# Forum: Ruby What's the correct and fast way to determine if a (gig) numb

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on 2007-02-11 20:30
```Hello,

I'm solving a math problem in Ruby.
I need to determine if a number is a perfect square.
If the number is small, you may do like the following.

def perfect_square? n
sqrt = n ** 0.5
sqrt - sqrt.to_i == 0
end

But Float number has limitation on precision.
Thus the function won't work correctly for big numbers like
(123456789123456789).

How would you solve such a case?
It should be fast as well as correct because I will use it repeatedly.

Sam```
on 2007-02-11 20:38
```Sam Kong wrote:
>
> But Float number has limitation on precision.
> Thus the function won't work correctly for big numbers like
> (123456789123456789).
>
> How would you solve such a case?
> It should be fast as well as correct because I will use it repeatedly.

Easy: compare integers rather than floats.

x = 123456789123456789

sqrt = Math::sqrt(x)
p(x == sqrt.floor**2)```
on 2007-02-11 22:48
```On Feb 11, 2007, at 8:30 PM, Sam Kong wrote:

>
> But Float number has limitation on precision.
> Thus the function won't work correctly for big numbers like
> (123456789123456789).
>
> How would you solve such a case?
> It should be fast as well as correct because I will use it repeatedly.

There are faster algorithms based on the fact that most non-squares
aren't quadratic residues modulo some integers. I remember some is
explained in Henri Cohen's "A Course in Computational Algebraic
Number Theory", but do not have the book at hand.

Perhaps you can take a look at the function that implements this test
in GNU MP, explained here,

http://www.swox.com/gmp/manual/Perfect-Square-Algorithm.html

or the one in Pari:

http://www.ufr-mi.u-bordeaux.fr/~belabas/pari/

-- fxn```
on 2007-02-12 19:32
```On Feb 11, 2007, at 10:48 PM, Xavier Noria wrote:

> There are faster algorithms based on the fact that most non-squares
> aren't quadratic residues modulo some integers. I remember some is
> explained in Henri Cohen's "A Course in Computational Algebraic
> Number Theory", but do not have the book at hand.

I've been able to consult the book today. There is a specialised
algorithm to compute integer roots using integer arithmetic, and the
integer square test itself. Thus, they guarantee a correct result. I
attach them below translated to Ruby.

Some operations would be written using bitwise stuff, but anyway the
code performs very poorly (compared to Math::sqrt) according to the
benchmarks. If performance is important a solution in C would be
worth exploring.

-- fxn

# From Henri Cohen's "A Course in Computational Algebraic Number
# Theory".

# Algorithm 1.7.1 (Integer Square Root) Given a positive integer n,
# this algorithm computes the integer part of the square root of n,
# i.e. the number m such that m^2 <= n < (m + 1)^2.
def isqrt(n)
x = n
loop do
y = ((x + n/x)/2)
if y < x
x = y
else
return x
end
end
end

# Cache the squares modulus 11, 63, 64, and 65. This is used to check
# for non-squares, since a square is a square mod k for all k. The
# choice of these numbers is based on the probability that a non-square
# is a square mod any of them, which is 6/715, less than a 1%.
\$squares = {}
[11, 63, 64, 65].each do |m|
\$squares[m] = [false] * m
(0...(m/2)).each {|i| \$squares[m][i**2 % m] = true}
end

# Algorithm 1.7.3 (Square Test). Given a positive integer n,
# this algorithm determines whether n is a square or not,
# and if it is, outputs the square root of n. We assume the
def issquare(n)
return false unless \$squares[64][n % 64]

r = n % 45045 # 45045 = 63*65*11
return false unless \$squares[63][r % 63]
return false unless \$squares[65][r % 65]
return false unless \$squares[11][r % 11]

q = isqrt(n)
return q**2 == n ? q : false
end

require 'benchmark'

\$r = 1000

# square of 32248581868698698768678697647823648238462348
\$s  =
103997103254216245831009973053627303273419242413513150637645310539211244
0875299413673104

# non-square, the previous number minus 1
\$ns =
103997103254216245831009973053627303273419242413513150637645310539211244
0875299413673103

# Just for the sake of curiosity, since the code based on Math::sqrt
is not correct.
Benchmark.benchmark do |x|
x.report("builtin is square (true)") do
1.upto(\$r) do
sqrt = Math::sqrt(\$s)
\$s == sqrt.floor**2
end
end
x.report("modular is square (true)") do
1.upto(\$r) do
issquare(\$s)
end
end
x.report("builtin is square (false)") do
1.upto(\$r) do
sqrt = Math::sqrt(\$ns)
\$ns == sqrt.floor**2
end
end
x.report("modular is square (false)") do
1.upto(\$r) do
issquare(\$ns)
end
end
end```
on 2007-02-12 20:56
```Hi Joel,

On Feb 11, 11:37 am, Joel VanderWerf <v...@path.berkeley.edu> wrote:
> > end
> x = 123456789123456789
>
> sqrt = Math::sqrt(x)
> p(x == sqrt.floor**2)

Yes. Your approach is better than mine.
But it gives a wrong answer for big numbers like 55833579873437812.

>
> --
>        vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Thanks anyway.

Sam```
on 2007-02-12 21:03
```Hi Xavier,

I really appreciate your kind help.
I put my comment inline.

