Equilibrium index of a sequence

Question

Equilibrium index of a sequence is an index such that the sum of elements at lower indexes is equal to the sum of elements at higher indexes. For example, in a sequence A:

A[0]=-7 A[1]=1 A[2]=5 A[3]=2 A[4]=-4 A[5]=3 A[6]=0

3 is an equilibrium index, because:

A[0]+A[1]+A[2]=A[4]+A[5]+A[6]

6 is also an equilibrium index, because:

A[0]+A[1]+A[2]+A[3]+A[4]+A[5]=0

(sum of zero elements is zero) 7 is not an equilibrium index, because it is not a valid index of sequence A.

The idea is to create a program that given a sequence (array), returns its equilibrium index (any) or -1 if no equilibrium indexes exist.

Answer 1

Python - 72 chars

A=input()
print[i for i in range(len(A))if sum(A[:i])==sum(A[i+1:])]or-1

Takes comma separated input

Answer 2

Golfscript 17 16

Since the form of the input isn't specified, this takes a string in Golfscript array format from stdin.

~0\{1$+.@+\}/])?

So run as e.g.

golfscript.ry eqindex.gs <<<"[-7 1 5 2 -4 3 0]"

The idea is very simple: it takes an array of A_i and maps to an array of A_i + 2 SUM_{j<i} A_j and then looks for the first index which is equal to the sum of the whole array.

---

For @mellamokb's challenge I offer:

~0\{1$+.@+\}/:S;]:A,,{A=S=},`

for 29 chars.

Answer 3

Haskell (95 83)

e l=[n|n<-[0..length l-1],sum(take n l)==sum(drop(n+1)l)]
main=interact$show.e.read

Reads a list in Haskell style from stdin, eg.

[-7,1,5,2,-4,3,0]

and returns a Haskell style list of the indices, eg.

[3,6]

The result is [], if there is no index.

Please tell me, if your spec wants a different behavior.

Edits:

• (95 → 83): list comprehension is more breve

Answer 4

C - 96

a[99],*p=a,s;main(){for(;scanf("%d",p)>0;s+=*p++
);for(;p>a;s-=*p)(s-=*--p)||printf("%d\n",p-a);}

Note that this prints the equilibrium indices in reverse order.

Sample usage:

$ ./equilibrium <<< "-7 1 5 2 -4 3 0"
6
3

Answer 5

Ruby (83 77)

a=*$<.map(&:to_i)
p (0...a.size).select{|x|a[0..x].reduce(:+)==a[x..-1].reduce(:+)}

Edit: Shorter version as suggested by Ventero:

a=$<.map &:to_i
p (0...a.size).select{|x|eval"#{a[0..x]*?+}==#{a[x..-1]*?+}"}

Input is one number per line, output is comma separated list of indexes in square brackets.

Answer 6

JavaScript (161)

P=parseInt;L=prompt().split(',');S=function(A)A.reduce(function(a,b)P(a)+P(b),0);R=[i for(i in L)if(S(L.slice(0,i))==S(L.slice(P(i)+1)))];alert(R.length>0?R:-1);

http://jsfiddle.net/6qYQv/1/

Answer 7

scala, 108

val l=readline().split(" ").map(w=>w.toInt)
for(i<-0 to l.length-1
if l.take(i).sum==l.drop(i+1).sum)yield i

Answer 8

Python - 114

i=map(lambda x:int(x),raw_input().split(" "));x=0
print map(lambda x:(sum(i[0:x])==sum(i[x+1::])),range(0,len(i)))

Python - 72

i=input()
print map(lambda x:sum(i[0:x])==sum(i[x+1::]),range(0,len(i)))

Prints whether or not the given index is an equilibrium index, does not print the integer indecies at which the array is balanced.

Answer 9

PHP, 134 chars

<?for($a=explode(",",fgets(STDIN));++$i<($c=count($a));$o.=$s==0?$i:"")for($n=$s=0;$n<$c;)$s+=$n<$i?$a[$n++]:-$a[++$n];echo$o?$o:"-1";

I have an itch that this is far from optimal PHP golfing, but just ran out of steam (brains). At least it's shorter than with array_sum and array_splice :-)