# comment
Single line comment
|
comment
# Ruby treats nil as a false value in conditions,
# you could write the following to process the lines in a file.
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Assignment
– An assignment statement sets the variable or attribute on its left side (the lvalue) to refer to the value on the right (the rvalue). It then returns that value as the result of the assignment expression. This means you can chain assignments, and you can perform assignments in some unexpected places.
– In older Ruby versions, the result of the assignment was the value returned by the attribute-setting method. In Ruby 1.8, the value of the assignment is always the value of the parameter; the return value of the method is discarded.
– Ruby assignments are effectively performed in parallel, so the values assigned are not affected by the assignment itself. The values on the right side are evaluated in the order in which they appear before any assignment is made to variables or attributes on the left.
– When an assignment has more than one lvalue, the assignment expression returns an array of the rvalues. If an assignment contains more lvalues than rvalues, the excess lvalues are set to nil. If a multiple assignment contains more rvalues than lvalues, the extra rvalues are ignored. If an assignment has just one lvalue and multiple rvalues, the rvalues are converted to an array and assigned to the lvalue.
– In common with many other languages, Ruby has a syntactic shortcut: a = a + 2 may be written as a += 2.
– Something you won’t find in Ruby are the autoincrement (++) and autodecrement (--) operators of C and Java. Use the += and -= forms instead.
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return value of the method is discarded
class Test
def val=(val)
@val = val
return 99
end
end
t = Test.new
a = t.val = 2
a #-->2
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Returning
Anything that can reasonably return a value does.
The if and case statements both return the value of the last expression executed
|
returning
a = b = c = 0 #--> 0
a = b = 1 + 2 + 3
a #-->6
b #-->6
a = (b = 1 + 2) + 3
a #-->6
b #-->3
File.open(name = gets.chomp)
[ 3, 1, 7, 0 ].sort.reverse #-->[7, 3, 1, 0]
if returns
song_type = if song.mp3_type == MP3::Jazz
if song.written < Date.new(1935, 1, 1)
Song::TradJazz
else
Song::Jazz
end
else
Song::Other
end
if returns in next lines
wer =
if (false)
1
else
2
end
puts wer #-> 2
case returns
rating = case votes_cast
when 0...10 then Rating::SkipThisOne
when 10...50 then Rating::CouldDoBetter
else Rating::Rave
end
multiple, multy multi return
def multy
return 1, 2, 3
end
w1, w2, w3 = multy
puts w1 #--> 1
puts w2 #--> 2
puts w3 #--> 3
conditions returns
part = [0, 1]
puts val = part && part[1] || '1' #-> 1
puts val = part && part[2] || '2' #-> 2
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Parallel assignment
An assignment expression may have one or more lvalues and one or more rvalues. This section explains how Ruby handles assignment with different combinations of arguments.
1. If the last rvalue is prefixed with an asterisk and is an object of class Array, the rvalue is replaced with the elements of the array, with each element forming its own rvalue.
2. If the assignment contains multiple lvalues and one rvalue, the rvalue is converted into an Array, and this array is expanded into a set of rvalues as described in (1).
3. Successive rvalues are assigned to the lvalues. This assignment effectively happens in parallel, so that (for example) a,b=b,a swaps the values in "a" and "b."
4. If there are more lvalues than rvalues, the excess will have nil assigned to them.
5. If there are more rvalues that lvalues, the excess will be ignored.
6. These rules are modified slightly if the last lvalue is preceded with an asterisk. This lvalue will always receive an array during the assignment. The array will consist of whatever rvalue would normally have been assigned to this lvalue, followed by the excess rvalues (if any).
7. If an lvalue is a parenthesized list, it is treated as a nested assignment statement, and the list is assigned from the corresponding rvalue as described by these rules. Collapse and expand arrays using parallel assignment
You can collapse and expand arrays using Ruby’s parallel assignment operator.
If the last lvalue is preceded by an asterisk, all the remaining rvalues will be collected and assigned to that lvalue as an array.
