Strings recipes for the Dart Cookbook

recipes/src/core/strings.md

+## Concatenating Strings
+
+### Problem
+
+You want to know how to concatenate strings in Dart. You tried using `+`, but

[sethladd, 2013/02/26 23:41:45]

A more concise way:

"You want to combine two or more strings"

Avoid "you want to know how to..."

+that resulted in an error.
+
+### Solution
+
+Use adjacent string literals:
+
+    'Dart'  'is' ' fun!'; // 'Dart is fun!'

[sethladd, 2013/02/26 23:41:45]

assign to variable

+        
+### Discussion
+
+Adjacent literals also work over multiple lines:
+
+        'Dart'
+        'is'
+        'fun!'; // 'Dart is fun!'
+
+They also work when using multiline strings:
+
+    '''Peanut
+    butter'''
+    '''and
+    jelly'''; // 'Peanut\nbutter and\njelly'
+        
+You can also concatenate adjacent single line literals with multiline strings:
+
+    'Peanut ' 'butter'
+    ''' and
+    jelly'''; // 'Peanut butter and\n jelly'
+
+#### Alternatives to adjacent string literals
+
+Use `concat()`:

[sethladd, 2013/02/26 23:41:45]

I'm not sure we want to show things that we generally don't recommend. Why show
this and not show StringBuffer? Shouldn't it be the other way around?

+
+    'Dewey'.concat(' Cheatem').concat(' and').concat( ' Howe'); // 'Dewey Cheatem and Howe'

[sethladd, 2013/02/26 23:41:45]

nice example :)

+
+Since `concat()` creates a new string every time it is invoked, a long chain of
+`concat()`s can be expensive; if you need to incrementally build up a long
+string, use a StringBuffer instead (see below).
+
+Use `join()` to combine a sequence of strings:
+
+    ['Dewey', 'Cheatem', 'and', 'Howe'].join(' '); // 'Dewey Cheatem and Howe'
+
+You can also use string interpolation (see below).
+
+
+## Interpolating expressions inside strings
+
+### Problem
+
+You want to create strings that contain Dart expressions and identifiers.
+
+### Solution
+
+You can put the value of an expression inside a string by using ${expression}.

[sethladd, 2013/02/26 23:41:45]

wrap in backticks?

[sethladd, 2013/02/26 23:41:45]

Shouldn't it be more declarative?

s/You can put/Put the

I don't know what proper cookbook style is.

+
+    var favFood = 'sushi';
+    'I love ${favFood.toUpperCase()}'; // 'I love SUSHI'
+
+You can skip the {} if the expression is an identifier:
+
+    'I love $favFood'; // 'I love sushi'
+      
+### Discussion
+
+An interpolated string‚ `string ${expression}` is equivalent to the
+concatenation of the strings ‚ 'string ' and `expression.toString()`.
+Consider this code:
+
+    var four = 4;
+    'The $four seasons'; // 'The 4 seasons'
+        
+It is equivalent to the following:
+
+    'The '.concat(4.toString()).concat(' seasons'); // 'The 4 seasons'
+        
+You should consider implementing a `toString()` method for user-defined

[sethladd, 2013/02/26 23:41:45]

This is a separate recipe. Consider breaking it out to a Problem: "How to
control the output of an object in string interpolation"

+objects. Here's what happens if you don't:
+
+    class Point {
+      num x, y;
+      Point(this.x, this.y);
+    }
+    
+    var point = new Point(3, 4);
+    'Point: $point'; // "Point: Instance of 'Point'"

[sethladd, 2013/02/26 23:41:45]

assign to variable. Creating objects without variables is jarring to me.

+
+Probably not what you wanted. Here is the same example with an explicit
+`toString()`:
+
+    class Point {
+      ...
+            
+      String toString() => "x: $x, y: $y";
+    }
+        
+    'Point: $point'; // 'Point: x: 3, y: 4'
+
+Interpolations are not evaluated within raw strings:
+
+    r'$favFood'; // '$favFood'

[sethladd, 2013/02/26 23:41:45]

This came out of left field. Can you add a recipe for "How do I use special
characters in my string that would otherwise be interpreted?" before this?

