DEV Community: Hakan Torun

Goldbach Conjecture And A Simple Approach in C

Hakan Torun — Sat, 02 Feb 2019 07:53:25 +0000

The Goldbach conjecture is the claim that every double integer greater than 2 can be written as the sum of two prime numbers. It is one of the biggest mathematical problems that cannot be proved.

In the original Goldbach hypothesis, Goldbach claims that every integer greater than 2 can be written as the sum of 3 prime numbers. This claim was put forward with the assumption that the number 1 is the prime number. But now 1 is not a prime number. This claim, which we treat as a Goldbach conjecture, consists of Euler's correction that “every double integer greater than 2 can be written as the sum of two prime numbers".

For example; 4,6,8,10 and 12 numbers can be write as 2 prime number pairs.

4 = 2 + 2
6 = 3 + 3
8 = 3 + 5
10 = 5 + 5

This example C code, which allows a double integer greater than 2 to be written as two prime numbers. E.g. 124620;

124620 = 19 + 124601


#include <stdio.h>

//Function declerations
int is_prime(int n);
void goldbach(int g);

int main(){
    int number = 0;
    while(1){
        printf("Enter even number:");
        scanf("%d",&number);
        if(number>2 && number%2==0){
            goldbach(number);
        }
        else{
            printf("Incorrect number!\n");
        }
        printf("\n");
    }
    return 0;
}


//Check is prime number?
int is_prime(int n){
    int flag = 1;
    for (int j = 2; j < n/2; j++)
    {
        if((n%j) == 0){
            return flag-1;
        }
    }
    return flag;
}


//Goldbach solution of a double integer greater than 2
void goldbach(int g){
    int i = 2;

    for (int j = g-i; j > 2; j--)
    {
        if(is_prime(i) == 1 && is_prime(j) == 1)
        {
            printf("%d = %d + %d\n",g,i,j);
            break;
        }
        i++;
    }
}

All two prime pairs. E.g. 16;

16 = 3 + 13
16 = 5 + 11
16 = 11 + 5
16 = 13 + 3

#include <stdio.h>

//Buffer for primes
int primes[100000] = {2};
int j = 0;

//Function declerations
int is_prime(int n);
void goldbach(int g);

int main(){
    int number = 0;
    while(1){
        printf("Enter even number:");
        scanf("%d",&number);
        if(number>2 && number%2==0){
            goldbach(number);
        }
        else{
            printf("Incorrect number!\n");
        }
        printf("\n");
    }
    return 0;
}

//Check is prime?
int is_prime(int n){
    int flag = 1;
    for (int j = 2; j < n/2; j++)
    {
        if((n%j) == 0){
            return flag-1;
        }
    }
    return flag;
}

//Goldbach solutions of a double integer greater than 2.
void goldbach(int g){
    int flag = 0;

    if(primes[j]<g){
        for (int i = primes[j]+1; i < g; i++)
        {
            if(is_prime(i) == 1){
                j++;
                primes[j] = i;
            }
        }       
    }

    for (int i = 0; i < j; i++)
    {
        for (int k = 0; k < j; k++)
        {
            if(primes[i] + primes[k] == g){
                printf("%d = %d + %d\n",g,primes[i],primes[k]);
                break;
            }
        }
    }

}

Binary Search Algorithm - C Programming Example

Hakan Torun — Fri, 18 Jan 2019 18:13:24 +0000

A binary search algorithm is a search algorithm that finds the position of searched value within the array. In the binary search algorithm, the element in the middle of the array is checked each time for the searched element to be found. If the middle element is not equal to the searched element, the search is repeated in the other half of the searched element. In this way, the search space is halved at each step.

The binary search algorithm works on sorted arrays. For non-sorted arrays, the array must first be sorted by any sorting algorithm in order to make a binary search.

The steps of binary search algorithm:

1- Select the element in the middle of the array.

