Question

How to write reverse number function using recursion in PHP

Answer 1

/*
number pow( number $base , number $exp )
float log10( float $arg )
*/

function rev_num($n) 
{
    return $n < 10 ? $n : ($n % 10) * pow(10, (int)log10($n)) + rev_num($n / 10);
}

printf("123 : %d<br />", rev_num(123));
/*
 * (n % 10) * pow(10, (int)log10(n)) + rev_num(n / 10);
 *    3     *     10 ^ 2 = 300       + rev_num(12)
 *    2     *     10 ^ 1 = 20        + rev_num(1)
 *    1     *     10 ^ 0 = 1         + rev_num(0)
 *   300 + 20 + 1 = 321; 
 */
     
printf("1234 : %d<br />", rev_num(1234));
/*
 * (n % 10) * pow(10, (int)log10(n)) + rev_num(n / 10);
 *    4     *     10 ^ 3 = 4000      + rev_num(123)
 *    3     *     10 ^ 2 = 300       + rev_num(12)
 *    2     *     10 ^ 1 = 20        + rev_num(1)
 *    1     *     10 ^ 0 = 1         + rev_num(0)
 *   4000 + 300 + 20 + 1 = 4321; 
 */
 
printf("1221 : %d<br />", rev_num(1221));


/*
run:

123 : 321
1234 : 4321
1221 : 1221

*/

 

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Answer 2

/*
number pow( number $base , number $exp )
float log10( float $arg )
*/

function rev_num($n) 
{
    return $n < 10 ? $n : ($n % 10) * pow(10, (int)log10($n)) + rev_num($n / 10);
}

echo "123 : " . intval(rev_num(123)) . "<br />";
/*
 * (n % 10) * pow(10, (int)log10(n)) + rev_num(n / 10);
 *    3     *     10 ^ 2 = 300       + rev_num(12)
 *    2     *     10 ^ 1 = 20        + rev_num(1)
 *    1     *     10 ^ 0 = 1         + rev_num(0)
 *   300 + 20 + 1 = 321; 
 */
     
echo "1234 : " . intval(rev_num(1234)) . "<br />";
/*
 * (n % 10) * pow(10, (int)log10(n)) + rev_num(n / 10);
 *    4     *     10 ^ 3 = 4000      + rev_num(123)
 *    3     *     10 ^ 2 = 300       + rev_num(12)
 *    2     *     10 ^ 1 = 20        + rev_num(1)
 *    1     *     10 ^ 0 = 1         + rev_num(0)
 *   4000 + 300 + 20 + 1 = 4321; 
 */
 
echo "1221 : " . intval(rev_num(1221)) . "<br />";


/*
run:

123 : 321
1234 : 4321
1221 : 1221

*/

 

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