+        
+## Incrementally building a string efficiently using a StringBuffer
+
+### Problem
+
+You want to collect string fragments and combine them in an efficient manner.
+
+### Solution
+
+Use a StringBuffer to programmatically generate a string. A StringBuffer 
+collects the string fragments, but does not generate a new string until
+`toString()` is called:
+
+    var sb = new StringBuffer();
+    sb.write("John, ");
+    sb.write("Paul, ");
+    sb.write("George, ");
+    sb.write("and Ringo");
+    sb.toString(); // "John, Paul, George, and Ringo"
+    

[sethladd, 2013/02/26 23:41:45]

assign to variable

+### Discussion
+
+In addition to `write()`, the StringBuffer class provides methods to write a
+list of strings (`writeAll()`), write a numerical character code
+(`writeCharCode()`), write with an added newline ('writeln()`), and more. Here

[sethladd, 2013/02/26 23:41:45]

Is this now writeCharUnit?

+is a simple example that show the use of these methods:
+
+    var sb = new StringBuffer();
+    sb.writeln("The Beatles:");
+    sb.writeAll(['John, ', 'Paul, ', 'George, and Ringo']);
+    sb.writeCharCode(33); // charCode for '!'.
+    sb.toString(); // 'The Beatles:\nJohn, Paul, George, and Ringo!' 
+
+Since a StringBuffer waits until the call to `toString()` to generate the
+concatenated string,  it represents a more efficient way of combining strings
+than `concat()`.  See the "Concatenating Strings" recipe for a description of
+`concat()`.
+
+## Converting between string characters and numbers

[sethladd, 2013/02/26 23:41:45]

Can you change the title? I saw "numbers" and I thought you meant like toInt()

