Dart data type 01 developed by Flutter

Dart data type 01 developed by Flutter
These articles are all learningDartThe study notes recorded in the process are some basic knowledge, almost no technical content, mainly for the convenience of later use when easy to refer to
My article on Flutter and Dart syntax series can be read if you are interested

Dart data type

The Dart supports the following built-in data types

numbers(digital)
strings(String)
booleans(Boolean)
lists(Also known asarrays)
maps
runesUsed to represent in a stringUnicodeCharacters)
symbols

The above data types can be initialized directly with literals

var str = 'this is a string' ;
var isStr = true;
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You can also initialize using constructors. Because each variable in Dart refers to an object — an instance of a class — some built-in types have their own constructors

// You can use the Map() constructor to create a Map, like this
var map = new Map(a);Copy the code

Dart Numbers supports two types of Numbers:

int: Integer value, whose value is usually located- 2 ^ 53and2 ^ 53Between, about9 * 10 ^ 16, that is, support for 16 digits
double: 64-bit(double precision) floating point number, consistentIEEE 754standard

int

Its value is usually located at- 2 ^ 53and2 ^ 53between
That’s between minus 9,007,199,254,740,992 and 9,007,199,254,740,992
In practice, more than 19 bits will cause an error
Refer to question 1533 for more information

/ / the following definitions may be an error, the star int variables defined in 9223372036854775807 or less than - 9223372036854775808
var bil = 12345678901234567890;
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Some common judgment attributes

  const m1 = 12;
  
  // Whether the value is negative or greater than 0 is false
  print(m1.isNegative);
  print(0.isNegative);

  // Is it finite
  print(b32.isFinite);
  print(m1.isFinite);

  // is infinite or infinitesimal
  print(m1.isInfinite);

  // Whether it is even
  print(m1.isEven);

  // Whether it is odd
  print(m1.isOdd);

  // Is it a NaN value
  print(m1.isNaN);
  
  // Symbol of data, -1.0: value less than 0, +1.0: value greater than 0, -0.0/0.0/NaN: value itself
  print(21.sign);  / / 1
  print(- 23.sign); // -1
  print(0.sign);   / / 0
  print(- 0.sign);  / / 0
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Int A commonly used function of a number type

  const m3 = 24;
  
  // Get the absolute value
  print(m3.abs());

  // To a string
  print(m3.toString());

  // power modulo; M3 to the fourth, modulo 3
  print(m3.modPow(4.3)); / / 1

  // return the greatest common divisor of m3 and 16
  print(m3.gcd(16));
  
  Return the remainder of m3 divided by 5
  print(m3.remainder(5));

  / / to double
  print(m3.toDouble());
  
  // Compare sizes: 0: same, 1: greater, -1: smaller
  print(m3.compareTo(30));
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double

Here are some ways to define a double:

var y = 1.1;
var exponents = 1.42 e5;
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Properties associated with type double

  // Is it a NaN value
  print(d0.isNaN);
  // is infinite or infinitesimal
  print(d0.isInfinite);
  // Is it finite
  print(d0.isFinite);
  // Whether the value is negative or greater than 0 is false
  print(d0.isNegative);

  // Hash code generated from the code unit
  print(d0.hashCode);
  
  // Symbol of data, -1.0: value less than 0, +1.0: value greater than 0, -0.0/0.0/NaN: value itself
  print(d0.sign);
  print(1.23.sign);
  print(0.0.sign);

  // Returns the type of the runtime
  print(d0.runtimeType);  // double
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The use of methods related to type double

  // To a string
  print(d0.toString());

  // Take the integer and discard the decimal point
  print(d0.toInt());

  // Compare sizes: 0: same, 1: greater, -1: smaller
  print(d0.compareTo(30));

  // Get the absolute value
  print(d0.abs());

  // Round off
  print(d0.round()); / / 13
  // round up
  print(d0.ceil());  / / 14
  // round down
  print(d0.floor()); / / 13

  Round ().todouble ()
  print(d0.roundToDouble()); / / 13.0
  print(d0.ceilToDouble());  / / 14.0
  print(d0.floorToDouble()); / / 13.0

  // Retain the specified decimal number (rounded), not complement 0, string return
  print(d0.toStringAsFixed(2)); / / 13.10

  // Retain the number of digits of the variable (the total number of digits before and after the decimal point), not enough to complement 0, extra rounding
  print(d0.toStringAsPrecision(10));  / / 13.09870000

