String

As you have seen in the previous week's lectures, string data types are represented in a single or double quote.

Examples

String name = "Godwin Ashong";

String town = "Teshie";

String country = "Ghana";

Joining strings

Two or more strings can be joined using the + operator

Examples

String firstname = "Godwin";

String lastname = "Ashong";

String fullname = firstname+lastname;

NOTE: If fullname is printed out, it will output GodwinAshong. In order to leave a space between the first name and last name, there must be an empty string with space in between the two strings. Thus:

String fullname = firstname+" "+lastname;

If the above fullname is printed out, it will output Godwin Ashong

As you have observed, ${} can be used to do computation within a string and the result displayed. You can refer to our previous examples which has that. The computation if there is any in the {} is perform and the result is output.

Example

Write a program which takes the first name and last name of a user and outputs the full name.

Code

// Importing dart:io file
import 'dart:io';
void main()
{
    print("Enter your first name");
    String fn = stdin.readLineSync();
    print("Enter your last name");
    String ln = stdin.readLineSync();
    String fname = fn+" "+ln;
    print("Your full name is ${fname}");
}

Result

Enter your first name

Michael

Enter your last name

Essien

Your full name is Michael Essien

String Properties

The properties listed in the following table are all read-only.

The isEmpty method can be handy when checking if an input field is empty or not

The Length method can be handy when checking the length of a password for password validation.

Methods to Manipulate Strings

Methods to Manipulate Strings

The String class in the dart: core library also provides methods to manipulate strings.

toLowerCase()

Converts all characters in this string to lower case.

Example

void main() {
   String name = "Abigal Adjei";  
   print('Name before toLowerCase() is called: ${name}');
   print('Name when toLower() is called: ${name.toLowerCase()}');
}

Output

Name before toLowerCase() is called: Abigal Adjei

Name when toLowerCase() is called: abigal adjei

toUpperCase()

Converts all characters in this string to upper case.

Example

void main() {
   String name = "Abigal Adjei";  
   print('Name before toUpperCase() is called: ${name}');
   print('Name when toUpperCase() is called: ${name.toUpperCase()}');
}

Output

Name before toUpperCase() is called: Abigal Adjei

Name when toUpperCase() is called: ABIGAL ADJEI

trim()

Returns the string without any leading and trailing whitespace.

Example

void main() {
   String fn = "           Abigal Atwei "; 
   String ln = "Adjei"; 
   print('Before trim() is called on fn:  Full name - ${fn+ln}');
   print('When trim() is called on fn: ${fn.trim()+ln}');
}

Output

Before trim() is called on fn:  Full name -            Abigal Atwei Adjei

When trim() is called on fn: Abigal AtweiAdjei

compareTo()

Compares this object to another.

Return Type

Returns an integer representing the relationship between two strings.

0 − when the strings are equal.

1 − when the first string is greater than the second

-1 − when the first string is smaller than the second

Example

void main() {
   String fruit = "Mango";
   String A = "A";
   String B = "B";
   String a = "a";
   print("When ${fruit} compared to Orange:${fruit.compareTo("Orange")}");
   print("When ${fruit} compared to orange:${fruit.compareTo("orange")}");
   print("When ${fruit} compared to pear:${fruit.compareTo("pear")}");
   print("When ${fruit} compared to Mango:${fruit.compareTo("Mango")}");
   print("When ${fruit} compared to mAnGo:${fruit.compareTo("mAnGo")}");
   print("When ${B} compared to ${A}:${B.compareTo(A)}");
   print("When ${a} compared to ${A}:${a.compareTo(A)}");
}

Output

When Mango compared to Orange:-1

When Mango compared to orange:-1

When Mango compared to pear:-1

When Mango compared to Mango:0

When Mango compared to mAnGo:-1

When B compared to A:1

When a compared to A:1

replaceAll()

Replaces all substrings that match the specified pattern with a given value.

