Looping and branching examples

This page contains some PHP examples that use loops and if/else code.

Contents 1 The 3N+1 Problem 2 Algorithm 3 PHP Code 4 The 3N+1 Form 5 HTML Form

The 3N+1 Problem

For N>0, repeatedly applying the following rules will lead to a series of integers that eventually converges at 1: If N is even, divide it by 2; if N is odd, multiply it by 3 and add 1. Write a PHP script to illustrate this problem.

Algorithm

1. Assign N a value > 1.
2. Loop while N > 1:
3.    Print N.
4.    If N is even
5.        N = N / 2
6.    Else
7.        N = 3 * N + 1
8. Done.

PHP Code

<?php
$n = 6;  // rand(2,100);
while ($n > 1) {
   echo "$n<BR>";
   if ($n % 2 == 0) {
      $n = $n / 2;
   } else {
      $n = 3 * $n + 1;
   }
}

?>

Click here for a ThreeNPlus1 solution.

The 3N+1 Form

This problem would be more interesting if the user could input the value of N rather than having it set in the program.

HTML Form

<FORM ACTION=".$self." METHOD="GET">
  <LABEL>N: 
     <INPUT TYPE="text" NAME="N" />
  </LABEL>
  <INPUT TYPE="submit" NAME="Go!" />
</FORM>

Click here for an example that creates a ThreeNPlus1 form solution.