The dart series: using numbers and strings in dart

Introduction

To be familiar with a language, the easiest way is to be familiar with the various core libraries provided by dart. Dart provides us with several commonly used libraries including dart:core, dart:async, dart:math, dart:convert, dart:html and dart:io.

Today I will introduce you to the use of numbers and strings in dart:core.

# number

Three types of numbers are defined in dart:core, namely num, int and double.

num is the general term for all numbers. Both int and double inherit from num and are subclasses of num.

In fact, there is also a data type called BigInt in dart:core. BigInt is an independent data type, not a subclass of num:

abstract class BigInt implements Comparable<BigInt>

The most common operation in numbers is to convert a string to a number. The conversion can call the parse method. First look at the definition of the parse method in num:

  static num parse(String input, [@deprecated num onError(String input)?]) {
    num? result = tryParse(input);
    if (result != null) return result;
    if (onError == null) throw FormatException(input);
    return onError(input);
  }

The input input can be decimal or hexadecimal, as shown below:

assert(int.parse('18') == 18);
assert(int.parse('0x05') == 5);
assert(double.parse('0.50') == 0.5);

num.parse will convert the corresponding character into int or double type:

assert(num.parse('18') is int);
assert(num.parse('0.50') is double);

The parse method can also pass in the base number corresponding to the string, such as decimal or hexadecimal:

assert(int.parse('11', radix: 16) == 17);

Above we talked about how to convert a string into a number, the following is how to convert a number into a string, num provides the toString() method, which can easily convert int and double into string.

assert(18.toString() == '18');

assert(3.1415.toString() == '3.1415');

For decimals, you can use toStringAsFixed to specify the number of decimal places:

assert(3.1415.toStringAsFixed(2) == '3.14');

If you want to use scientific notation, you can use toStringAsPrecision:

assert(314.15.toStringAsPrecision(2) == '3.1e+2');

String

All strings are encoded in UTF-16 in dart. String in dart defines many commonly used and very useful methods.

For example, query in a string:

assert('www.flydean.com'.contains('flydean'));

assert('www.flydean.com'.startsWith('www'));

assert('www.flydean.com'.endsWith('com'));

assert('www.flydean.com'.indexOf('flydean') == 4);

To intercept a substring from a string:

assert('www.flydean.com'.substring(4, 11) == 'flydean');

To intercept the string according to specific characters:

var parts = 'www.flydean.com'.split('.');
assert(parts.length == 3);

So what is the corresponding Chinese support in dart? Because all characters in dart are expressed in UTF-16, if a UTF-16 unit can represent the corresponding character, then Chinese is no problem in use:

  assert('你好吗？'.substring(1,2) == '好');
  assert('你好吗？'[1] == '好');

However, some characters cannot be represented by a UTF-16 unit. At this time, you need to use the characters package to process specific characters.

Convert the string to uppercase or lowercase:

assert('www.flydean.com'.toUpperCase() ==
    'WWW.FLYDEAN.COM');

// Convert to lowercase.
assert('WWW.FLYDEAN.COM'.toLowerCase() ==
    'www.flydean.com');

Dart provides the trim() method, which can intercept the spaces before and after the string:

assert('  www.flydean.com  '.trim() == 'www.flydean.com');

StringBuffer

In addition to the displayed string to create characters, dart also provides the StringBuffer class, through which we can create strings freely:

var sb = StringBuffer();
sb
  ..write('www.flydean.com ')
  ..writeAll(['is', 'very', 'good'], ' ')
  ..write('.');

var fullString = sb.toString();

The above code output: "www.flydean.com is very good."

Among them, writeAll() connects the incoming character array with a specific connector.

Summarize

The above is the introduction of numbers and strings in dart.

This article has been included in http://www.flydean.com/14-dart-number-string/

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