Example

void main(){
    String text = "My name is Adjei. Adjei is a name of a person in Ghana";
    print("Text before replaceAll is called:${text}");
    print("Text when replaceAll('Adjei','Ama') is called: ${text.replaceAll('Adjei','Ama')}");
}

Output

Text before replaceAll is called:My name is Adjei. Adjei is a name of a person in Ghana

Text when replaceAll('Adjei','Ama') is called: My name is Ama. Ama is a name of a person in Ghana

split()

Splits the string at matches of the specified delimiter and returns a list of substrings.

Example

void main(){
    String date = "2020-07-31";
    print("When split by the pattern - is ${date.split("-")}");
}

Output

When split by the pattern - is [2020, 07, 31]

substring()

Returns the substring of this string that extends from start Index, inclusive, to end Index, exclusive.

Example

void main(){
    String text = "I am a text";
    print("When part of the text is extracted ${text.substring(0,4)}");
    print("When part of the text is extracted ${text.substring(5,8)}");
}

Output

When part of the text is extracted I am

When part of the text is extracted a t

toString()

Returns a string representation of this object.

Example

void main(){
    int age = 12;
    print("You are ${age.toString()} years old");
}

Output

You are 12 years old

List

If you know other programming languages you may had come across an Array. Dart programming represents an array in a form of a

list. A List is simply an ordered group of objects. The dart:core library provides the List class that enables creation and manipulation of lists.

Figure: A List

The position of each of the content is known as index and the content is known as an element

The list position starts from 0. Hence the last index of a list is 1 less than the size of the list. 

Thus:

last index = size of list - 1

As you can see the size of the list (array) above is 10. Hence the last index will be:

last index = 10 - 1 = 9

Lists classifications

• Fixed Length List
• Growable List

Fixed Length List

A fixed length list cannot change at runtime. The programmer specifies the length(size) of the list at the time of declaration.

Syntax

var list_name = new List(size_of_the_list);

Examples

var names = new List(4);

var ages = new List(20);

Assigning value to list index

list_name[index] = value;

Using the names list example above, we could write

names[0] = "Kofi Agyenum";

names[1] = "Adjei Mensah";

names[2] = "Davida Agbedo";

names[3] = "Joseph Anum";

It is illegal to write:

names[4], names[5] etc because our list is of the size 4 which ends at index 3, thus last index = 4 - 1 = 3. Hence any index more than 3 is illegal.

Growable List

A growable list's length can change at run-time. Thus the size is unlimited. You declare a list as growable if you don't know how many elements will the list contain.

Syntax

var list_name = [value_1,value_2,value_3];

or 

var list_name = new List();

You can add new element(s) to the growable list by using the add() method.

NOTE: You can't use list_name[index] = value for a growable list.

You can access the value of the element in the list by using the index of the list.

Thus value = list_name[index]

You can use the length method to get the size of growable list.

The size of the list will be nameOfList.length

You can use a for loop to loop through each index of the list as shown below:

Examples

void main(){
    var names = new List(3);
    /*
    names.add("Kofo Agyenum");
    names.add("Atwei Adjetey");
    */
    names[0] = "Kofo Agyenum";
    names[1] = "Atwei Adjetey";
    names[2] = "Etonam Adjovi";
    print("*********** Names ****************");
    for(int i=0;i<names.length;i++){
      print(names[i]);
    }
    print("Growable List Method 1");
    var ages = [10,11];
    print("Adding new value to list");
    ages.add(23);
    print("*********** Ages ****************");
    for(int i=0;i<ages.length;i++){
      print(ages[i]);
    }
    print("Growable List Method 2");
    var courses = new List();
    courses.add("Maths");
    courses.add("English");
    courses.add("Science");
    print("*********** Courses ****************");
    for(int i=0;i<courses.length;i++){
      print(courses[i]);
    }
  }