  /** toStringAsExponential * 1.toStringAsExponential(); // 1e+0 * 1.toStringAsExponential(3); / / 1.000 e+0 * 123456. ToStringAsExponential (); / / 1.23456 e+5 * 123456. ToStringAsExponential (3); / / 1.235 e+5 * 123. ToStringAsExponential (0); // 1e+2 */


  /** toStringAsPrecision * 1.toStringAsPrecision(2); / / 1.0 * 1 e15. ToStringAsPrecision (3); / / 1.00 e+15 * 1234567. ToStringAsPrecision (3); / / 1.23 e+6 * 1234567. ToStringAsPrecision (9); / / 1234567.00 * 12345678901234567890. ToStringAsPrecision (20); // 12345678901234567168 * 12345678901234567890.toStringAsPrecision(14); / / e+19 1.2345678901235 * 0.00000012345. ToStringAsPrecision (15); / / 1.23450000000000 * 0.0000012345 e-7 toStringAsPrecision (15); / / 0.00000123450000000000 * /
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Booleans

To represent Boolean values,DartThere is a name forboolThe type of.
Only two objects are of type Boolean:trueandfalseObject, both of which are also compile-time constants
whenDartWhen you need a Boolean, it’s onlytrueObjects are considered to betrueAnd all the other values areflase; As 1."aString", as well assomeObjectEquivalents are considered to befalse

  const m = 1;
  if (m) {
    print('Is a Boolean');
  } else {
    print('Not a Boolean');
  }
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inDartIs the code that determines that the above statement is valid
But in theDartCheck mode run, the above code will throw an exception, indicatingmThe variable is not a Boolean value
Therefore, it is not recommended to use the above method to judge

Strings

Dart strings are UTF-16-encoded sequences of characters that can be created using either single or double quotation marks:

var s1 = 'Single quotes work well for string literals.';
var s2 = "Double quotes work just as well.";
// When there are single quotes inside single quotes (double quotes inside double quotes), you must use backslash \ bar escape
var s3 = 'It\'s easy to escape the string delimiter.';
var s4 = "It's even easier to use the other delimiter.";
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Concatenation of strings

You can concatenate strings by simply writing adjacent strings together

var string = 'name''+''age'
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Concatenate adjacent strings with +

var string1 = 'name' + '+' + 'age';
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Reference variables

inDartThe use of$Symbols refer to variables or expressions
Expression reference:${expression}If the expression is a variable{}Can be omitted

  const num1 = 12;
  // Reference the expression
  const ageStr0 = 'age = $num1';
  const ageStr1 = 'age = ${num1} is my age';
  // Reference the expression
  const ageStr2 = 'age = ${num1 * num1}';
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Multiline string

Triple quotation marks using single or double quotation marks

  const line1 = Tens of thousands of roads, safety first, driving is not standard, the family two lines of tears.;
  const line2 = """ Thousands of roads, safety first, driving is not standard, family tears """;
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Escape symbol

Declare the raw string (prefixed with r) by prefixing the string with the character r, or preceded by \ to avoid escaping \. This is especially useful in regular expressions

  // Escape characters
  print(R 'escape character, \n');
  print('Escape character, \\n');
  print('Escape character, \n');
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Attribute is introduced

  const string0 = 'https://www.titanjun.top/';
  
  // Get each character of the string according to the index
  print(string0[1]);

  // Whether the string is empty
  print(string0.isEmpty);
  print(' '.isEmpty); // true
  // Whether the string is not empty
  print(string0.isNotEmpty);
  print(' '.isNotEmpty);  // false

  // Returns the iterable of the string Unicode code
  print(string0.runes);
  // Returns a list of UTF-16 code units for strings
  print(string0.codeUnits);
  // Returns the hash code generated from the code unit
  print(string0.hashCode);
  // The length of the string
  print(string0.length); 
  // Returns the runtime type of the object
  print(string0.runtimeType);  // String
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Methods to introduce

  const string0 = 'https://www.titanjun.top/';
  
  // String comparison
  print('titan'.compareTo('jun'));
  
  // Case conversion
  print(string0.toUpperCase());
  print(string0.toLowerCase());

  // Truncate string (leading index and ending index)
  print(string0.substring(0.5)); // https
  // The index is truncated to the end by default
  print(string0.substring(12));  // titanjun.top/

  // Split the string
  print(string0.split('. '));  // [https://www, titanjun, top/]
  print(string0.split(new RegExp(r"t")));  // [h, , ps://www., i, anjun., op/]

  // Remove TAB Spaces and newlines from the string
  const string1 = '\t\ttitanjun top\n';
  print(string1.trim());
  // Remove TAB Spaces and newlines at the beginning of the string
  print(string1.trimLeft());
  // Remove TAB Spaces and newlines at the end of the string
  print(string1.trimRight());
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endsWith

