Kingwood Computer Programming

Learn to Code

Help Example: Three

► QUESTION: I was working on the first Project Euler problem:

Multiples of 3 and 5

Problem 1

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9.
The sum of these multiples is 23.

Find the sum of all the multiples of 3 or 5 below 1000.

Here is my code, it seems like it should work but instead of the correct sum (which is: 233,168) I get zero. Can you help me figure out what is wrong?

  <?
      $maxValue = 1000;

      $sum = 0;

      for($i = 0; $i < maxValue; $i ++)
      {
         if($i % 3 == 0 || $i % 5 == 0)
            $sum += $i;
      }

      printf("The sum is: %d\n", $sum);
   ?>

► ANSWER:The problem with your code is here:

      for($i = 0; $i < maxValue; $i ++)
                       ^^^^^^^^

In php, all variables are prefixed with a '$', earlier in your program you had:

      $maxValue = 1000;

Which does prefix 'maxValue' with a '$' but when it is missing, php interprets things differently. To understand what is going on, consider this piece of code:

  <?
      define("forty_two", 42);

      printf("The value of the define 'forty_two' is: %d\n", forty_two);
   ?>

When executed, this code produces:

      The value of the define 'forty_two' is: 42

The feature being used is php's "define" feature which lets you create a named constant. Notice that when the constant is used, it is *not* preceded by a '$' (which makes sense as a 'define' is not a variable).

Alright, so when php sees a "thing" without a '$', one possible interpretation is that that "thing" is a named constant. In your code though, you never defined a constant 'maxValue' (only a variable) so trying to use that "constant" confused php and it defaults to using zero for any undefined, named constant. This is why your code says that '$sum' is zero, because the code (to php) reads as if it said:

  <?
      $maxValue = 1000;

      $sum = 0;

      for($i = 0; $i < 0; $i ++)
      {
         if($i % 3 == 0 || $i % 5 == 0)
            $sum += $i;
      }

      printf("The sum is: %d\n", $sum);
   ?>

Ok, so adding '$' before 'maxValue' in the for loop will fix it, but this is a bit annoying isn't it? The program ran and produced output but in no way warned you that you might be doing something a little odd.

As it turns out, php does have a way to address this - here's your same program with one small addition:

  <?
      error_reporting(-1);

      $maxValue = 1000;

      $sum = 0;

      for($i = 0; $i < maxValue; $i ++)
      {
         if($i % 3 == 0 || $i % 5 == 0)
            $sum += $i;
      }

      printf("The sum is: %d\n", $sum);
   ?>

When executed, the code above outputs:

   Notice: Use of undefined constant maxValue - assumed 'maxValue' in
   euler.php on line 8

   The sum is: 0

Much better! It not only informed us of the issue but told us the precise line number where it occurred. So what does 'error_reporting(-1)' do? Php has a system of flags it uses to allow you to select which errors it informs you of. They use named constants (like 'E_ERROR' or 'E_WARNING') and provide fine-grained control over php's error reporting output.

So why do we pass -1 to error_reporting? Because error_reporting's constants are all bit flags and -1 is a number with *every* bit set. So, saying:

   error_reporting(-1);

Tells php:

   "Inform me of every possible error and warning that you know about"

While this might *seem* extreme, the errors and warnings you encounter as a consequence of using hightened error_reporting levels will almost be legitimate issues that need to be corrected.

► Reference Files