Check if number is a sum of consecutive numbers or not

Question

Write a program that checks if a given positive integer can be represented as sum of two or more consecutive positive integers.

Example:

43 can be represented as 21 + 22

10 = 1+2+3+4

but 4 cannot be represented in this way.

Input spec: positive integer (as argument or stdin)

Output spec: true or false

sample i/o

$./check 43
true
$./check 4
false

solution expected is creative, fast, golfed

Answer 1

JavaScript (31)

alert(!!((n=~~prompt())&(n-1)))

From my testing, I believe this gives correct solutions.

http://jsfiddle.net/3e9FZ/

Edit: (SPOILER ALERT!) Here is the justification for this answer:

1. All odd numbers greater than 1 can be trivially written as the sum of two consecutive numbers (ex 15 = 7+8, 23=11+12, etc.).
2. For even numbers having an odd factor where the odd factor is less than twice the even factor. For example, 4*7, because 7 < (2*4 = 8). Simply add 7 numbers with 4 at the center, 1+2+3+4+5+6+7.
3. For even numbers having an odd factor where the odd factor is more than twice the even factor. For example, 4*9, because 9 > (2 * 4 = 8). Double the even factor, and halve the odd factor to get 8*4.5. You will add the 8 numbers centered at 4.5, i.e., 1+2+3+4+5+6+7+8.
4. The only numbers left are the even numbers having no odd factor, i.e., the powers of two. The formula for the sum of a consecutive set of numbers is (avg * count). Now if the count is odd, then avg is a whole number, and (avg * count) has an odd factor. If count is even, then avg must be #.5, and thus avg * 2 is odd, and so avg * count has an odd factor. Therefore, any sum using the formula (avg * count) must have an odd factor, which powers of two do not, and therefore have no solution.

Answer 2

Golfscript, 20 chars

~.(&!!"falsetrue"5/=

Someone had to.

Answer 3

C

return 1 or 0:

int i(int j){return((j%2)?!(j==1):i(j/2));}

Answer 4

JavaScript (81)

n=prompt(r=i=1)|0;while(r&++i*i/2<n)if(i%2&!(n%i)|!(i%2|(n+i/2)%i))r=!r;alert(!r)

I'm cheating with & and | - even though they are technically bit-wise operations, they do the job quite nicely because the result is a 1/0 which are valid conditionals!

http://jsfiddle.net/HCqK2/4/

Answer 5

C, 67

As mellamokb suggested all numbers, except powers of two, can be written as sum of positive consecutive numbers:

main(int i,char**a){
printf((i=atoi(a[1]))&i-1?"true\n":"false\n");
}

Answer 6

Haskell, 74

Felt like doing the brute force way.

import List
main=print.f=<<readLn
f n=elem n$map sum$tails=<<inits[1..n-1]

Answer 7

Ruby (23)

only 3 chars longer than golfscript :)

p 0<(n=$*[0].to_i)&n-1

output:

$ ruby gc2958.rb 4
false
$ ruby gc2958.rb 43
true
$ wc gc2958.rb
  1       2      23 gc2958.rb

Answer 8

Python, 24 chars

Shortest python atm, borrowing bin() from @Fraxtil :)

bin(input()).count('1')>1

Answer 9

Python, 33 chars

It seems that there's a way shorter way to do this though:

s=input()
n=2
while n<s:n*=2
n!=s

Answer 10

Python, 34

bool(int(bin(2*int(input()))[3:]))

I took an alternative approach to the solution, but can't seem to squeeze anything else out of it. This code takes the input, multiplies it by two (this fixes the input case of 1 which breaks otherwise), converts it to binary, strips out the first 3 characters (0b1), converts the remainder to an integer (which is 0 iff the input was a power of two), and then converts that to a boolean.

As mentioned above, you can remove the 2* to get a 32-char solution that fails on an input of 1 but is otherwise perfect.

Answer 11

Perl:

print(((log($ARGV[0])/log(2))=~ /\./ )?true:false);

$perl Soc.pl 45

false

$perl Soc.pl 8

true

Answer 12

J, 12

It doesn't follow the spec 'true'/'false'. Rather I stick with 1/0. It checks if log base 2 of a number has a decimal mark.

+/=&'.'":2^.

   +/=&'.'":2^.1
0
   +/=&'.'":2^.4
0
   +/=&'.'":2^.43
1

Answer 13

Mathematica 31 39 35

True unless n = 1 or n is a power of 2.

n>1\[And]\[Not]FactorInteger@n~MatchQ~{{2,_}}

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And and Not are single characters in Mathematica: