Common Collection Methods

Most of the programming languages have arrays as a way to list items together. However, Dart has a collection of data structures similar to array. These are supported by the Dart library ‘ dart : core ’ and some other classes.

Such dart collections are classified as follows :

1. List : It is an ordered list of objects that is implemented using List class. This class enables the creation and manipulation of lists.

There are following two types of lists :

• Fixed length list : The size of this list is fixed and can not be changed at runtime.
• Growable list : The size of this list is dynamic and can be changed at runtime.

2. Set : It represents a collection of objects in which every object can appear only once.

3. Maps : It consists of a key-value pair that can belong to any data type. It is completely dynamic, that means it can grow and shrink at runtime.

4. Queue : It is a collection from where we can add and delete items from both ends.

Here are some of the common collection methods :

1. isEmpty( ) or isNotEmpty( ) :

These two functions are used to check whether a list, set, or map has items or not, that is, whether it is empty or not.

Consider the following example that explains the concept of these functions :

Example – 1 :

void main( )
{ 
  // declaraing a list ' coffees '
  var coffees = [ ] ;
  
  // declaring a list ' teas '
  var teas = [ ' green ', ' black ', ' chamomile ', ' earl grey ' ] ;
  
  // checking whether the list ' coffees ' is empty or not
  print( " Is the list of coffees empty ? : " ) ;
  print( coffees.isEmpty ) ;
  
  // checking whether the list ' teas ' is empty or not
  print( " Is the list of teas empty ? : " ) ;
  print( teas.isEmpty ) ;
}

Output :

Is the list of coffees empty ? : 
    true
    Is the list of teas empty ? : 
    false

2. forEach( ) :

This function is used to apply a certain function or condition to each item in a list, set, or map.

Consider the following example that explains the concept of this  function :

Example – 2 :

void main( )
{   
    // defining the list ' teas '
    var teas = [ ' green ', ' black ', ' chamomile ', ' earl grey ' ] ;  
    
    // converting the each item of the list in the upper case
    var t = teas.map( ( tea ) => tea.toUpperCase( ) ) ;
  
    // printing each item of the list
    t.forEach( print ) ;
}

Output :

GREEN 
BLACK 
CHAMOMILE 
EARL GREY

3. where( ) :

This function is used to retrieve all the items that match to a particular condition listen in the where( ) function. Similarly, the functions any( ) and every( ) can be used to check whether some or each item in the list matches a condition.

Consider the following example that explains the concept of this function :

Example – 3 :

void main( )
{
   var colours = [ ' green ', ' black ', ' red ', ' white ' ] ;
  
  // black is the darkest colour of all listed colors
  bool isdark( String colName ) => colName == ' black ' ;
  
  // Use where( ) function to retrive the item that returns true
  // from the isdark( ) function. 
  // Here, it returns the colour that is dark in the list
  print( colours.where( isdark ) ) ;
  
  // Use any( ) function to check whether any of the item in the
  // collection returns satisfies the isdark( ) function
  // Here, it returns true if any of the colour is dark in the list
  print( colours.any( isdark ) ) ;
  
  // Use every( ) function to check whether all the items in a
  // collection satisfies the isdark( ) function
  // Here, it returns true if every colour is dark in the list
  print( colours.every( isdark ) ) ;
}

Output :

( black )
true
false