CODEWITHSUNDEEP
c++
ymgpwcca
ymgpwcca

Reputation: 25

How to use pow() in while loop?

I need to find the highest integer x (x > 1) so that 2^x <= n (n is input from keyboard).

I wrote this but it loops forever

#include <cmath>

...

double x, result;
x = 2.0;
result = pow(2.0, x);

while (result <= n) {
    x++;
}
cout << x;

...

Upvotes: 1

Views: 402

Answers (4)

user12450543
user12450543

Reputation:

must guard it to ensure correct result with test,

//..
#include <cmath>
using namespace std;


double result, x = 1.0,
n = 8;

while ((result=pow(2.0, x)) < n)
   x++;
if (result > n) x--;

cout << x <<"\n";  // with sample n = 8 or 8.7 etc as well

3

Upvotes: 2

H S
H S

Reputation: 1221

Adding code to @David's answer. This is how you should update result in loop, so that your existing infinite loop terminates.

  //Assuming n is initialized somewhere else.
  double x, result;
  x = 0.0;
  result = 0.0; //Initialize Result variable first, so that while loop condition passes.
  while (result <= n)
  { 
      x++;
      result = pow(2.0, x + 1);
  }

  cout << x; // x contains exponent of 2
  //result contains next 2 raise to power greater than n

Moreover, Since you're calculating max power of 2 which is less than to equal to n. Another way to do same:

int x = log2(n);

Yet another way to do this via bit shifting:

int x;
for (x = 0; 1 << (x + 1) <= n; x++);
// Same as before  : x contains exponent of 2

UPDATE : Edited answer to address issue mentioned in comments. Thanks to @Adrian, I realized my silly mistake.

Upvotes: 2

eerorika
eerorika

Reputation: 238341

I wrote this but it loops forever

while (result <= n) {
    x++;
}

Here is an analogy:

You stack antique clocks into a box. You've been instructed to stop stacking when the box contains ten marshmallows. How much time will it take for you to complete that task? Answer: Infinitely long, because what you're doing does not have an effect on the condition of stopping the repetition.

Your program has the same problem: The loop repeats until result <= n is false. But the loop modifies neither variable, so therefore if the loop is ever entered, then it will never stop.

Upvotes: 3

David Schwartz
David Schwartz

Reputation: 182761

You need to update the value of result in the loop, not just once before you start the loop.

Upvotes: 4

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