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Variables and Data Types

Perl has three different variables: scalar, array, and associative array.

For some "strongly typed" languages such as C or Fortran, each variable must be explicitly declared before you use them. For C, it works like this:

    int i;
    i=67;

But for Perl, the variables do not need to be decalred before their use. You can use them directly. The intepreter can recognize each variable by its prefix.

Scalar Variables

Scalar variables can be a number, a string or undefined. Scalar varaibles are prefixed by $ sign. Here are some scalar variable example:

   $str1="www";        #assign string "www" to scalar variable $str1
   $str2="cgi";        #assign string "cgi" to scalar variable $str2
   $str3=$str1.$str2;  #concatenate two strings so $str3="wwwcgi"
   $int=5;             #assign integer 5 to scalar variable $int
   $int=3+2;           #here $int=5
   $int=3*4;           #here $int=12
   $int=5;$int++;      #here $int=5+1=6
   $int=5;$int+=4;     #here $int=5+4=9

Array of Scalar

Array of scalar should be prefixed by @ sign. But when you specify one element in an array, you can also prefix it with a $ sign. Here are some array of scalar examples:

   @name1=("viky","jeff");     #assign "viky","jeff" two strings to array @name1
   @name2=@name1;              #now @name2=("viky","jeff")
   @name3=("john",@name1);     #now @name3=("john","viky","jeff")
   ($one,@name4)=@name3;       #now @one="john" ,@name4=("viky","jeff")    
   @name1=();                  #now @name1 becomes an empty array          
   @int=(2,6,7,8,9);           #Five numbers are assigned to array @int
   $x=@int;                    #assign an array to an scalar variable will return the 
                               #number of elements in the array, so $x=5
   $x=$#int;                   # $# is a special variable which will return the index of the 
                               last element in an array, so $x=4 
   ($x)=@int                   #$x is equal to the first element of array, so $x=2
   $b=$int[0];                 #$b is equal to the first element in array, so $b=2
   $c=@int[0];                 #$c is also equal to the first element in array, so $c=2
                               # $int[0] works the same way as @int[0] 
   $int[0]=5;                  #assign 5 to be the first element of array, so @int=(5,6,7,8,9)
   $int[0,1]=[1,3]             #asisgn 1 to the first element, 3 to be the second element, so @int=(1,3,7,8,9)
   @int[0,1]=@int[1,0]         #the first two element exchange ,now @int=(3,1,7,8,9)
   @data=@int[0,1]             #here @data=(1,3)
   $int[5]=10                  # assigns the sixth element to @int so @int=(1,3,7,8,9,10)

Associative arrays, sometimes called "hashes", are arrays that are indexed not on ordered integers, but on arbitrary string values. Typically, elements of an associative array are referred to as "key" and "value" pairs - the "key" is used to find the element of the array that has the "value". An associate array is prefixed with the percent(%) sign. Here is the format for an associate array:

   %arrayname=(key1,value1,key2,value2,key3,value3...);

For each key in asscoiate array, there is a value corresponding to it:

Add or change a pair of element:
```
 $arrayname{key}=value
```
To add a pair of key-value to associate array, first prefix the array name with dollar sign($), and embrace the key inside the {} then assign the value to the key. If the key is already in this array, then the key's value will change.
Get the the value of a key :
```
  $value=$arrayname{key}
```
To delete a pair of key-value use the following statement:
```
    delete $arrayname{key}
```
where the delete is a function provided by Perl.

Here are some associate array examples:

  %name=('john','biology','jeff','physics');    # suppose john teaches biology and jeff teaches physics
  $a=$name{'john'};            # here $a='Biology'
  $b=$name{'jeff'};            # here $b='physics'
  $name{'bill'}='math';        # add a new pair of element : key is bill, its value is math.
                               # now %name=('john','biology','jeff','physics','bill','math')          
  $name{'bill'}='English';     # Because we have already have a key 'bill' in the associate array, 
                               # so the 'bill' value change to 'English' 
                               # now %name=('john','biology','jeff','physics','bill','English')
  delete $name{'jeff'};        # now  #now %name=('john','biology','bill','English')
  @X=%name                     # associate array assign to array @X
  %name=@X                     # array assign to associate array.

Variables and Data Types

Scalar Variables

Array of Scalar

Associate Array