- The
method
System.currentTimeMillis()
will return the following value:
- Problem description:
- Print the current time as: HH:MM:SS GMT
- Use a concrete example
to determine the steps
used to find (hours, minutes, seconds):
Let n = 100000000 millisec 1. Find total number of seconds since midnight, Jan 1 1970: n = n / 1000 = 100000000 / 1000 = 100000 (total # seconds) 2. Find seconds in current time: seconds = n % 60 = 100000000 % 1000 = 40 *** part of answer n = n / 60 = 100000 / 60 = 1666 (total # minutes) 3. Find minutes in current time: minutes = n % 60 = 1666 / 60 = 46 *** part of answer n = n / 60 = 1666 % 60 = 27 (total # hours) 3. Find hours in current time: hours = n % 24 = 27 % 24 = 3 *** part of answer
- Algorithm in pseudo code:
n = System.currentTimeMillis(); // n = total # milli sec n = n / 1000; // n is now = total # sec seconds = n % 60; // Compute seconds in current time n = n / 60; // n is now = total # minutes minutes = n % 60; // Compute minutes in current time n = n / 60; // n is now = total # hours hours = n % 24; // Compute hours in current time
- Java program:
public class ShowCurrentTime { public static void main(String[] args) { long nMillis; // We need long for accuracy !!! long n, hours, minutes, seconds; nMillis = System.currentTimeMillis(); n = nMillis/1000; // Total # Second; seconds = n % 60; // Seconds n = n / 60; // Total # minutes; minutes = n % 60; // Minutes n = n / 60; // Total # hours; hours = n % 24; // Hours System.out.print(hours); System.out.print(":"); System.out.print(minutes); System.out.print(":"); System.out.print(seconds); System.out.println(" GMT"); } }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile: javac ShowCurrentTime.java
- To run: java ShowCurrentTime