Output

*********** Names ****************

Kofo Agyenum

Atwei Adjetey

Etonam Adjovi

Growable List Method 1

Adding new value to list

*********** Ages ****************

10

11

23

Growable List Method 2

*********** Courses ****************

Maths

English

Science

Example

Write a program which asks a user to enter favorite fruits. The user should enter q when he wants to quit entering or any other letter to continue entering. Store the entered fruits in a list and show the content after a user enters q

Code

// importing dart:io file
import 'dart:io';
void main(){
   var fruits = new List();
   String q = "c";
   while(q.toLowerCase() !="q"){
       print("Enter your favorite fruit");
       String fruit = stdin.readLineSync();
       fruits.add(fruit);
       print("Enter q to quit or any other letter to add more");
       q= stdin.readLineSync();
   }
   //Print out the content of the list
   print("********* Your favorite fruits ***********");
   for(int i=0;i<fruits.length;i++){
      print(fruits[i]);
   }
}

Output

Enter your favorite fruit

Apple

Enter q to quit or any other letter to add more

c

Enter your favorite fruit

Mango

Enter q to quit or any other letter to add more

c

Enter your favorite fruit

Pineapple

Enter q to quit or any other letter to add more

q

********* Your favorite fruits***********

Apple

Mango

Pineapple

List Properties

The following table lists some commonly used properties of the List class in the dart:core library.

Example

import 'dart:io';
void main(){
     var fees = new List();
     if(fees.isEmpty){
       print("No one paid yesterday");
       fees.add(1200);
       fees.add(1000);
       fees.add(800);
       print("${fees.length} paid today");
       print("First amount paid:${fees.first}");
       print("Last amount paid:${fees.last}");
       print("Reverse payment order");
       print("First amount paid:${fees.reversed.first}");
       print("Last amount paid:${fees.reversed.last}");
     }
     print("Test isNotEmpty Method");
     var sick = new List();
     sick.add("Kofi");
     if(sick.isNotEmpty){
        print("${sick.length} is(are) sick");
     }
     if(sick.length==1){
       print("Only one person is sick and the student is ${sick.single}");
     }
}

Output

No one paid yesterday

3 paid today

First amount paid:1200

Last amount paid:800

Reverse payment order

First amount paid:800

Last amount paid:1200

Test isNotEmpty Method

1 is(are) sick

Only one person is sick and the student is Kofi

Updating List

Replacing by range

List.replaceRange(int start_index,int end_index,Iterable ) 

Where,

Start_index − an integer representing the index position to start replacing.

End_index − an integer representing the index position to stop replacing.

− an iterable object that represents the updated values.

Example

import 'dart:io';
void main(){

     var fees = new List();

     fees.add(1000);

     fees.add(800);

     fees.add(1500);

     fees.add(700);

     fees.replaceRange(1,2,[1400,800]);

     print("Fees amount at 1 is :${fees[1]}");

     print("Fees amount at 2 is :${fees[2]}");

}

Outcome

Fees amount at 1 is :1400

Fees amount at 2 is :800

Replacing by index

To replace the element of an index, simply assign new value to that index like the way assignment is done for fixed list.

Thus:

list[index] = value;

Example

import 'dart:io';
void main(){
     var fees = new List();
     fees.add(1000);
     fees.add(800);
     fees.add(1500);
     fees.add(700);
     fees[1] = 1400;
     fees[2] = 800;
     print("Fees amount at 1 is :${fees[1]}");
     print("Fees amount at 2 is :${fees[2]}");
}

Output

Fees amount at 1 is :1400

Fees amount at 2 is :800

Removing element from list

List.remove()

The List.remove() function removes the first occurrence of the specified item in the list. This function returns true if the specified value is removed from the list.

Syntax

List.remove(Object value)

Where,

value − represents the value of the item that should be removed from the list.

