Optimized way to find if a number is a perfect square - algorithm

Question

I had a question in my assignment to find whether a number was perfect square or not:
Perfect square is an element of algebraic structure which is equal to
the square of another element.
For example: 4, 9, 16 etc.
What my friends did is, if n is the number, they looped n - 1 times calculating n * n:
// just a general gist
int is_square = 0;
for (int i = 2; i < n; i++)
{
if ((i * i) == n)
{
std::cout << "Yes , it is";
is_square = 1;
break;
}
}
if (is_square == 0)
{
std::cout << "No, it is not";
}
I came up with a solution as shown below:
if (ceil(sqrt(n)) == floor(sqrt(n)))
{
std::cout << "Yes , it is";
}
else
{
std::cout << "no , it is not";
}
And it works properly.
Can it be called as more optimized solution than others?

Answer 1

The tried and true remains:
double sqrt(double x); // from lib
bool is_sqr(long n) {
long root = sqrt(n);
return root * root == n;
}

Answer 2

You would need to know the complexity of the sqrt(x) function on your computer to compare it against other methods. By the way, you are calling sqrt(n) twice ; consider storing it into a variable (even if the compiler probably does this for you).
If using something like Newton's method, the complexity of sqrt(x) is in O(M(d)) where M(d) measures the time required to multiply two d-digits numbers. Wikipedia
Your friend's method does (n - 2) multiplications (worst case scenario), thus its complexity is like O(n * M(x)) where x is a growing number.
Your version only uses sqrt() (ceil and floor can be ignored because of their constant complexity), which makes it O(M(n)).
O(M(n)) < O(n * M(x)) - Your version is more optimized than your friend's, but is not the most efficient. Have a look at interjay's link for a better approach.

Answer 3

#include <iostream>
using namespace std;
void isPerfect(int n){
int ctr=0,i=1;
while(n>0){
n-=i;
i+=2;
ctr++;
}
if(!n)cout<<"\nSquare root = "<<ctr;
else cout<<"\nNot a perfect square";
}
int main() {
isPerfect(3);
isPerfect(2025);
return 0;
}

Answer 4

I don't know what limitations do you have but perfect square number definition is clear
Another way of saying that a (non-negative) number is a square number,
is that its square roots are again integers
in wikipedia
IF SQRT(n) == FLOOR(SQRT(n)) THEN
WRITE "Yes it is"
ELSE
WRITE "No it isn't"
examples sqrt(9) == floor(sqrt(9)), sqrt(10) == floor(sqrt(10))

Answer 5

I'll recommend this one
if(((unsigned long long)sqrt(Number))%1==0) // sqrt() from math library
{
//Code goes Here
}
It works because.... all Integers( & only positive integers ) are positive multiples of 1
and Hence the solution.....
You can also run a benchmark Test like;
I did with following code in MSVC 2012
#include <iostream>
#include <conio.h>
#include <time.h>
#include <math.h>
using namespace std;
void IsPerfect(unsigned long long Number);
void main()
{
clock_t Initial,Final;
unsigned long long Counter=0;
unsigned long long Start=Counter;
Start--;
cout<<Start<<endl;
Initial=clock();
for( Counter=0 ; Counter<=100000000 ; Counter++ )
{
IsPerfect( Counter );
}
Final=clock();
float Time((float)Final-(float)Initial);
cout<<"Calculations Done in "<< Time ;
getch();
}
void IsPerfect( unsigned long long Number)
{
if(ceil(sqrt(Number)) == floor(sqrt(Number)))
//if(((unsigned long long)sqrt(Number))%1==0) // sqrt() from math library
{
}
}
Your code took 13590 time units
Mine just 10049 time units
Moreover I'm using few extra steps that is Type conversion
like
(unsigned long long)sqrt(Number))
Without this it can do still better
I hope it helps .....
Have a nice day....

Answer 6

Your solutions is more optimized, but it may not work. Since sqrt(x) may return the true square root +/- epsilon, 3 different roots must be tested:
bool isPerfect(long x){
double k = round( sqrt(x) );
return (n==(k-1)*(k-1)) || (n==k*k) || (n==(k+1)*(k+1));
}

Answer 7

This is simple python code for finding perfect square r not:
import math
n=(int)(input())
giv=(str)(math.sqrt(n))
if(len(giv.split('.')[1])==1):
print ("yes")
else:
print ("No")