Similarly, if the last rvalue is an array, you can prefix it with an asterisk, which effectively expands it into its constituent values in place. (This is not necessary if the rvalue is the only thing on the right side—the array will be expanded automatically.)
|
parallel assignment
a, b = b, a #swap the values in two variables
x = 0 #-> 0
a, b, c = x, (x += 1), (x += 1) #-> [0, 1, 2]
a, b, c = 1, "cat", [ 3, 4, 5 ]
a = [1, 2, 3, 4]
b, c = a #-> b == 1, c == 2
b, *c = a #-> b == 1, c == [2, 3, 4]
b, c = 99, a #-> b == 99, c == [1, 2, 3, 4]
b, *c = 99, a #-> b == 99, c == [[1, 2, 3, 4]]
b, c = 99, *a #-> b == 99, c == 1
b, *c = 99, *a #-> b == 99, c == [1, 2, 3, 4]
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Nested assignments
Parallel assignments have one more feature worth mentioning. The left-hand side of an assignment may contain a parenthesized list of terms. Ruby treats these terms as if they were a nested assignment statement. It extracts out the corresponding rvalue, assigning it to the parenthesized terms, before continuing with the higher-level assignment.
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nested assignments
b, (c, d), e = 1,2,3,4 #-> b == 1, c == 2, d == nil, e == 3
b, (c, d), e = [1,2,3,4] #-> b == 1, c == 2, d == nil, e == 3
b, (c, d), e = 1,[2,3],4 #-> b == 1, c == 2, d == 3, e == 4
b, (c, d), e = 1,[2,3,4],5 #-> b == 1, c == 2, d == 3, e == 5
b, (c,*d), e = 1,[2,3,4],5 #-> b == 1, c == 2, d == [3, 4], e == 5
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Comparison
== Test for equal value.
=== Used to compare the each of the items with the target in the when clause of a case statement.
<=> General comparison operator. Returns -1, 0, or +1, depending on whether its receiver is less than, equal to, or greater than its argument.
<, <=, >=, > Comparison operators for less than, less than or equal, greater than or equal, and greater than.
=~ Regular expression pattern match.
eql? True if the receiver and argument have both the same type and equal values. 1 == 1.0 returns true, but 1.eql?(1.0) is false.
equal? True if the receiver and argument have the same object ID.
– Both == and =~ have negated forms, != and !~. However, these are converted by Ruby when your program is read. a != b is equivalent to !(a == b), and a !~ b is the same as !(a =~ b). This means that if you write a class that overrides == or =~ you get a working != and !~ for free. But on the flip side, this also means that you cannot define != and !~ independent of == and =~, respectively.
– You can use a Ruby range as a boolean expression. A range such as exp1..exp2 will evaluate as false until exp1 becomes true. The range will then evaluate as true until exp2 becomes true. Once this happens, the range resets, ready to fire again. We show some examples of this on page
| |
document here, here document Here document
A here document consists of lines in the source up to, but not including, the terminating string that you specify after the << characters. Normally, this terminator must start in the first column. However, if you put a minus sign after the << characters (<<-), you can indent the terminator.
Strings can continue across multiple input lines, in which case they will contain newline characters. It is also possible to use here documents to express long string literals. Whenever Ruby parses the sequence <<identifier or <<quoted string, it replaces it with a string literal built from successive logical input lines. It stops building the string when it finds a line that starts with the identifier or the quoted string. You can put a minus sign immediately after the << characters, in which case the terminator can be indented from the left margin. If a quoted string was used to specify the terminator, its quoting rules will be applied to the here document; otherwise, double-quoting rules apply.
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here document
aString = <<END_OF_STRING
The body of the string
is the input lines up to
one ending with the same
text that followed the '<<'
END_OF_STRING
indent the terminator
print <<-STRING1, <<-STRING2
Concat
STRING1
enate
STRING2
produces: Concat
enate
quoting rules
a = 123
print <<HERE
Double quoted \
here document.
Sum = #{a + 1}
HERE
print <<-'THERE'
This is single quoted.
The above used #{a + 1}
THERE
produces: Double quoted here document.
Sum = 124
This is single quoted.
The above used #{a + 1}
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null nil
Object, just like any other, that happens to represent nothing. spaceship <=>
Compares two values, returning -1, 0, or +1 depending on whether the first is less than, equal to, or greater than the second.
| |
$_ variable
Global variable $_
|
This uses some behind-the-scenes magic behavior: gets assigns the last line read to the global variable $_, the ~ operator does a regular expression match against $_, and print with no arguments prints $_.
file = File.open("ordinal")
while file.gets
print if ~/third/ .. ~/fifth/
end
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BEGIN and END Blocks
Every Ruby source file can declare blocks of code to be run as the file is being loaded (the BEGIN blocks) and after the program has finished executing (the END blocks).