Example

import 'dart:io';
void main(){
     var fees = new List();
     fees.add(1000);
     fees.add(800);
     fees.add(1500);
     fees.add(700);
     fees.remove(700);
     print(fees);
}

Output

[1000, 800, 1500]

List.removeAt()

The List.removeAt function removes the value at the specified index and returns it.

Syntax

List.removeAt(int index)

Where,

index − represents the index of the element that should be removed from the list.

Example

import 'dart:io';
void main(){
     var fees = new List();
     fees.add(1000);
     fees.add(800);
     fees.add(1500);
     fees.add(700);
     print("Starting list");
     print(fees);
     fees.remove(700);
     print("After removing 700");
     print(fees);
     fees.removeAt(1);
     print("After removing index 1");
     print(fees);
}

Output

Starting list

[1000, 800, 1500, 700]

After removing 700

[1000, 800, 1500]

After removing index 1

[1000, 1500]

List.removeLast()

The List.removeLast() function removes the last element and returns the new list

Example

import 'dart:io';
void main(){
     var fees = new List();
     fees.add(1000);
     fees.add(800);
     fees.add(1500);
     fees.add(700);
     print("Before removing last element");
     print(fees);
     fees.removeLast();
     print("After removing last element");
     print(fees);
}

Output

Before removing last element

[1000, 800, 1500, 700]

After removing last element

[1000, 800, 1500]

List.removeRange()

The List.removeRange() function removes the items within the specified range. 

List.removeRange(int start, int end)

Where,

Start − represents the starting position for removing the items.

End − represents the position in the list to stop removing the items.

Example

import 'dart:io';
void main(){
     var fees = new List();
     fees.add(1000);
     fees.add(800);
     fees.add(1500);
     fees.add(700);
     fees.add(750);
     fees.add(900);
     print("Before removing range from 1,4");
     print(fees);
     fees.removeRange(1,4);
     print("After removing");
     print(fees);
}

Output

Before removing range from 1,4

[1000, 800, 1500, 700, 750, 900]

After removing

[1000, 750, 900]

Map

The Map object is a simple key/value pair. Keys and values in a map may be of any type. 

A Map is a dynamic collection. In other words, Maps can grow and shrink at runtime.

Maps can be declared in two ways

• Using Map Literals
• Using a Map constructor

Declaring a Map using Map Literals

To declare a map using map literals, you need to enclose the key-value pairs within a pair of curly brackets "{ }".

Thus

var identifier = { key1:value1, key2:value2,...,key_n:value_n}

Example

var login = {'username':'admin','password':'root'}

Declaring a Map using a Map Constructor

To declare a Map using a Map constructor, you have to follow two steps. 

First, declare the map and second, initialize the map.

Thus:

var identifier = new Map()

Now, assign values to map keys

map_name[key] = value

Example

var login = new Map();

login['username'] = "admin";

login['password'] = "root";

Adding value to map

Syntax

identifier[key] = value;

Example

login['email'] = 'test@example.com';

Note − A map value can be any object including NULL.

Example

import 'dart:io';
void main(){
     var login = {"username":"admin","password":"root"};
     print("Map Content");
     print(login);
     print("Accessing content");
     print("Username:${login['username']}");
     print("Password:${login['password']}");
     print("Adding new field");
     login['email'] = "test@example.com";
     print("Print new content");
     print(login);
     print("Constructor Method");
     var details = new Map();
     details['firstname'] = "Joseph";
     details['lastname'] = "Mensah";
     details['age'] = 20;
     print(details);
     print("Adding new field");
     details['pic'] = "https://kuulchat.com/images/pic.jpg";
     print("Print new content");
     print(details);
}

Outcome

Map Content

{username: admin, password: root}

Accessing content

Username:admin

Password:root

Adding new field

Print new content

{username: admin, password: root, email: test@example.com}

Constructor Method

{firstname: Joseph, lastname: Mensah, age: 20}

Adding new field

Print new content

{firstname: Joseph, lastname: Mensah, age: 20, pic: https://kuulchat.com/images/pic.jpg}

