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 02282013, 12:56 AM #1Member
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Using method to compute the sum of n numbers, not getting the return value
m(i) = i / ( i + 1) where i goes from 1 to 20
The program is supposed to show a table with i and the corresponding value m(i) listed.
i.e.
i m(i)
_____________
1 0.5000
2 1.1667
.
.
.
.
20 .....
I got my code to compile but I am getting the same value for all values of i (1 to 10).
Java Code:public class HW5_13New { public static double sum(int i) { double add = 0; for ( i = 1; i <= 20; i++ ) { add = (double)i / ( i + 1); } return add; } public static void main(String[] args) { System.out.println("i m(i)"); System.out.println("____________________"); for ( int i = 1; i <= 20; i++) { double ans = sum (i); System.out.print("\n" + i + " " +" " + ans); } } }
Last edited by abi; 02282013 at 01:31 AM.
 02282013, 01:12 AM #2Moderator
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Re: Using method to compute the sum of n numbers, not getting the return value
Java Code:public static int sum(int i)
In code like this you need to give some thought to what sort of thing (int or double) you intend each variable be.
I am having trouble taking the value from the Sum method into the main method.
 02282013, 01:22 AM #3Member
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Re: Using method to compute the sum of n numbers, not getting the return value
I had been completely oblivious of that. Just didn't occur to me. Rookie mistakes I guess. So I made that correction all of my variables are doubles now.
I guess I didn't describe my problem adequately enough. Yes it does compile, that is what i meant when I said it displays. After I made the variable type correction it shows 0.5 for all the i (1  20).
Thanks man.
 02282013, 02:29 AM #4Senior Member
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Re: Using method to compute the sum of n numbers, not getting the return value
Looks like you simply keep assigning a new value to add. So the only value saved is the last one in the loop.
JimThe Java™ Tutorial  SSCCE  Java Naming Conventions
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 02282013, 03:30 AM #5Member
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Re: Using method to compute the sum of n numbers, not getting the return value
I don't see a way to get around that. Could you tell me how I could store the value of each i to add?
 02282013, 03:38 AM #6Senior Member
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Re: Using method to compute the sum of n numbers, not getting the return value
Here is your sum method:
Java Code:public static double sum(int i) { double add = 0; for ( i = 1; i <= 20; i++ ) { add = (double)i / ( i + 1); } return add; }
To update add, do the following:
Java Code:add = add + (double)i/(i+1);
JimThe Java™ Tutorial  SSCCE  Java Naming Conventions
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 02282013, 03:56 AM #7Moderator
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Re: Using method to compute the sum of n numbers, not getting the return value
I'm glad you've got the double/int thing sorted out.
Jim's right, you've got to "accumulate" the result. You can also use += for that.
Is there some reason you're not doing the whole thing in main()? I'm a fan of using separate methods, but here you are calculating the same thing over and over again.
 02282013, 04:18 AM #8Member
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Re: Using method to compute the sum of n numbers, not getting the return value
I am learning to use separate methods and the assignment requires that I use it.
Wouldn't that result in a total sum.
I want this:
say it first takes i = 1 and comes out with 0.5
so add = 0.5
it displays 1 0.5
then
now it goes to i = 2 and gets 0.6667
so wouldnt add = 0.5 + 0.0667
and so on until it reaches 20Last edited by abi; 02282013 at 04:30 AM.
 02282013, 04:20 AM #9Member
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Re: Using method to compute the sum of n numbers, not getting the return value
Couldn't that result in a total sum.
I want this:
say it first takes i = 1 and comes out with 0.5
so add = 0.5
it displays 1 0.5
then
now it goes to i = 2 and gets 0.6667
so wouldnt add = 0.5 + 0.0667
and so on until it reaches 20Last edited by abi; 02282013 at 04:30 AM.
 02282013, 04:28 AM #10Senior Member
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Re: Using method to compute the sum of n numbers, not getting the return value
Ok, perhaps you don't want to update "add." I reread the first thread. The loop in the sum method confused me. You don't need it.
Just simply return the value i/(i+1) from the sum method. The reason your method is messing up is because the i in the for loop is clobbering your
method argument. And since you aren't really summing anything you might want to change the name (but it isn't necessary for operation).
Regards,
JimThe Java™ Tutorial  SSCCE  Java Naming Conventions
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 02282013, 04:43 AM #11Member
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Re: Using method to compute the sum of n numbers, not getting the return value
Java Code:public class HW5_13New { public static double sum(int i) { double add += (double)i / ( i + 1); } return add; } public static void main(String[] args) { System.out.println("i m(i)"); System.out.println("____________________"); for ( int i = 1; i <= 20; i++) { double ans = sum (i); System.out.print("\n" + i + " " +" " + ans); } }
sample
i m(i)
1 0.5
2 0.6667 < 0.5 + 0.6667
.
.
so on
i need
1 0.5
2 1.667
...
I tried a few things with this updated code but it isnt happening.
 02282013, 04:54 AM #12Senior Member
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Re: Using method to compute the sum of n numbers, not getting the return value
OK, let me try and understand. You want to create a list of values as follows:
i i/(i+1)
1 1/2
2 2/3
3 4/5
...
...
19 19/20
20 20/21
Am I correct? If so check out my post #10
JimLast edited by jim829; 02282013 at 04:58 AM.
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 02282013, 05:00 AM #13Member
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Re: Using method to compute the sum of n numbers, not getting the return value
Sorry for being unclear.
No that is not correct but that is what I am getting currently.
What I need is for it to add up as it goes from 1  20 and display the sum as it goes along
i i/(i+1)
1 1/2
2 (1/2)+ (2/3)
3 (1/2)+ (2/3) + (3/4)
...
 02282013, 05:18 AM #14Senior Member
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Re: Using method to compute the sum of n numbers, not getting the return value
OK, got it. What you need to to is sum of the values in add as we have told you. But you want to increase the number of terms you are summing for each call. This is controlled by the termination for value n which is now passed to sum. If you want to include the nth iteration in sum change to from < n to <= n.
Java Code:public static double sum(int n) { double add = 0; for (int i = 1; i < n; i++) { add += (double)i / ( i + 1); } return add }
Now you can play with it to fine tune it to the proper output.
Regards,
JimThe Java™ Tutorial  SSCCE  Java Naming Conventions
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 02282013, 06:09 AM #15Member
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Re: Using method to compute the sum of n numbers, not getting the return value
Ok I got that and came up with this. The only problem now is that it displays
1 0.0
2 0.5
3 1.16
...
do you see the trend?the values on the right have been pushed down per say.I poked at it but i can't see how to get it to start at 1  0.5.
Java Code:public class HW5_13New { public static double sum(int n) { double add = 0; for (int i = 1; i < n; i++) { add += (double)i / ( i + 1); } return add; } public static void main(String[] args) { System.out.println("i m(i)"); System.out.println("____________________"); for ( int n = 1; n <= 20; n++) { sum (n); System.out.print("\n" + n + " " +" " + sum(n)); } } }
 02282013, 03:07 PM #16Senior Member
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Re: Using method to compute the sum of n numbers, not getting the return value
Try changing the loop comparison in the sum method from "<" to "<=".
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 02282013, 07:59 PM #17Member
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