Odtnhj

#include <stdio.h>
#include <math.h>

int main()
 int num,i;
 scanf("%d",&num);

 int a[num];
 for (i=0; i<num; i++)
 scanf("%d",&a[i]);
 

 for (i=0; i<num+1; i++)
 if (pow(2,i) == a[i])
 printf("%dn",a[i]);
 
 

I am trying to create a program that displays an integer in an array if it is a power of two. ex; an array contains the integer (1,2,4,9) it will print 1,2 and 4 only.

What is wrong with this code? It works on some test cases, but on most it doesn't.

If value is a power of two, only one bit is set:

#include <stdbool.h>
#include <limits.h>

bool is_pow_of_2(unsigned value)

 int count = 0;
 for (unsigned shift = 0; shift < sizeof(value) * CHAR_BIT; ++shift) 
 if (value & 1 << shift) // test if bit shift is set
 ++count;
 if (count > 1) // if there a more than one bits set in value
 return false; // it cannot be a power of 2
 

 return true;

Plan B:

bool is_pow_of_2(signed value)
{
 if(value <= 0)
 return false;

 // ...

To check if a number is a power of n with a recursive function :

function isPowOfn(int n, int number) 
 return (number == 1 ? TRUE : (number % n != 0 ? FALSE : isPowOfn(n, number / n)));

or (as suggested by WhozCraig)

function isPowOfn(int n, int number) (value % n == 0 && isPowOfn(n, number / n))

Input-output examples :

in: 2, 6 
processing: 3 
out: FALSE 

in: 2, 16 
processing: 8 - 4 - 2 - 1 
out: TRUE 

in: 2, 162 
processing: 81
out: FALSE

in: 3, 81
processing: 27 - 9 - 3 - 1
out: TRUE

in: 5, 244140625
processing: 48828125 - 9765625 - 1953125 - 390625 - 78125 - 15625 - 3125 - 625 - 125 - 25 - 5 - 1
out: TRUE

in: 4, 67957741992
processing: 16989435498
out: FALSE

/* Function to check if x is power of 2*/
bool isPowerOfTwo (int x) 
 
 /* First x in the below expression is for the case when x is 0 */
 return x && (!(x&(x-1))); 
 

Explanation: A power of 2 has only 1 bit set. If we subtract 1 from a power of 2 then all unset bits after the only set bit become set; and the set bit becomes unset.

If x is a power of 2, x&(x-1) will be 0. (!(x&(x-1))) will be 1. And the return expression will be 1.

The logical AND (&&) of this expression with x is to ensure that this will not be true if x is 0.

The other methods proposed to test if a number is a power of two make assumptions about the representation of numbers. This should be avoided. Always write generic code that is easy to understand and works everywhere. Avoid recursion too if possible.

Too many programmers try to be clever. You should consider other methods only if this function is a bottleneck in your program and if your compiler is not smart enough. As a beginner you are obviously not in that situation.

bool is_power_of_2 (int n)

 if (n < 1) 
 return false;
 

 while (n % 2 == 0) 
 n = n / 2;
 

 return (n == 1);