On Feb 12, 10:32 am, Xavier Noria <f...@hashref.com> wrote:
> attach them below translated to Ruby.
>
>        return x
>      end
>    end
> end

Actually you posted this message, I implemented an algorithm myself.
It's just the way we calculate sqrt with pen and paper.
It might be slow but correct even for big numbers.

def max_digit n, m
result = 0
(0..9).each do |i|
break if ((n * 10) + i) * i > m
result = i
end
result
end

def perfect_square? n
arr = []
a = n
while true
a, b = a / 100, a % 100
arr.unshift b
break if a == 0
end
remain = 0
carried = 0
arr.each do |i|
num = remain * 100 + i
digit = max_digit(carried, num)
remain = num - (carried * 10 + digit) * digit
carried = (carried * 10 + digit) + digit
end
remain == 0
end

I felt ashamed to see the code you posted.OTL

>
>    return false unless \$squares[11][r % 11]
> \$s  =
> Benchmark.benchmark do |x|
>    end
>    end
> end

Sam```
on 2007-02-12 21:14
```Sam Kong wrote:
>>>   sqrt - sqrt.to_i == 0
>> sqrt = Math::sqrt(x)
>> p(x == sqrt.floor**2)
>
> Yes. Your approach is better than mine.
> But it gives a wrong answer for big numbers like 55833579873437812.

Wrong how?

irb(main):001:0> x = 55833579873437812
=> 55833579873437812
irb(main):002:0> sqrt = Math::sqrt(x)
=> 236291303.0
irb(main):003:0> sqrt.floor**2 - x
=> -3

Ok, I can see that one problem with my approach is that I should have
on 2007-02-12 22:08
```On Feb 12, 12:12 pm, Joel VanderWerf <v...@path.berkeley.edu> wrote:
> >>>   sqrt = n ** 0.5
>
> irb(main):002:0> sqrt = Math::sqrt(x)
> => 236291303.0
> irb(main):003:0> sqrt.floor**2 - x
> => -3
>
> Ok, I can see that one problem with my approach is that I should have
> used #round instead of #floor.

I think even if you use #round, the problem won't go away.
Float type cannot generate correct result due to its limited
precision.

Sam```
on 2007-02-12 22:25
```On Feb 12, 2:07 pm, "Sam Kong" <sam.s.k...@gmail.com> wrote:
> I think even if you use #round, the problem won't go away.
> Float type cannot generate correct result due to its limited
> precision.

For example:
irb(main):004:0>
x=981723462487562983749812734972342334123435635465656432452
=> 981723462487562983749812734972342334123435635465656432452

irb(main):005:0> y=x**2
=>
963780956798569484937549463180492938080866374138542452022484415498543617865176348383792466015930367123924038732304

irb(main):006:0> z=Math.sqrt(y)
=> 9.81723462487563e+056

irb(main):007:0> z==x
=> true

irb(main):010:0> z==(x+1000)
=> true

irb(main):011:0> z==(x+10000)
=> true```
on 2007-02-12 22:34
```Sam Kong wrote:
>>>>> def perfect_square? n
>>>> sqrt = Math::sqrt(x)
>> => -3
>>
>> Ok, I can see that one problem with my approach is that I should have
>> used #round instead of #floor.
>
> I think even if you use #round, the problem won't go away.
> Float type cannot generate correct result due to its limited
> precision.

I agree, but I don't think the problem shows up until you have much
larger numbers. Is that the case for your program?

In this case, 55833579873437812 cannot be exactly represented by a
float, but it doesn't matter for purposes of this calculation.```
on 2007-02-13 14:00
```On Feb 12, 10:33 pm, Joel VanderWerf <v...@path.berkeley.edu> wrote:
> >>>>> If the number is small, you may do like the following.
> >>>> x = 123456789123456789
> >> irb(main):003:0> sqrt.floor**2 - x
> larger numbers. Is that the case for your program?
>
> In this case, 55833579873437812 cannot be exactly represented by a
> float, but it doesn't matter for purposes of this calculation.
>
> --
>        vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407
Best is use GMP. Other methods are binary search. Or bigdecimal with
required accuracy```
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