– A program may include multiple BEGIN and END blocks. BEGIN blocks are executed in the order they are encountered. END blocks are executed in reverse order.
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BEGIN & END blocks
BEGIN {puts "begin code 1"}
END {puts "end code 1"}
puts "line 2"
BEGIN {puts "begin code 2"}
END {puts "end code 2"}
puts "line 3"
produces:
begin code 1
begin code 2
line 1
line 2
line 3
end code 2
end code 1
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General Delimited Input
There are alternative forms of literal strings, arrays, regular expressions, and shell commands that are specified using a generalized delimited syntax.
– All these literals start with a percent character, followed by a single character that identifies the literal's type.
– Following the type character is a delimiter, which can be any character.
– If the delimiter is one of the characters "(", "[", "{", or "<", the literal consists of the characters up to the matching closing delimiter, taking account of nested delimiter pairs. For all other delimiters, the literal comprises the characters up to the next occurrence of the delimiter character.
– Delimited strings may continue over multiple lines.
%q – Single-quoted string
%Q, % – Double-quoted string
%w – Array of tokens
%W – Array of tokens with substitutions
%r – Regular expression pattern
%x – Shell command
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using
var = 'Exp'
%q/this is a string #{var}/ #-> this is a string #{var}
%Q-string #{var}- #-> string Exp
%(a (nested) string var=#{var}) #-> a (nested) string var=Exp
(%w<q va\ r e r t y>)[1] #-> va r
['q', var, 'e', 'r', 't', 'y'][1] #-> Exp
puts %q{def fred(a)
a.each { |i| puts i }
end}
produces:
def fred(a)
a.each { |i| puts i }
end
%w %W samples
value = 1
a = %w{ #{value} } # => ["\#{value}"]
b = %W{ #{value} } # => ["1"]
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: Symbols
A Ruby symbol is the internal representation of a name. You construct the symbol for a name by preceding the name with a colon.
– A particular name will always generate the same symbol, regardless of how that name is used within the program.
– Other languages call this process "interning" and call symbols "atoms".
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symbols
:Object
:myVariable
|
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about
The class Array holds a collection of object -references. Each object reference occupies a position in the array, identified by a non-negative integer index.
It’s more efficient to access array elements, but hashes provide more flexibility
– Literals of class Array are created by placing a comma-separated series of object references between square brackets.
– A trailing comma is ignored.
– Arrays of strings can be constructed using a shortcut notation, %w, which extracts space-separated tokens into successive elements of the array. A space can be escaped with a backslash. This is a form of general delimited input. Indexing elements
– Ruby array index starts at zero.
– Index an array with a non-negative integer, and it returns the object at that position or returns nil if nothing is there. Index an array with a negative integer, and it counts from the end.
– You can also index arrays with a pair of numbers, [ -start, -count ]. This returns a new array consisting of references to count objects starting at position start. - You can index arrays using ranges, in which start and end positions are separated by two or three periods. The -two-period form includes the end position, and the -three-period form does not. remove Modifying elements in array
– The [ ] operator has a corresponding [ ]= operator, which lets you set elements in the array. If used with a single integer index, the element at that position is replaced by whatever is on the right side of the assignment. Any gaps that result will be filled with nil.
