Results 1 to 8 of 8
Thread: sum of elements in array
 07122010, 06:29 PM #1Member
 Join Date
 May 2010
 Posts
 44
 Rep Power
 0
sum of elements in array
The task is to write a program that returns the index of the cell in the array where the sum of the elements in the array until that cell's element equals the sum of the elements after it until the end. This is for any size array.
If there is no index like that, return 1.
for example: 2 1 6 5 4 returns 2 since 2+1+6=5+4 and 6 is in place 2
however: 3 2 7 returns 1...
Attempt:
I tried writing a program that would check the array like this...
Java Code:2+1+6+5 =4, 2+1+6=5+4, 2+1=6+5+4, 2=1+6+5+4.
Java Code:public class EqualSum { public static int equalSum(int[]a){ int counter=0; // counter saves the values of a[j] for (int j=a.length1; j>0; j){ if (sum(a, a[j1]) == (a[j] + counter)) // this checks if the sum from the first cell through the j1 cell equals the j cell. Since a[j] gets 1 smaller every time, counter keeps the values of the previous a[j]'s... return j1; counter += a[j]; } return 1; } public static int sum(int[]array, int until){ int counter=0; for (int i=0; i<until; i++) counter+= array[i]; return counter; } }
Thanks.
 07122010, 07:59 PM #2
 Join Date
 Sep 2008
 Location
 Voorschoten, the Netherlands
 Posts
 14,414
 Blog Entries
 7
 Rep Power
 25
This little problem is so full of preconditions and loop invariant conditions that defining them solves your problem. Here goes:
suppose that 'left' is the sum of the array elements a[0]+a[1] ... a[j] and 'right' is the sum of the elements a[j+1]+a[j+2] ...
suppose the length of the array 'a' is 'n'.
suppose we have a method sum(int[] a) that calculates the sum of all elements in array 'a'.
For starters we have j= n1, left= sum(a) and right= 0.
If left < right we have to return 1 (but we can ignore this condition)
If left == right we return j
If left > right we repeat the test after we have adjusted the values as follows:
left= a[j]
right+= a[j]
j= j1
if after the adjustments j < 0 we have to return 1
All this babbling translates to code as:
Java Code:public static int equalSum(int[] a) { int right = 0; int left= sum(a); for (int j = a.length  1; j >= 0; j) { if (left == right) return j; right+= a[j]; left= a[j]; } return 1; }
JosLast edited by JosAH; 07122010 at 08:51 PM.
 07122010, 08:27 PM #3Can anyone find the error here
For example:Java Code:public static int sum(int[]array, int until){ int counter=0; for (int i=0; i<until; i++) counter+= array[i]; System.out.println("counter=" + counter + ", for until=" + until); // show values return counter; }
 07122010, 08:40 PM #4
 Join Date
 Sep 2008
 Location
 Voorschoten, the Netherlands
 Posts
 14,414
 Blog Entries
 7
 Rep Power
 25
 07152010, 04:42 AM #5Senior Member
 Join Date
 Feb 2010
 Location
 Waterford, Ireland
 Posts
 748
 Rep Power
 8
 07152010, 07:53 AM #6
 Join Date
 Sep 2008
 Location
 Voorschoten, the Netherlands
 Posts
 14,414
 Blog Entries
 7
 Rep Power
 25
 07172010, 01:59 AM #7Senior Member
 Join Date
 Feb 2010
 Location
 Waterford, Ireland
 Posts
 748
 Rep Power
 8
I agree with your philosophy, I like math but I find it hard to get enthusiastic about it, I think math courses are thought way too abstract and teachers very rarely enthuse about a problems application just it's solution. I tried researching math topics for myself and their uses but even that area seem's fairly limited, any recommendations of books or websites?
 07172010, 08:36 AM #8
 Join Date
 Sep 2008
 Location
 Voorschoten, the Netherlands
 Posts
 14,414
 Blog Entries
 7
 Rep Power
 25
Agreed, math by itself is dry as straw, but it's a nice toolkit: it helps you solving real world problems; that's what I like about Donald Knuth's books: he abstracts away from a problem and drills down to the bottom of its mathematical foundations, then builds up a solution in the math domain and creates a program for it, solving the original problem. The entire analysis and construction process was in the math domain where only rock hard proofs count, there's no room for speculations, engineering or just plain guessing.
kind regards,
Jos
Similar Threads

How to Insert elements into one array
By zlich in forum New To JavaReplies: 8Last Post: 01022010, 03:37 PM 
Looping Array Elements
By enzyme in forum New To JavaReplies: 3Last Post: 11262009, 05:35 PM 
Get value of a String into elements of an Array.
By mainy in forum New To JavaReplies: 1Last Post: 08012009, 09:17 PM 
How to check whether two elements are available in an array?
By venkatteshb in forum New To JavaReplies: 8Last Post: 08272008, 10:45 PM 
Help with array of elements
By zoe in forum New To JavaReplies: 1Last Post: 07242007, 05:33 PM
Bookmarks