+
+### Problem 
+
+You want to convert string characters into numerical code units and back.
+
+### Solution
+
+Use `string.codeUnits()` to access the sequence of Unicode UTF-16 code units
+that make up a string:
+    
+    'Dart'.codeUnits.toList(); // [68, 97, 114, 116]
+    
+     var smileyFace = '\u263A'; // ☺
+     smileyFace.codeUnits.toList(); // [9786]
+     
+The number 9786 represents the code unit '\u263A'.
+     
+Use the `runes` getter to access a string's code points:
+ 
+    'Dart'.runes.toList(); // [68, 97, 114, 116]
+     smileyFace.runes.toList(); // [9786]
+ 
+### Discussion
+
+Notice that using `runes` and `codeUnits()` produces identical results
+in the examples above. That is because each character in both 'Dart' and
+`smileyFace` fits within 16 bits, resulting in a code unit corresponding
+neatly with a code point.
+
+Consider an example where a character cannot be represented within 16-bits,
+the Unicode character for a Treble clef ('\u{1F3BC}'). This character consists
+of a surrogate pair: '\uD83C', '\uDFBC'. Getting the numerical value of this
+character using `codeUnits()` produces the following result:
+
+    var clef = '\u{1F3BC}'; // 🎼
+    clef.codeUnits.toList(); // [55356, 57276]
+
+The numbers 55356 and 57276 represent `clef`'s surrogate pair, '\uD83C' and
+'\uDFBC', respectively.
+
+#### Using the runes getter
+
+You can also use `runes` to convert a string to its corresponding numerical values:
+
+    clef.runes.toList(); // [127932]
+    
+The number 127932 represents the code point '\u1F3BC'.
+
+#### Using codeUnitAt() to access individual characters
+
+To access the 16-Bit UTF-16 code unit at a particular index, use
+`codeUnitAt()`:
+
+    'Dart'.codeUnitAt(0); // 68
+    smileyFace.codeUnitAt(0); // 9786
+    
+The number 9786 represents the code unit '\u263A', the `smileyFace`
+characrter.
+
+Using `codeUnitAt()` with the multi-byte `clef` character leads to problems:
+
+    clef.codeUnitAt(0); // 55356
+    clef.codeUnitAt(1); // 57276
+
+In either call to `clef.codeUnitAt()`, the values returned represent strings
+that are only one half of a UTF-16 surrogate pair.  These are not valid UTF-16
+strings.
+
+#### Converting numerical values to strings
+
+You can generate a new string from code units using the factory 
+`String.fromCharCodes(charCodes)`:
+
+    new String.fromCharCodes([68, 97, 114, 116]); // 'Dart'
+        
+    var heart = '\u2661'; // ♡
+    new String.fromCharCodes([73, 32, 9825, 32, 76, 117, 99, 121]);
+    // 'I ♡ Lucy'
+
+The charCodes can be UTF-16 code units or runes.
+
+The Unicode character for a Treble clef is '\u{1F3BC}', with a rune value of 
+127932. Passing either code units, or a code point to `String.fromCharCodes()`
+produces the `clef` string:
+
+    new String.fromCharCodes([55356, 57276]); // 🎼
+    new String.fromCharCodes([127932]), // 🎼
+        
+You can use the `String.fromCharCode()` factory to convert a single code unit
+to a string:
+
+    new String.fromCharCode(127932); // 🎼
+
+Creating a string with only one half of a surrogate pair is permitted, but not
+recommended.
+
+## Determining if a string is empty
+
+### Problem
+
+You want to know if a string is empty. You tried ` if(string) {...}`, but that
+did not work.
+
+### Solution
+
+Use `string.isEmpty`:
+
+    var emptyString = '';
+        emptyString.isEmpty; // true
+        
+A string with a space is not empty:
+ 
+    var space = " ";
+    space.isEmpty; // false
+   
+### Discussion 
+
+Don't use `if (string)` to test the emptiness of a string. In Dart, all
+objects except the boolean true evaluate to false. `if(string)` will always
+be false.
+
+Don't try to explicitly test for the emptiness of a string:
+
+    if (emptyString == anotherString) {...}
+    
+This may work sometimes, but if `string` has an empty value that is
+not a literal `''`, the comparisons will fail:
+    
+    emptyString == '\u0020'; // false
+    emptyString == '\u2004'; // false
+    
+## Removing leading and trailing whitesapce
+
+### Problem
+
+You want to remove leading and trailing whitespace from a string.
+
+### Solution
+
+Use `string.trim()`:
+
+    var space = '\n\r\f\t\v';
+    var string = '$space X $space';