– If the index to [ ]= is two numbers (a start and a length) or a range, then those elements in the original array are replaced by whatever is on the right side of the assignment. If the length is zero, the right side is inserted into the array before the start position; no elements are removed. If the right side is itself an array, its elements are used in the replacement. The array size is automatically adjusted if the index selects a different number of elements than are available on the right side of the assignment. arr.length -> Integer
arr.size -> Integer
Returns length of array .to_a
Converts a range to a list
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arrays
a = [ 1, 'cat', 3.14 ] # create array with three elements
a[0] # --> 1 access the first element
a[2] = nil # set the third element
a #--> [1, "cat", nil]
arr = [ fred, 10, 3.14, "This is a string", barney("pebbles"), ]
%w( first sec\ ond ) #-> ["first", "sec ond"]
b = Array.new # array constructor
b.class #--> Array
b.length #-->0
b[0] = "second"
b[1] = "array"
b #-->["second", "array"]
c = [] # array short constructor
c.class #--> Array
string to array
a = %w{ ant bee cat dog elk }
a[0] #--> "ant"
a[3] #--> "dog"
array indexation (-1 = the last)
a = [ 1, 3, 5, 7, 9 ]
a[1] #-->3
a[2] #-->5
a[-1] #-->9
a[99] #-->nil
subarray – arr[start, count]
a = [ 1, 3, 5, 7, 9 ]
a[1, 3] #-->[3, 5, 7]
a[3, 1] #-->[7]
a[3, 2] #-->[7, 9]
subarray – two-period three-period
a = [ 1, 3, 5, 7, 9 ]
a[1..3] #-->[3, 5, 7]
a[1...3] #-->[3, 5]
a[3..3] #-->[7]
a[3..1] #-->[]
set elements in array
a = [ 1, 3, 5, 7, 9 ]
a[1] = "bat" #-->[1, "bat", 5, 7, 9]
a[2] = "cat" #-->[1, "bat", "cat", 7, 9]
a[3] = [ 9, 8 ] #-->[1, "bat", "cat", [9, 8], 9]
a[6] = 99 #-->[1, "bat", "cat", [9, 8], 9, nil, 99]
set ranges elements in array, replacing elements
a = [ 1, 3, 5, 7, 9 ] #-->[1, 3, 5, 7, 9]
a[2, 2] = ’cat’ #-->[1, 3, "cat", 9]
a[2, 0] = ’dog’ #-->[1, 3, "dog", "cat", 9] inserting elements
a[1, 1] = [ 9, 8, 7 ] #-->[1, 9, 8, 7, "dog", "cat", 9]
a[0..3] = [] #-->["dog", "cat", 9] removing elements
a[5..6] = 99, 98 #-->["dog", "cat", 9, nil, nil, 99, 98]
ranges to array
(1...10).to_a #-->[1, 2, 3, 4, 5, 6, 7, 8, 9]
('bar'..'bat').to_a #-->["bar", "bas", "bat"]
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arr.sort -> Array
arr.sort {|a,b| block } -> Array
Returns a new array created by sorting arr.
Comparisons for the sort will be done using the <=> operator or using an optional code block.
The block implements a comparison between a and b, returning -1, 0, or +1. arr.sort! -> arr
arr.sort! {|a,b| block } -> arr
Same as Array.sort, but modifies the receiver in place.
arr is effectively frozen while a sort is in progress.
|
Array sort
a = [ "d", "a", "e", "c", "b" ]
a.sort #-> ["a", "b", "c", "d", "e"]
a.sort {|x,y| y <=> x } #-> ["e", "d", "c", "b", "a"]
Array sort!
a = [ "d", "a", "e", "c", "b" ]
a.sort! #-> ["a", "b", "c", "d", "e"]
a #-> ["a", "b", "c", "d", "e"]
|
arr.reverse -> Array
Returns a new array using arr's elements in reverse order. arr.reverse! -> arr or nil
Same as reverse, but returns nil if arr is unchanged (arr.length is zero or one).
|
reverse
["a", "b", "c"].reverse #->["c", "b", "a"]
[1].reverse #->[1]
reverse!
a = ["a", "b", "c"]
a.reverse! #->["c", "b", "a"]
a #->["c", "b", "a"]
[ 1 ].reverse! #->nil
|
reverse_each
Same as Array.each, but traverses arr in reverse order.
|
reverse_each
a = ["a", "b", "c"]
a.reverse_each {|x| print x, " " }
produces:
c b a
|
arr.clear -> arr
Removes all elements from arr.
|
clear
a = ["a", "b", "c", "d", "e"]
a.clear #-> []
|
arr.collect {| obj | block } -> Array
Returns a new array by invoking block once for every element, passing each element as a parameter to block.
