Forums Register Login

Using Postfix notation......

+Pie Number of slices to send: Send
Folks, the listing below is a small program that calculates the factorial for a given range of numbers.
public class TestFactorial
{
public static void main (String[] args)
{
for (int i = 0; i < 9; i++)
System.out.println(" f(" + i + ") = " + f(i));
}
static long f (int n)
{
long f = 1;
while (n > 1)
f *= n--;
return f;
}
}

Now, the thing is, I'm having a little problem getting my head around the line
<B> f *= n--; </B>
This is an example of a postfix operation, right?
So, f is multiplied by the value of n BEFORE its value is decreased by 1.
Going by this, and using the value of n= 3 - how does it work?
+Pie Number of slices to send: Send
It's equivalent to the following:
Don't write code like f *=n--; !
+Pie Number of slices to send: Send
Here's another way using a for loop
[ March 25, 2003: Message edited by: Barry Gaunt ]
+Pie Number of slices to send: Send
And of course the well know recursive factoral solution:

Expanding on that to cover all the bases:


Don't write code like f *=n--; !


Passionate about not writing confusing code are we? I absolutely agree, make it clear.
Michael Morris
+Pie Number of slices to send: Send
Note that n-- is not postfix notation (aka Polish notation). It is an example of using -- as a postfix operator.
+Pie Number of slices to send: Send
Wassisname said:

Passionate about not writing confusing code are we?


Must be this SCJP certi going to me 'ead.
+Pie Number of slices to send: Send
 

Originally posted by Barry Gaunt:
Here's another way using a for loop


OK, looking at this code.....
Taking the number 3 as an example
What happens??
OK...
The integer 3 enters the method as m.
And then within the for loop below
for ( int n = m ; n > 1 ; n-- )
We FIRSTLY decrease the value of 3 by 1, i.e., so we get 2. And then we multily this by f, i.e., the value 1. - 1 X 2 = 2
Then do the same thing again with the previously decremented value of 2, using the for loop, so we get 1.
And then we multiply this by f, i.e., the value 1. - 1 X 1 = 1
But, 2 x 1 = 2
How do we get 6???
+Pie Number of slices to send: Send
 


for ( int n = m ; n > 1 ; n-- )
We FIRSTLY decrease the value of 3 by 1,


Um, .... no. That isn't how a 'for' loop works. The "n--" doesn't hit until after the first iteration of the loop.
+Pie Number of slices to send: Send
Cheers!
Got it now!
Forgot to see that the postfix expression was relevant to the for loop.
Cheers folks!
+Pie Number of slices to send: Send
Don't know if postfix has anything to do with it. These loops will behave the same:

and
+Pie Number of slices to send: Send
 

Originally posted by Steve Jensen:
Cheers!
Got it now!
Forgot to see that the postfix expression was relevant to the for loop.
Cheers folks!


In fact, the postfix operator has NOTHING to do with it. This is all about how a for loop works. When the loop starts, the first part is executed only (commonly int i = 0 . The last section (commonly i++) isn't executed until the end if the loop. And finally the comparison, or middle part, (commonly i < SOME_FINAL_VALUE), is executed after the increment.
HTH
Layne
What's that smell? I think this tiny ad may have stepped in something.
a bit of art, as a gift, that will fit in a stocking
https://gardener-gift.com


reply
reply
This thread has been viewed 1702 times.
Similar Threads
Converting long hexadecimal strin to decimal
Highest Number of Consecutive Primes
Brackets { } .........
Hoare-Calculus
August Newsletter Puzzle
More...

All times above are in ranch (not your local) time.
The current ranch time is
Mar 29, 2024 05:55:40.