Tag Archives: Mathematics

How to Setup Different Error Messages for Each Zend Form Element Validator

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Anyone who has worked with that has come across this problem. I’d like to show different error message on each validator attached to a Zend_Form_Element. Let’s say we validate an text input field. We want it to contain only digits, but also we’d like to display different messages when the field is empty and when the user has entered something that is different from digits.

It can be done by attaching to the form element two validators: Zend_Validate_Digits and Zend_Validate_NotEmpty, but first let’s see how to change the default “Value is required and can’t be empty” error message of a form field.

$element = $form->createElement('text', 'phone');
$element->setLabel('Please, enter your phone number:')
	->setRequired(true)
	->addValidator('Digits');
$form->addElement($element);

Here we validate the field with Zend_Validate_Digits and we have set it to be required. Thus everything containing characters, i.e. “my123name” or “007bond”, will be false, while “1234” will be true.

Zend Framework
To show different error messages you've to attach them per validator and not per form element!

Continue reading How to Setup Different Error Messages for Each Zend Form Element Validator →

Powerful PHP: Less Known String Manipulation

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Yet another thing that’s great in PHP is the power you have when doing some string manipulation/operation. Here’s something that is really useful, but I think it remains a bit unknown. Let’s imagine you need to take the first (or whatever) character of a string. Most developers go to the obvious:

$str = 'hello world';
echo substr($str, 0, 1); // outputs "h"

But here’s something better and cleaner.

echo $str{0}; // outputs "h"

This code chunk return the first character of $str, but it can be used with the same success for any other character of the string. In my opinion this is more cleaner and its really syntactically self documented.

This approach can be useful when trying to check whether the first symbol for instance is “?” or “/”.

From SVG to Geo Coordinates – A Complete Guide

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Why This Task Is Not Trivial?

First of all what do we have? There is a vector shape, which may represent a map, which we’d like to convert into a GEO map. In other words there is a SVG file containing the source shape, that you’d like to convert in geoJSON or whatever collection of geo points. This is not trivial, of course, first of all because there’s no an algorithm or automation that can do this, and because everybody knows that the resulting map will be only approached, but will never be so accurate as the vector shape. This is because in a vector shape you may contain Bézier Curves, which I’ll show a little bit later in this post, that are difficult to represent in geo coordinates.

So the first task will be to find an approaching algorithm. However there are two things that are optimistic:

  1. You can’t effectively represent Bézier curves in geo coordinates, but even if you could do it there’s no practical need, because the collection of geo coordinates will be huge and this will slow down you’re application. Remember that geoJSON is yet again possibly used by your browser and the amount of geo points will be proportionally slowing down the app.
  2. As you may know Google’s visualization map is representing the World’s countries again with quite approached maps. Take a look at the following image – the country borders are quite sharpened:

Google Visualization Map

So far we know that we need an approaching algorithm that will convert vector lines and possibly curves in geo coordinates, but before we proceed we’ve to understand the SVG format. Continue reading From SVG to Geo Coordinates – A Complete Guide →

Construct a Sorted PHP Linked List

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This is really a draft, but I somehow decided to post it. Here’s a simple linked list in PHP, where only the add() method is slightly different. When you have an item to add it’s placed on the “right” place – the result is a sorted list.

Few notes before the code! This list is designed for integers, but I plan to “extend” it somehow and it can be improved a lot. In my future posts I’ll post something more about the complexity of the algorithm and an analysis of it.

class Node
{
    public $data;
    public $next;
    public $prev;
 
    public function __construct($data, $prev, $next)
    {
        $this->data = $data;
        $this->prev = $prev;
        $this->next = $next;
    }
}
 
class LinkedList
{
    protected $front = null;
 
    public function add($data)
    {
        if (!$this->front) {
            $node = new Node($data, null, null);
            $this->front = &$node;
        } else {
            if ($data < $this->front->data) {
                $node = new Node($data, null, $this->front);
                $this->front = $node;
                return;
            }
 
            $current = $this->front;
            while ($current) {
                if ($current->data < $data && isset($current->next) && $current->next->data > $data) {
                    $node = new Node($data, $current, $current->next);
                    $current->next = $node;
                }
                if ($current->data < $data && !isset($current->next)) {
                    $node = new Node($data, $current, $current->next);
                    $current->next = $node;
                }
                $current = $current->next;
            }
        }
    }
 
    public function printl()
    {
        $current = &$this->front;
        while($current) {
            echo $current->data, '<br />';
            $current = $current->next;
        }
    }
}
 
$list = new LinkedList();
$list->add(13);
$list->add(14);
$list->add(15);
$list->add(11);
$list->add(12);
$list->add(17);
$list->add(10);
$list->add(16);
 
$list->printl();

Friday Algorithms: Input Data and Complexity

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Size of ..

As we all know most of the cases a program execution time depends most from the input data. As I wrote in my post the degree of the input data estimates the algorithm complexity.

Of course 3*n² is a bit faster than 5*n², but in general both functions are similar and have the same complexity. In this case we tend to say that the size of the input data is n.

Size of the input data
The size of the input data affects the complexity of the algorithm

It is easy to bind this to arrays or other simple data structures. For example when sorting an array with n elements, the size of the input data is n, but sometimes there is not such an obvious relation between an algorithm and the size of the input data.

For instance when we’ve to search a path into a graph, than maybe the best way to describe the input data is by summing both the number of vertices and the number of edges.

Conclusion

Think, think, think
Think, think, think

However this is important when dealing with algorithms, because a mistake in the estimation of the input data size will result in wrong algorithm estimation at all