CODEWITHSUNDEEP
c++function
zhijun huang
zhijun huang

Reputation: 11

how it return the boolean result?

class Solution
{
public:
  bool isPowerOfThree (int n)
  {
    double temp = log10 (n) / log10 (3);
      return !(temp - (int) temp);

  }
};

in the leetcode problem 326:Power of Three Given an integer, write a function to determine if it is a power of three.

I don't understand how it returns the boolean result.

I expect someone can tell me how to understand this following code: return ! (temp - (int) temp);

Upvotes: 1

Views: 88

Answers (3)

R Sahu
R Sahu

Reputation: 206717

The solution relies on temp being a whole number when n is some power of 3 and temp being a number with some fractional value when n is not a power of 3.

Let's say n is 9.
Then, temp will be 2.0.
Then, (temp-(int)temp) will be 0 and !(temp-(int)temp) will be true.

Let's say n is 10.
Then, temp will be 2.0959.
Then, (temp-(int)temp) will be 0.0959 and !(temp-(int)temp) will be false.

Unfortunately, floating point computations are not that precise. It will be better to use:

double temp = log10 (n) / log10 (3);
double diff = (temp - (int) temp);`
return (std::abs(diff) < tolerance);

where tolerance can be a small number, such as 1.0E-6.

Update

My experiment with cygwin/g++ on my computer and at ideone.com suggests that the tolerance can be 1.0e-6 for a large set of numbers but it needs to be almost 1.0e-11 or smaller for INT_MAX. See https://ideone.com/BgnQxV.

Upvotes: 1

Oblivion
Oblivion

Reputation: 7374

temp is a double, and (int)temp truncates it to an int. Let's say temp is 1.5, temp - (int)temp is 0.5. since the return type is bool the function will cast the resulting 0.5 to a bool !(0.5) which should be false.

Words from @Nautatava "everything except !(0), !(false) and !(null) gives false. Thus !(anything other than 0) is false."

Upvotes: 1

paddy
paddy

Reputation: 63481

This code is simply using integer truncation to test whether the number temp is a whole number. If it is, then temp - (int)temp will be zero. Since that would then mean the number is a power of three, then the unary "not" operator ! will return true. And of course ! used on any non-zero (true) value will be false.

Upvotes: 0

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