+    string.trim(); // 'X'
+
+The String class has no methods to remove leading and trailing whitespace. But
+you can always use regExps.
+
+Remove only leading whitespace:
+
+    string.replaceFirst(new RegExp(r'^\s+'), ''); //  'X $space'
+   
+Remove only trailing whitespace:
+
+    string.replaceFirst(new RegExp(r'\s+$'), ''); // '$space X'
+
+## Calculating the length of a string
+
+### Problem
+
+You want to get the length of a string, but are not sure how to 
+correctly calculate the length when working with Unicode.
+
+### Solution
+
+Use string.length to get the number of UTF-16 code units in a string:
+        
+    'I love music'.length; // 12
+        
+### Discussion
+
+For characters that fit into 16 bites, the code unit length is the same as the
+rune length:
+
+    var hearts = '\u2661'; // ♡
+    
+    hearts.length; // 1
+    hearts.runes.length; // 1
+  
+If the string contains any characters outside the Basic Multilingual
+Plane (BMP), the rune length will be less than the code unit length:
+  
+    var clef = '\u{1F3BC}'; // 🎼
+    clef.length; // 2
+    clef.runes.length; // 1
+    
+    var music = 'I $hearts $clef'; // 'I ♡ 🎼'
+    music.length; // 6
+    music.runes.length // 5
+
+Use `length` if you want to number of code units; use `runes.length` if you 
+want the number of distinct characters.
+
+## Getting the character at a specific index in a string
+
+### Problem
+
+You want to be able to access a character in a string at a particular index.
+
+### Solution
+
+For strings in the Basic Multilingual Plane (BMP), use [] to subscript the
+string:
+
+    'Dart'[0]; // 'D'
+
+    var hearts = '\u2661'; // ♡
+    hearts[0]; '\u2661' // ♡
+
+For non-BMP characters, subscripting yields invalid UTF-16 characters:
+
+    var coffee = '\u{1F375}'; // 🍵
+    var doughnuts = '\u{1F369}'; // 🍩
+    var healthFood = '$coffee and $doughnuts'; // 🍵 and 🍩
+ 
+    healthFood[0]; // Invalid string, half of a surrogate pair.
+    
+You can slice the string to get the first 2 code units:
+ 
+    healthFood.slice(0, 2); // 🍵
+
+#### The safer approach: subscript runes
+
+You can always subscript runes and be sure that you are dealing with complete
+characters:
+
+    healthFood.runes.first; // 127861
+
+The number 127861 represents the code point for coffee, '\u{1F375}' (🍵 ). 
+
+Contrast this with the result of subscripting `codeUnits`:
+
+    healthFood.codeUnits.first; // 55356
+
+The number 55356 represents the first of the surrogate pair for '\u{1F375}'. 
+This is not a valid UTF-16 string.
+
+If you are dealing with non-BMP characters, avoid subscripting `codeUnits`. 
+
+
+## Splitting a string
+   
+### Problem
+
+You want to split a string into substrings.
+
+### Solution
+
+To split a string into a list of characters, map the string runes:
+
+    "dart".runes.map((rune) => new String.fromCharCode(rune)).toList(); 
+    // ['d', 'a', 'r', 't']
+    
+    var smileyFace = '\u263A'; // ☺
+    var happy = 'I am $smileyFace'; // 'I am ☺'
+    happy.runes.map((charCode) => new String.fromCharCode(charCode)).toList(); 
+    // [I,  , a, m,  , ☺]
+        
+You can also use string.split(''):
+
+    'Dart'.split(''); // ['D', 'a', 'r', 't']
+    smileyFace.split('').length; // 1
+        
+Do this only if you are sure that the string is in the Basic Multilingual
+Plane (BMP). Since `split('')` splits at the UTF-16 code unit boundaries,
+invoking it on a non-BMP character yields the string's surrogate pair:
+
+    var clef = '\u{1F3BC}'; // 🎼, not in BMP.
+    clef.split('').length; // 2
+        
+The surrogate pair members are not valid UTF-16 strings.
+
+
+### Split a string using a regExp
+
+The `split()` method takes a string or a regExp as an argument. Here is an
+example of using `split()` with a regExp:
+
+    var nums = "2/7 3 4/5 3~/5";
+    var numsRegExp = new RegExp(r'(\s|/|~/)');
+    nums.split(numsRegExp); // ['2', '7', '3', '4', '5', '3', '5']
+
+In the code above, the string `nums` contains various numbers, some of which
+are expressed as fractions or as int-divisions. A regExp is used to split the
+string to extract just the numbers.
+ 
+You can perform operations on the matched and unmatched portions of a string