The result of block is used as the given element in the new array. arr.collect! {| obj | block } -> arr
Invokes block once for each element of arr, replacing the element with the value returned by block.
|
collect
a = [ "a", "b", "c", "d" ]
a.collect {|x| x + "!" } #-> ["a!", "b!", "c!", "d!"]
a #-> ["a", "b", "c", "d"]
collect!
a = [ "a", "b", "c", "d" ]
a.collect! {|x| x + "!" } #-> ["a!", "b!", "c!", "d!"]
a #-> ["a!", "b!", "c!", "d!"]
|
| |
about
Hashes (sometimes known as associative arrays, maps, or dictionaries) are similar to arrays in that they are indexed collections of object references. However, while you index arrays with integers, you can index a hash with objects of any type: strings, regular expressions, and so on. When you store a value in a hash, you actually supply two objects — the index, normally called the key, and the value. You can subsequently retrieve the value by indexing the hash with the same key. The values in a hash can be objects of any type.
a hash by default returns nil when indexed by a key it doesn’t contain. Sometimes you’llwant to change this default. For example, if you’re using a hash to count the number of times each key occurs, it’s convenient to have the default value be zero. This is easily done by specifying a default value when you create a new, empty hash.
Compared with arrays, hashes have one significant advantage: they can use any object as an index. However, they also have a significant disadvantage: their elements are not ordered, so you cannot easily use a hash as a stack or a queue. Also thes are not so efficient as arrays.
Hash does not support multiple keys with the same value
|
hash
inst_section = {
'cello' => 'string',
'clarinet' => 'woodwind',
'drum' => 'percussion',
}
h = {} # ={}
h.length #-->0 =length
h['dog'] #-->nil
hash
h = { 'dog' => 'canine', 'cat' => 'feline', 'donkey' => 'asinine' }
h.length #-->3 length
h['dog'] #-->"canine"
h['cow'] = 'bovine'
h[12] = 'dodecine'
h['cat'] = 99
h #-->{"cow"=>"bovine", "cat"=>99, 12=>"dodecine",
# "donkey"=>"asinine", "dog"=>"canine"}
|
Hash[ [ key => value ]* ] -> Hash
Creates a new hash populated with the given objects.
– Equivalent to the literal { key, value, ... }.
– Keys and values occur in pairs, so there must be an even number of arguments.
|
Hash[
Hash["a", 100, "b", 200] #->{"a"=>100, "b"=>200}
Hash["a" => 100, "b" => 200] #->{"a"=>100, "b"=>200}
{ "a" => 100, "b" => 200 } #->{"a"=>100, "b"=>200}
|
Hash.new( Object=nil ) -> Hash
Returns a new, empty hash.
– If Object is specified, it will be used as the default value.
|
Hash.new
h = Hash.new("Go Fish")
h["a"] = 100
h["b"] = 200
h["a"] #->100
h["c"] #->"Go Fish"
0 by defaul instead NIL
histogram = Hash.new(0)
histogram['key1'] --> 0
histogram['key1'] = histogram['key1'] + 1
histogram['key1'] --> 1
|
hsh[ aKeyObject ] -> aValueObject
Element Reference
– Retrieves the aValueObject stored for aKeyObject.
– If not found, returns the default value.
|
Element Reference
h = { "a" => 100, "b" => 200 }
h["a"] #->100
h["c"] #->nil
|
hsh[ aKeyObject ] = aValueObject -> aValueObject
hsh.store( aKeyObject, aValueObject ) -> aValueObject (Synonym)
Element Assignment
– Associates the value given by aValueObject with the key given by aKeyObject. – aKeyObject should not have its value changed while it is in use as a key (a String passed as a key will be duplicated and frozen).
|
Element Assignment
h = { "a" => 100, "b" => 200 }
h["a"] = 9
h["c"] = 4
h #->{"a"=>9, "b"=>200, "c"=>4}
|
hsh == anOtherHash -> true or false
Equality
– Two hashes are equal if they each contain the same number of keys and if each key-value pair is equal to (according to Object#== ) the corresponding elements in the other hash.
|
Equality
h1 = { "a" => 1, "c" => 2 }
h2 = { "a" => 1, "c" => 2, 7 => 35 }
h3 = { "a" => 1, "c" => 2, 7 => 35 }
h4 = { "a" => 1, "d" => 2, "f" => 35 }
h1 == h2 #->false
h2 == h3 #->true
h3 == h4 #->false
|
hsh.default -> Object
Returns the "default value" that is, the value returned for a key that does not exist in the hash.
