Lisp for the C++ programmer: Changing an element of a list

Here is the example of changing an element of a Common Lisp List and its equivalent code in C++ typed as List comment.

 ; #include <cstdlib>  
 ; #include <cstdio>  
 ;  
 ; int main(){  
 ;    double *d = (double*) malloc (sizeof(double) * 3);  
 ;    d[0] = 1;  
 ;    d[1] = 2;  
 ;    d[2] = 4;  
 ;  
 ;    puts("Current List:");  
 ;    printf("%f %f %f\n", d[0], d[1], d[2]);  
 ;  
 ;    puts("New List:");  
 ;    d[0] = 100;  
 ;    printf("%f %f %f\n", d[0], d[1], d[2]);  
 ;    return(0);  
 ;}  
   
   
 (setq aList (list 1 2 3))  
 (print "Current List:")  
 (print aList)  
   
 (setf (elt aList 0) 100)  
 (print "New List:")  
 (print aList)  
   
   

Lisp for the C++ programmer: for loop

Here is the example of for loop. Equivalent C++ code is commented on the top the Common Lisp code as comments.

 ; for (int i = 0; i <= 10; i++){  
 ;    printf("%f\n", i);  
 ;    printf("%f\n", i * 2);  
 ; }  
   
 (loop for i from 0 to 10  
     do  
     (progn  
         (print i)  
         (print (* i 2))  
     )  
 )  
   

Lisp for the C++ programmer: Function definitions and function calls

Here is the simple Common Lisp example, in which a sum and an ArithmaticMean functions defined. Both C++ and Common Lisp code here calculate the arithmetic mean of 1,2,3,4,5 and 6 which is 3.5. C++ code is commented as in the Common Lisp file.

 ; double sum (double *d, int len){  
 ;    double mysum = 0.0;  
 ;    for (int i = 0; i < len; i++){  
 ;        mysum += d[i];  
 ;    }  
 ;    return(mysum);  
 ; }  
 ;  
 ; double ArithmeticMean (double *d, int len){  
 ;    return ( sum(d) / len );  
 ; }  
   
 ; int main(){  
 ;    double *mylist = (double*) malloc(sizeof(double) * 6);  
 ;    for (int i = 0; i < 6; i++){  
 ;        d[i] = (double)i;  
 ;    }  
 ;    printf("%f\n", ArithmeticMean(d, 6);  
 ;    return(0); 
 ; }  
   
   
   
 (defun sum (aList)  
     (setq mysum 0.0)  
     (dotimes (i (length aList))  
         (setq mysum (+ mysum (nth i aList )))  
     )  
     mysum  
 )  
   
 (defun ArithmeticMean (aList)  
     (/ (sum aList) (length aList))  
 )  
   
 (print (ArithmeticMean '(1 2 3 4 5 6)))  
   

Lisp for the C++ programmer: cond expression

Here is the example for the cond expression of Common Lisp and its C++ equivalent.

 ; int x = 10;  
 ; if (x < 10) {  
 ;    puts("x is smaller than 10");  
 ; } else if (x > 10){  
 ;    puts("x is bigger than 10");  
 ; } else if (x == 10){  
 ;    puts("x equals to 10"));  
 ; }  
   
 (setq x 10)  
   
 (cond  
     ((< x 10) (print "x is smaller than 10"))  
     ((> x 10) (print "x is bigger than 10"))  
     ((= x 10) (print "x equals to 10"))  
 )  
   

Singleton Design Pattern in PHP Example

Singleton Pattern

Hello! As we all know that, design patterns used to solve any issue in software engineering. All of design patterns have typical skills. For example, in the singleton pattern a class can distribute one instance of itself to other classes. In this article I'll show you how to design a PDO database class using singleton design pattern in PHP programming language.

//singleton design pattern
class DB {
 
 private static $status = NULL;
 private static $info = 'mysql:host=localhost;dbname=dbname';
 private static $username = 'username';
 private static $password = 'password';
 
 private function __construct() { }
 
 public static function connect() {
  if(!self::$status) {
   try {
    self::$status = new PDO(self::$info, self::$username, self::$password);
    self::$status->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    self::$status->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
   }
   catch (PDOException $e) {
    throw new Exception($e->GetMessage()); 
   }
  }
  return self::$status;
 }
 
 public function __destruct() {
  self::$status = NULL; 
 }
}

First, We define status as a static variable, because we want to control whether connection is exists via only one variable. Aftet that define database information as private static variables. After all we create our connection static function, that is connect().

Here is the way is actually, if status variable is not null, code try to connect PDO database, otherwise connection is exists and will return true,NOT null.
Finally, in destruct function, connection has been closed by null value. When we want to connect database, we call the singleton like:

DB::connect();

If class has any other function, then like:

DB::connect()->functionName($param);

See you!