2- Compare the selected element to the searched element, if it is equal to the searched element, terminate.

3- If the searched element is larger than the selected element, repeat the search operation in the major part of the selected element.

4- If the searched element is smaller than the selected element, repeat the search in the smaller part of the selected element.

5- Repeat the steps until the smallest index in the search space is less than or equal to the largest index.

For example; 2,3,4,5,6,7,8,9,22,33,45.

The following steps will be followed to find the number 4 with binary search in a sequential array of these numbers.

The middle element of the array is selected as 7 and compared with the searched element 4. The searched element (4) is not equal to the middle element (7), the part at the center of the array which is less than 7.

Our new search array: 2,3,4,5,6.

The middle element of our new search array is 4 and the search is completed.

With binary search algorithm, it is possible to find the searched value to log2N comparisons in an N-element array.

A sample C code will be as follows if we try to implement the binary search algorithm in a sequential array as in the example.

C Example:


#include <stdio.h>
int main()
{
    int array[11] = {2,3,4,5,6,7,8,9,22,33,45};
    int low       = 0;
    int high      = 10;
    int flag      = 0;
    int s         = 4;

    while(low <= high){
        int index = low+(high-low)/2;
        if(array[index] == s){
            flag = 1;
            printf("Founded: %d \n",index);
            break;
        }
        else if(array[index] < s){
            low   = index+1;
        }
        else{
            high = index-1;
        }
    }
    if(flag == 0){
        printf("Not Found!\n");
    }

    return 0;
}

Memcached Installation and PHP Memcache Example

Hakan Torun — Mon, 20 Aug 2018 21:56:29 +0000

Web servers with high traffic and with high CRUD operations will be under heavy load. In such a case, if the code is written to use resources efficiently enough, one of the first needs to serve under heavy load is a cache mechanism.

What is Memcahced?

Memcached is a cost-effective, open-source, high-performance, distributed caching system written for the popular blogging service LiveJournal, and is often used in Web applications. It is a simple but powerful caching system. Today, Facebook is actively used and contributed to the development. Popular web applications and services (Wikipedia, Youtube, Flickr) are actively used also.

Memcached is an in-memory store. All caching is on memory. Therefore, critical data should not be stored only on memcached. Data on memcached is stored as key-value pairs. Memcached is a life saver, when Web applications that are particularly has high level database operation.

Memcached Installation

In Debian-based Linux distros;

apt-get install php-memcached
apt-get install memcached

In Fedora/RedHat based Linux distros;

yum install php-memcached
yum install memcached

With that commands memcached and PHP-memcached packages can be easily installed. Memcached works on TCP/IP. Communication with memcached can be provided between any softwares.

Starting Memcached Deamon

memcached -d -m 512 -l 127.0.0.1 -p 11211 -u nobody

This command will start a memcached service that uses a cache space of 512 MB and listens on the TCP and UDP port 11211. The cache space created when initializing the memcached service is fixed. Similar to memory management in operating system architectures, it will overwrite the first written data if the cache space is full. You can define a maximum 4GB cache space on x86 systems.

To get more details about the service after starting memcached;

echo "stats settings" | nc localhost 11211

By default, the memcached key lengths are 250 characters. Value can be a maximum of 1MB.

PHP Memcache Example

To connect to memcached with PHP;

$memcache = new Memcache;
$memcache->connect('127.0.0.1', 11211) or die ("Unable to connect to Memcached");

To write data on memcached;

//$exampleValue = 2;
//$exampleValue = "Example String";
$exampleValue = array('0','1','2','3');

$memcache->set('exampleKey', $exampleValue, false, 1800);
//The first parameter is the key, the second parameter value, the third parameter, memcached compression is true/False, the fourth parameter is the cache time in seconds

Get value of a key on memcached;

$exampleValue = $memcache->get('exampleKey');

In a sample scenario, a value that does not change very frequently in our web application is stored in the database. This value will need to be read from the database whenever the page is rendered or in each API call. Let us want to keep the cache with memcached instead of reading this value every time from the database.