Determines whether a string ends with a character (string). The argument does not accept a regular expression

  const str1 = 'titanjun.top';
  print(str1.endsWith('p'));  //true
  print(str1.endsWith('/'));  //false
  print(str1.endsWith('top'));  //true
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startsWith

bool startsWith(Pattern pattern, [int index = 0]);
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Determines whether a string begins with a character (string) and accepts a regular expression

  const str1 = 'titanjun.top';
  print(str1.startsWith('h'));  //false
  print(str1.startsWith('tit')); //true
  print(str1.startsWith('it'.1)); //true
  print(str1.startsWith(new RegExp(r'[A-Z][a-z]'), 1)); //false
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indexOf

int indexOf(Pattern pattern, [int start]);
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Gets the index of the specified character (string) based on its first occurrence in the original string, from left to right
You can start at the specified index, starting at 0 by default
If there is no character (string) in the original string, the return value is: -1

  const str2 = 'https://www.titanjun.top/';
  print(str2.indexOf('titan')); / / 12
  print(str2.indexOf('t'.5));  / / 12
  print(str2.indexOf(new RegExp(r'[a-z]'))); / / 0
  // If no characters are changed, -1 is printed
  print(str2.indexOf('ppp'));  // -1
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lastIndexOf

int lastIndexOf(Pattern pattern, [int start]);
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The effect is the same as indexOf, except that the order of indexOf is left to right and that of lastIndexOf is right to left

  const str2 = 'https://www.titanjun.top/';
  print(str2.lastIndexOf('t'.20));  / / 14
  print(str2.indexOf(new RegExp(r'[a-z]'))); / / 0
  // If no characters are changed, -1 is printed
  print(str2.indexOf('ppp'));  // -1
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Fill a placeholder

String padLeft(int width, [String padding = ' ']);
String padRight(int width, [String padding = ' ']);
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Fill in placeholders before and after strings
Parameter one: The number of bits of the string you want to get
Parameter two: Supplementary character if the number of bits is insufficient

  const str3 = '12';
  print(str3.padLeft(2.'0')); / / 12
  print(str3.padRight(3.'0')); / / 120
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contains

bool contains(Pattern other, [int startIndex = 0]);
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Checks whether the string contains a character
Checks if the character at the specified index is a character

bool contains(Pattern other, [int startIndex = 0]);

const str = 'Dart strings';
print(str.contains('D'));
print(str.contains(new RegExp(r'[A-Z]')));
print(str.contains('D'.0));
print(str.contains(new RegExp(r'[A-Z]'), 0));
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Substitution characters

// Can only be replaced once. Parameter 3 is the starting index value. Default is 0
String replaceFirst(Pattern from, String to, [int startIndex = 0]);

// Replace all qualified characters (strings)
String replaceAll(Pattern from, String replace);

// Replace a range of characters
String replaceRange(int start, int end, String replacement);

// The following is an example:
// Replace the string
  const str4 = 'titanjun12--0123';
  print(str4.replaceFirst('t'.'T'));  // Titanjun12--0123
  print(str4.replaceFirst('12'.'21'.10));   //titanjun12--0213

  // Replace all
  print(str4.replaceAll('12'.'21'));  //titanjun21--0213
  print(str4.replaceAll('t'.'T'));  //TiTanjun12--0123

  // Interval substitution
  print(str4.replaceRange(0.5.'top'));  //topjun12--0123
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List

A List object in Dart is an array in other languages

Create an array

// Create a List of specified length without adding/removing elements
List([int length]);

// Create a fixed-length List by specifying the length, and use fill to initialize the value at each position without adding/removing elements
List.filled(int length, E fill, {bool growable: false});

// Create a List of all elements,
// When growable is true (the default), the constructor returns a growable List. Otherwise, it returns a List of fixed length
List.from(可迭代 elements, {bool growable: true})

// Generate a List of all values
// Create a List of fixed length unless growable is true(the default)
List.generate(int length, E generator(int index), {bool growable: true})

// Create one that contains all elements without changing its length or elements
List.unmodifiable(可迭代 elements)
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List

If the parameters are setlength(lengthIt cannot be negative ornull), then createdListIt’s a fixed length
Elements can be modified, the number of elements cannot be modified, and elements cannot be deleted or added

var l1 = new List(3);  //[null, null, null]
print(l1.length);  / / 3

// The following is an error
l1.length = 1;
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If length is not set, the length of the List is 0 and is growable

// Both ways are the same
var l10 = new List(a);var l11 = [];

