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Thread: Factorial in java
 04052011, 02:43 AM #1Member
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Factorial in java
Hi, im new to this forum and also Java. Im currrently doing my degree 1st year. My lecturer gave me a question on factorial and im unable to solve it.
i need to explain why any number of threads can be executing inside of it without causing interference in the program below.
Public int factorial (int n)
{
If (n<0)
Return 1;
Else if (n ==0)
Return 1;
Int result =1;
For (i=1; i<=n;i++)
{
Int tem = result;
Temp= temp *I;
Result=temp
}
Return result;
}
Please help me to understand this and what should i do to complete it.:confused:
 04052011, 02:49 AM #2
What threads are you talking about? Also if you post code please make sure it is correct Java code.
 04052011, 03:25 AM #3
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looks like someone is typing on a cell phone. Are you learning threads in your class yet? Your question isn't entirely clear, can you please clarify? Also, make sure you use code tags.
[ code]
YOUR CODE HERE
[/code]
 04052011, 04:18 AM #4Member
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Im learning thread chapter in class. This is the question that my lecturer gave to me :
Question2
Given the following method, explain why any number of threads can be executing inside of it without causing interference.
Java Code:Public int factorial (int n) { If (n<0) Return 1; Else if (n ==0) Return 1; Int result =1; For (i=1; i<=n;i++) { Int tem = result; Temp= temp *I; Result=temp } Return result; }
 04052011, 04:27 AM #5
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That code is very wrong, I am going to assume you are posting this from a cell phone and laziness is the culprit.
I don't have an amazing understanding of threads, but my understanding is that deadlock, and other problems come from when one thread is accessing something another thread is accessing at the same time. This is just a simple method call, it is thread safe because it isn't really changing anything, it is just running a method and returning an answer. Because of this, multiple threads should be able to run it without bumping into each other.
Like I said though, my thread understanding is far from exceptional, so others may have better explanations.
 04052011, 04:51 AM #6Member
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Don't know what all that threads you are talking about is... but I'll try to answer the factorial part... First of all I guess you know what a factorial is (if you don't read it).
Keep consistency in your variables. Java is case sensitive so "Result" is not the same as "result". Use curly brackets to make out code blocks too... That said:
Java Code:public int factorial(int n){ { if (n < 0) { return 1; } else if (n == 0) { return 1; } int result = 1; for (i = 1; i <= n; i++){ int temp = result; // you can replace all these temp = temp * i; // lines with a simple result = temp; // result *= i; } return result; } }
Factorial applies only to positive numbers so if a negative n is given the method returns 1. This is probably for testing reasons after the method is calledJava Code:if (factorial < 0) {//the method returned 1 and you should so some error handling or something}
If n > 0 then you create a loop from 1 to n and you multiply the number with itself for every increment of i. So lets say n = 5. The factorial of 5! = 1 x 2 x 3 x 4 x 5 = 120. Then you return that value.
I'm hope I made that clear for you...
 04052011, 12:49 PM #7Member
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Santeron,
You have answered my question. I understand the basic knowledge on factorial but i did not understand the question. Your explanation has helped me to understand and give a basic idea on the program. I'm working on it. Will get back here if i have any problem. :)
 04052011, 02:52 PM #8
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cenosillicaphobia: the fear for an empty beer glass
 04062011, 09:40 AM #9Member
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I have tried to complete the code give by my lecturer but im only getting 1 as the output. Please help me with this code :
Java Code:public class Factorial { public static long factorial( int n ) { if( n <= 0 ) return 1; if(n==0) return 1; else return n * factorial( n  1 ); } public static void main( String [ ] args ) { int result=0; int Temp; int temp=result; for( int i = 1; i <= 9; i++ ) Temp=temp*i; result=temp; System.out.println(factorial(result)); } }
 04062011, 09:47 AM #10
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The first if statement should only be true if n < 0. If it is as you have it it will never reach the termination condition you want. I don't understand why you have such a weird main method. The original method you had was correct except for some syntax problems. I thought your assignment was to explain why multiple threads can run this in parallel.
 04062011, 09:57 AM #11Member
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Yes, i have to explain but i tried to complete the program so that i can explain it more clearly with example of output.
 04062011, 09:00 PM #12Member
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I seriously don't get what you have done on those two methods... When you write some code read it line by line to see what values you have in your vars. So you start you main and you have result=0, Temp not initialized and temp=result=0. You get in the loop:
 int = 1
Temp = temp * i = 0 * 1 = 0
result= temp = 0
call factorial(result)  int = 2
Temp = temp * i = 0 * 2 = 0
result = temp = 0
call factorial(result)  rest of loop
 So first of all you never multiply with a 0 value. Initialize result = 1.
 Second I told you before. Temp != temp. Java is case sensitive and those two variables are totally different.
 Why do you keep sending variable values from one variable to another? Why don't you just write System.out.println(factorial(i));
Your syntax is wrong and you have a problem in logic. Let's follow this, too. 3 possible routes.
 n is less than 0. first if is true so it returns 1
 n is equal to 0. first if is true again(?) and returns 1. So erase the "=" from the "<=". You should also use else if and not if. It's cleaner code.
 n is greater than 0 so else is executed... and you call factorial method again with a n1 argument. So if n=5 you are going to call factorial(4). Following the same logic that will call factorial(3), then that will call factorial(2) which will lead to factorial(0) which is 1. Why do that?
Just make a for loop inside else and calculate your result.
Java Code:if (condition) { //code } else if (condition) { //code } else { /code } //else if and else are of course optional
Last edited by santeron; 04062011 at 09:07 PM.
 04072011, 01:32 AM #13
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Any reason why you changed from doing it with a loop to doing it recursively?
Also, don't worry about main just yet. Here is some pseudocode.
Java Code:if n < 0 return 1 end if if n == 0 return 1 end if declare result loop multiply loop var by result and set result to answer end loop return result
Java Code:if n < 0 return 1 end if if n == 0 return 1 end if else return value times the recursive call end else
 04092011, 12:22 PM #14Member
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Guys thanks for the reply. I have one last question.
1) explain why any number of threads can be executing inside of it without causing interference
Java Code:Public int factorial (int n) { If (n<0) Return 1; Else if (n ==0) Return 1; Int result =1; For (i=1; i<=n;i++) { Int tem = result; Temp= temp *I; Result=temp } Return result; }
Is it correct if i were to say
1) In factorial function, the current number in current loop will not be repeated and the current number will increase because the current number will be multiplied by previous number.
 04092011, 12:34 PM #15
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Check out jos's post number 8 for a good explanation.
 04092011, 12:40 PM #16
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I noticed it before: my replies seem to be invisible; I answer a question but the OP goes on and on ignoring my reply and asking the same question over and over again. OPs should read the replies given to them and if they don't understand the reply they should ask instead of bluntly ignoring the reply.
kind regards,
Joscenosillicaphobia: the fear for an empty beer glass
 04092011, 12:42 PM #17
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Agreed. However, your post seemed very simple to understand.
 04092011, 12:46 PM #18
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 04092011, 01:04 PM #19Member
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