//Database connections etc.

$memcache     = new Memcache;
$memcache->connect('127.0.0.1', 11211) or die ("Unable to connect to Memcached");

$exampleValue = $memcache->get('exampleKey');

if($exampleValue === true){
    return $exampleValue;
}
else{
    $exampleValue = $db->get_var(sprintf("SELECT exampleValue FROM %s WHERE id = '%d'",'exampleTable',1));
    if($exampleValue){
        $memcache->set('exampleKey', $exampleValue, false, 1800);
        return $exampleValue;
    }
    else{
        return false;
    }
}

In this way, the disk I/O operations will be reduced due to the database, accessing directly through the memory can be achieved performance gains. The correct implementation of Memcached, the correct selection of data to be cached on Memcached, and the pre-detection of processes that could create a race condition can reduce the load that may occur on Web servers.

Python Scrapy Installation And Example Crawler

Hakan Torun — Thu, 09 Aug 2018 20:25:28 +0000

Scrapy is an open source framework written in Python that allows you to extract data from structural content such as html and xml. It is able to scraping and crawling on websites especially fast enough. First of all, you should install python packet manager, pip.

Installing with using pip:

pip install scrapy

Starting a new project:

scrapy startproject tutorial

When a scrapy project is created, a file / directory structure will be created as follows.

tutorial/
    scrapy.cfg            # deploy configuration file

    tutorial/             # project's Python module, you'll import your code from here
        __init__.py

        items.py          # project items definition file

        pipelines.py      # project pipelines file

        settings.py       # project settings file

        spiders/          # a directory where you'll later put your spiders
            __init__.py

Example spider:

import scrapy

class QuotesSpider(scrapy.Spider):
    name = "quotes"
    start_urls = [
        'http://quotes.toscrape.com/page/1/',
        'http://quotes.toscrape.com/page/2/',
    ]

    def parse(self, response):
        for quote in response.css('div.quote'):
            yield {
                'text': quote.css('span.text::text').extract_first(),
                'author': quote.css('small.author::text').extract_first(),
                'tags': quote.css('div.tags a.tag::text').extract(),
            }

Scrapy spiders can scan and extract data on one or more addresses. With Scrapy selectors, the desired fields can be selected and filtered. Scrapy is supported by xpath scrapy in selectors.

For crawling:

scrapy crawl quotes

For writing output to json file:

scrapy crawl quotes -o qutoes.json

Email Address Format, PHP filter_var Function And RFC 5321

Hakan Torun — Thu, 09 Aug 2018 09:19:35 +0000

The email address standarts is set by Rfc 5321. But in practice, things change a bit. Rfc5321 tells us that an email address might look like this:

local-part@domain

The domain part can be any domain or sub-domain, or even a local domain that is not tld. But there are the following restrictions for the local part.

- A-Z, a-z latin letters
- 0-9 digits
- Not to be the first and last character, dot (.) character
- Special characters !#$%&'*+-/=?^_`{|}~

We are familiar with the - and _ characters between special characters, but other special characters are not common at all. While Google's email service does not allow special characters from popular email services, Microsoft's email service allows only - and _ special characters. For a different service, the following address may be possible.

john`doe{software-developer}@domain.sub-domain.com

When the PHP filter_var function is used to controlling the mail address format

filter_var('john`doe{software-developer}@domain.sub domain.com',FILTER_VALIDATE_EMAIL)

This function will not return a false for the format described in Rfc 5321. But if the domain part is gmail, we will accept an email address that should not be, because the gmail implementation is different. When you try to verify with a non-standard library or a different method, things will be unbearable. In a possible scenario, a validation for popular email services will fail for another email service implemented according to Rfc 5321. In a multilingual system, if you have email services used in different languages and languages, the result is a fully disaster!