// All work
l10.length = 3;
l10.add(1);
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When creating a growable List with a specified length, the length is allocated only after creation

List growableList = new List().. length =500;
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filled

Creates a fixed-length object by specifying the lengthListAnd initializes the value for each position
All the elements are the samefillValue. If the specified value is a mutable object, thenListAll elements are the same object and are modifiable

  var l2 = new List.filled(3.'l');  //[l, l, l]
  var l3 = new List.filled(2[]);/ / [[], []]
  l3[0].add(12);   
  print(l3);      / / [[12], [12]]
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from

Create one that contains allelementstheList
elementstheIteratorSpecifies the order of elements.
whengrowablefortrue(default), the constructor returns a growableList. Otherwise, it returns a fixed lengthList

var l5 = new List.from([1.2.3.4]);
l5.add(5);
print(l5);   // [1, 2, 3, 4, 5]

// The following add method returns an error
var l5 = new List.from([1.2.3.4], growable: false);
l5.add(5);
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generate

Generates a value that contains all valuesListCreates an element based on the index value
growableforfalseIs created whenListIt’s a fixed length

var l4 = new List.generate(3, (int i) => i * i);
l4.add(14);
print(l4);
// [0, 1, 4, 14]
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unmodifiable

Create one that contains allelementsUnmodifiableList
unmodifiableListYou can’t change its length or elements
If the element itself is immutable, thenListIt’s also immutable

var l6 = new List.unmodifiable([1.2.3.4]);
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The List properties

  var arr1 = [1.2.3.4];
  // The first and last elements of the array
  print(arr1.first);  / / 1
  print(arr1.last);   / / 4

  // Check whether the array is empty
  print(arr1.isNotEmpty);  // true
  print(arr1.isEmpty);     // false

  // Array length, number of elements
  print(arr1.length);  / / 4

  // Return the List in reverse order
  print(arr1.reversed);  // [4, 3, 2, 1]

  // Returns an Iterator that is allowed to iterate over all elements of the Iterable
  print(arr1.iterator);

  // The runtime type of the object
  print(arr1.runtimeType);   // List<int>

  // Get the hash value of the object
  print(arr1.hashCode);
  
  // Get the element by index
  print(arr1[2]);

  // Modify the element according to the index
  arr1[1] = 11;
  print(arr1);
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The List method

increase

  // Add elements
  arr1.add(5);

  // Add an array
  arr1.addAll([10.12]);
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To find the

  var arr2 = ['one'.'two'.'three'.'one'.'four'];

  // Whether to contain an element
  print(arr2.contains('one'));  // true
  // Check whether the array has elements that meet the criteria
  print(arr2.any((item) => item.length > 4));  // true
  // Check whether all elements of the array meet the criteria
  print(arr2.every((item) => item.length > 4));  // false
  
  // Convert to Map with index as Key and corresponding element as Value
  print(arr2.asMap());  // {0: one, 1: two, 2: three, 3: one, 4: four}
  
  // Randomly shuffle the elements in the List
  arr2.shuffle();

  // Get the element by index, equivalent to arr2[3]
  print(arr2.elementAt(3));

  // Get the index of the element, starting at index 0 by default
  print(arr2.indexOf('one'));  / / 0
  // Start with the second index
  print(arr2.indexOf('one'.2));  / / 3
  // If it cannot be found, return -1
  print(arr2.indexOf('five'));  // -1

  // Get the index of the element from the back
  print(arr2.lastIndexOf('one'));
  print(arr2.lastIndexOf('one'.3));
  print(arr2.lastIndexOf('five'));
  
  // Returns the first element that satisfies the condition
  print(arr3.firstWhere((item) => item == 'one'));
  
  // Find the element that matches the condition, if there is only one element that matches the condition, return that element
  // If no element is matched, or if more than one element is matched, an exception is thrown
  print(arr2.singleWhere((item) => item.length == 5));  //three
  
  // Return all elements except the original count
  arr2 = ['one'.'two'.'three'.'four'];
  print(arr2.skip(2)); // (three, four)

  // Return all elements that do not meet this condition
  print(arr2.skipWhile((item) => item.length == 3));  //(three, four)

  // Return a new List of objects from start (inclusive) to end (exclusive), unchanged
  print(arr2.sublist(1.3));
  // Do not specify end, default to array end
  print(arr2.sublist(2));
  
  // Get the elements of an interval, return an array
  print(arr2.getRange(1.3));    // ['two', 'three']
  
  // Concatenate arrays into strings
  print(arr2.join());  //onetwothreefour
  print(arr2.join(The '-'));  //one-two-three-four