+when using `split()` with a regExp:
+
+    'Eats SHOOTS leaves'.splitMapJoin((new RegExp(r'SHOOTS')),
+      onMatch: (m) => '*${m.group(0).toLowerCase()}*',
+      onNonMatch: (n) => n.toUpperCase()); // 'EATS *shoots* LEAVES'
+      
+The regExp matches the middle word ("SHOOTS"). A pair of callbacks are
+registered to transform the matched and unmatched substrings before the
+substrings are joined together again.
+
+## Changing string case
+
+### Problem
+
+You want to change the case of strings.
+
+### Solution
+
+Use `string.toUpperCase()` and `string.toLowerCase()` to covert a string to 
+lower-case or upper-case, respectively:
+
+    var string = "I love Lucy";
+    string.toUpperCase(); // 'I LOVE LUCY!'
+    string.toLowerCase(); // 'i love lucy!'
+ 
+### Discussion
+ 
+Case changes affect the characters of bi-cameral scripts like Greek and French:
+ 
+    var zeus = '\u0394\u03af\u03b1\u03c2'; // Δίας (Zeus in modern Greek)
+    zeus.toUpperCase(); // 'ΔΊΑΣ'
+    
+    var resume = '\u0052\u00e9\u0073\u0075\u006d\u00e9'; // Résumé
+    resume.toLowerCase(); // 'résumé'
+    
+They do not affect the characters of uni-case scripts like Devanagari (used for
+writing many of the languages of India):
+
+    var chickenKebab = '\u091a\u093f\u0915\u0928 \u0915\u092c\u093e\u092c'; 
+    // चिकन कबाब (in Devanagari)
+    chickenKebab.toLowerCase();  // चिकन कबाब 
+    chickenKebab.toUpperCase();  // चिकन कबाब 
+    
+If a character's case does not change when using `toUpperCase()` and
+`toLowerCase()`, it is most likely because the character only has one
+form.
+
+## Determining whether a string contains another string
+
+### Problem
+
+You want to find out if a string is the subset of another string.
+
+### Solution
+
+Use `string.contains()`:
+
+    var string = 'Dart strings are immutable';
+    string.contains('immutable'); // True.
+
+You can indicate a startIndex as a second argument:
+
+    string.contains('Dart', 2); // False
+    
+### Discussion
+
+The String library provides a couple of shortcuts for testing whether a string
+is a substring of another:
+
+    string.startsWith('Dart'); // True.
+    string.endsWith('e'); // True.
+        
+You can also use `string.indexOf()`, which returns -1 if the substring is
+not found within a string, and its matching index, if it is:
+
+        string.indexOf('art') != -1; // True, `art` is found in `Dart`
+
+You can also use a regExp and `hasMatch()`:
+
+    new RegExp(r'ar[et]').hasMatch(string); //  True, 'art' and 'are' match.
+    
+
+## Finding matches of a regExp pattern in a string
+
+### Problem
+
+You want to use regExp to match a pattern in a string, and 
+want to be able to access the matches.
+
+### Solution
+
+Construct a regular expression using the RegExp class and find matches using
+the `allMatches()` method:
+
+    var string = 'Not with a fox, not in a box';
+    var regExp = new RegExp(r'[fb]ox');
+    List matches = regExp.allMatches(string);
+    matches.map((match) => match.group(0)).toList(); // ['fox', 'box']
+     
+You can query the object returned by `allMatches()` to find out the number of
+matches:
+ 
+    matches.length; // 2
+
+To find the first match, use `firstMatch()`:
+        
+        regExp.firstMatch(string).group(0); // 'fox'
+
+
+## Substituting strings based on regExp matches
+
+### Problem
+
+You want to match substrings within a string and make substitutions based on
+the matches.
+
+### Solution
+
+Construct a regular expression using the RegExp class and make replacements
+using `replaceAll()` method:
+
+    'resume'.replaceAll(new RegExp(r'e'), '\u00E9'); // 'résumé'
+
+If you want to replace just the first match, use 'replaceFirst()`:
+
+    '0.0001'.replaceFirst(new RegExp(r'0+'), ''); // '.0001'
+        
+The RegExp matches for one or more 0's and replaces them with an empty string.
+
+You can use `replaceAllMatched()` and register a function to modify the
+matches:
+
+    var heart = '\u2661'; // ♡
+    var string = "I like Ike but I $heart Lucy";
+    var regExp = new RegExp(r'[A-Z]\w+');
+    string.replaceAllMapped(regExp, (match) => match.group(0).toUpperCase()); 
+    // 'I like IKE but I ♡ LUCY'