Map – Properties

The Map class in the dart:core package defines the following properties −

Keys

Returns an iterable object representing keys

Values

Returns an iterable object representing values

Length

Returns the size of the Map

isEmpty

Returns true if the Map is an empty Map

isNotEmpty

Returns true if the Map is an empty Map

Example

import 'dart:io';
void main(){
     var details = new Map();
     details['firstname'] = "Joseph";
     details['lastname'] = "Mensah";
     details['age'] = 20;
     print(details);
     print("Adding new field");
     details['pic'] = "https://kuulchat.com/images/pic.jpg";
     print("Print new content");
     print(details);
     print("Map Properties");
     print("Map keys");
     print(details.keys);
     print("Map values");
     print(details.values);
     print("Map Size");
     print(details.length);
     if(details.isNotEmpty){
       print("Details map is not empty");
     }
     var emptyMap = new Map();
     if(emptyMap.isEmpty){
       print("Empty Map is empty");
     }
}

Output

{firstname: Joseph, lastname: Mensah, age: 20}

Adding new field

Print new content

{firstname: Joseph, lastname: Mensah, age: 20, pic: https://kuulchat.com/images/pic.jpg}

Map Properties

Map keys

(firstname, lastname, age, pic)

Map values

(Joseph, Mensah, 20, https://kuulchat.com/images/pic.jpg)

Map Size

4

Details map is not empty

Empty Map is empty

Map - Functions

Following are the commonly used functions for manipulating Maps in Dart.

addAll()

Adds all key-value pairs of other to this map.

clear()

Removes all pairs from the map.

remove()

Removes key and its associated value, if present, from the map.

forEach()

Applies f to each key-value pair of the map.

Applies the specified function on every Map entry. In other words, forEach enables iterating through the Map’s entries.

Syntax

Map.forEach(void f(K key, V value));

Parameters

f(K key, V value) − Applies f to each key-value pair of the map.

Calling f must not add or remove keys from the map

Return Type − void.

Example

void main(){
     var details = new Map();
     details['firstname'] = "Joseph";
     details['lastname'] = "Mensah";
     details['age'] = 20;
     print(details);
     print("Adding new field");
     details['pic'] = "https://kuulchat.com/images/pic.jpg";
     print("Print new content");
     print(details);
     var address = new Map();
     address['street'] = "12 B Mango Street";
     address['town'] = "Teshie";
     address['city'] = "Accra";
     address['country'] = "Ghana";
     print("Address Map");
     print(address);
     details.addAll(address);
     print("After adding address to details");
     print(details);
     print("Remove country from details");
     details.remove("country");
     print("After removing country from details");
     print(details);
     print("Calling foreach to loop through each key and value");
     details.forEach((k,v) => print('${k}: ${v}'));
     print("Calling clear to clear all keys and values");
     details.clear();
     print("After clearing details map");
     print(details);
     
}

Output

{firstname: Joseph, lastname: Mensah, age: 20}

Adding new field

Print new content

{firstname: Joseph, lastname: Mensah, age: 20, pic: https://kuulchat.com/images/pic.jpg}

Address Map

{street: 12 B Mango Street, town: Teshie, city: Accra, country: Ghana}

After adding address to details

{firstname: Joseph, lastname: Mensah, age: 20, pic: https://kuulchat.com/images/pic.jpg, street: 12 B Mango Street, town: Teshie, city: Accra, country: Ghana}

Remove country from details

After removing country from details

{firstname: Joseph, lastname: Mensah, age: 20, pic: https://kuulchat.com/images/pic.jpg, street: 12 B Mango Street, town: Teshie, city: Accra}

Calling foreach to loop through each key and value

firstname: Joseph

lastname: Mensah

age: 20

pic: https://kuulchat.com/images/pic.jpg

street: 12 B Mango Street

town: Teshie

city: Accra

Calling clear to clear all keys and values

After clearing details map

{}