  // Return the original count of elements
  print(arr2.take(2));
  // Returns the array elements that match the condition until the condition value is false to stop filtering
  arr2 = ['one'.'two'.'three'.'four'.'ten'];
  print(arr2.takeWhile((item) => item.length == 3));  //(one, two)
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delete

  var arr2 = ['one'.'two'.'three'.'one'.'four'];
  
  // Delete the specified element
  // Return true if the element exists
  print(arr2.remove('two'));  // true
  print(arr2);   // [one, three, one, four]
  // If there is no such element, return false
  print(arr2.remove('five'));  // false

  // Delete by index, return the value of the deleted element
  print(arr2.removeAt(1));  // three
  print(arr2);   // [one, one, four]

  // Remove the last element and return the value of that element
  print(arr2.removeLast());  // four
  print(arr2); // [one, one]

  [start, end] [start, end]
  arr2.addAll(['six'.'seven'.'eight']);
  arr2.removeRange(1.3);
  print(arr2);  // [one, seven, eight]

  // Remove all elements that match the criteria
  arr2.removeWhere((item) => item.length == 3);
  print(arr2);  // [seven, eight]
  
  // Remove all elements in the List that do not meet the criteria
  arr2.retainWhere((item) => item.length > 3);
  print(arr2);
  
  // Delete all elements
  arr1.clear();
  print(arr1);  / / []
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insert

  var arr3 = [1.3.4];
  // Insert an element somewhere
  arr3.insert(1.10);
  print(arr3); / / [1, 10, 3, 4]

  // Insert an array
  arr3.insertAll(2[12.32]);
  print(arr3);
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Important method

/ / filter Iterable < E >where(bool test(E element)) => new WhereIterable<E>(this, test); Iterable<T> map<T>(T f(E E)) => new MappedIterable<E, T>(this, f); Void sort([int compare(E a, E b)]); // iterative calculation, initialValue: initialValue, combine: T fold<T>(T initialValue, T combine(T previousValue, E element)) // Combine (T previousValue, E element) Compute function E reduce(E combine(E value, E element)) // Perform function operation void in iterative order for each element of the setforEach(void f(E element)) // Expand Each element of Iterable to 0 or more elements.Copy the code

Let’s take a look at how each of these functions is used

  var arr2 = ['one'.'two'.'three'.'four'];
  
  // The filter operation returns all elements that meet the criteria
  print(arr2.where((item) => item.length == 3));  //(one, two, ten)
  
  // Map a new array. The argument is a function
  var array = arr2.map((item) {
    return item + The '-';
  });
  print(array.toList());  // [one-, ten-, two-, four-, three-]
  
  // Sort by default from small to large
  arr2.sort();
  print(arr2);  //[four, one, ten, three, two]
  // Set the criteria for sorting
  arr2.sort((item1, item2) {
    // If the result of the two comparisons is 0, the results may be different after sorting
    return item1.length.compareTo(item2.length);
  });
  print(arr2);  //[one, ten, two, four, three]

  // iterative calculation, initialValue: initialValue, combine: calculation function
  var arr4 = [1.2.3.4];
  // Set the initial value
  var result1 = arr4.fold(10, (prev, value) => prev + value);  / / 20
  var result2 = arr4.fold(2, (prev, value) => prev * value);  / / 48
  // The initial value is the value of the first element. The iterable must have at least one element. If it has only one element, the element returns directly
  var result3 = arr4.reduce((value, element) => value * element);  / / 24

  // Operate on each element
  arr2.forEach((item) {
    print(item);
  });
  
  // expand, return a new Iterable in the order of the elements generated by calling f on each element
  var pairs = [[1.2], [3.4]].var flattened = pairs.expand((pair) => pair).toList();
  print(flattened); // => [1, 2, 3, 4];

  var input = [1.2.3];
  var duplicated = input.expand((i) => [i, i]).toList();
  print(duplicated); // => [1, 1, 2, 2, 3, 3]
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Because the space is too long, the remaining data types will continue to be studied and recorded in the next article

reference

Dart official website syntax introduction – Chinese version
Dart official website syntax introduction – English version
Dart Language Chinese community

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Dart data type 01 developed by Flutter

Dart data type

int

double

Booleans

Strings

Concatenation of strings

Reference variables

Multiline string

Escape symbol

Attribute is introduced

Methods to introduce

endsWith

startsWith

indexOf

lastIndexOf

Fill a placeholder

contains

Substitution characters

List

Create an array

List

filled

from

generate

unmodifiable

The List properties

The List method

increase

To find the

delete

insert

Important method

reference

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