– Defaults to nil. hsh.default = Object -> hsh
Sets the "default value"
|
default
h = { "a" => 100, "b" => 200 }
h.default = "Go fish"
h.default #->Go fish
h["a"] #->100
h["z"] #->"Go fish"
|
hsh.delete( aKeyObject ) -> aValueObject
hsh.delete( aKeyObject ) {| aKeyObject | block } -> aValueObject
Deletes and returns a key-value pair from hsh whose key is equal to aKeyObject.
– If the key is not found, returns the default value.
– If the optional code block is given and the key is not found, pass in the key and return the result of block.
|
delete
h = { "a" => 100, "b" => 200 }
h.delete("a") #->100
h.delete("z") #->nil
h.delete("z") { |el| "#{el} not found" } #->"z not found"
|
hsh.delete_if {| key, value | block } -> hsh
Deletes every key-value pair from hsh for which block evaluates to true.
|
delete_if
h = { "a" => 100, "b" => 200, "c" => 300 }
h.delete_if {|key, value| key >= "b" } #->{"a"=>100}
|
hsh.sort -> Array
hsh.sort {|a, b| block } -> Array
Converts hsh to a nested array of [key, value] arrays and sorts it, using Array.sort.
|
Hash sort
h = { "a" => 20, "b" => 30, "c" => 10 }
h.sort #-> [["a", 20], ["b", 30], ["c", 10]]
h.sort {|a,b| a[1]<=>b[1]} #-> [["c", 10], ["a", 20], ["b", 30]]
|
hsh.clear -> hsh
Removes all key-value pairs from hsh.
|
clear
h = {"a" => 100, "b" => 200}
h.clear #-> {}
|
hsh.each {| key, value | block } -> hsh
hsh.each_pair {| key, value | block } -> hsh (Synonym)
Calls block once for each key in hsh, passing the key and value as parameters.
|
each
h = { "a" => 100, "b" => 200 }
h.each {|key, value| print key, " is ", value, "\n" }
produces:
a is 100
b is 200
|
hsh.each_key {| key | block } -> hsh
Calls block once for each key in hsh, passing the key as a parameter.
|
each_key
h = { "a" => 100, "b" => 200 }
h.each_key {|key| puts key }
produces:
a
b
|
hsh.each_value {| value | block } -> hsh
Calls block once for each key in hsh, passing the value as a parameter.
|
each_value
h = { "a" => 100, "b" => 200 }
h.each_value {|value| puts value }
produces:
100
200
|
hsh.empty? -> true or false
Returns true if hsh contains no key-value pairs.
| |
hsh.fetch( aKeyObject [, aDefObject ] ) -> Object
hsh.fetch( aKeyObject ) {| aKeyObject | block } -> Object
Returns a value from the hash for the given key.
If the key can't be found, there are several options:
– With no other arguments, it will raise an IndexError exception;
– if aDefObject is given, then that will be returned;
– if the optional code block is specified, then that will be run and its result returned.
|
fetch
h = { "a" => 100, "b" => 200 }
h.fetch("a") #->100
h.fetch("z", "go fish") #->"go fish"
h.fetch("z") { |el| "go fish, #{el}"} #->"go fish, z"
The following example shows that an exception is raised if the key is not found and a default value is not supplied.
h = { "a" => 100, "b" => 200 }
h.fetch("z")
produces:
prog.rb:2:in `fetch': key not found (IndexError)
from prog.rb:2
|
hsh.has_key?( aKeyObject ) -> true or false
Synonyms:
hsh.include?( aKeyObject ) -> true or false
hsh.key?( aKeyObject ) -> true or false
hsh.member?( aKeyObject ) -> true or false
Returns true if the given key is present in hsh.
|
has_key?
h = { "a" => 100, "b" => 200 }
h.has_key?("a") #->true
h.has_key?("z") #->false
|
hsh.has_value?( aValueObject ) -> true or false
hsh.value?( aValueObject ) -> true or false (Synonym)
Returns true if the given value is present for some key in hsh.
|
has_value?
h = { "a" => 100, "b" => 200 }
h.has_value?(100) #->true
h.has_value?(999) #->false
|
hsh.index( aValueObject ) -> aKeyObject
Returns the key for a given value.
– If not found, returns the default value.
|
index
h = { "a" => 100, "b" => 200 }
h.index(200) #->"b"
h.index(999) #->nil
|
hsh.indexes( [ key ]+ ) -> Array
hsh.indices( [ key ]+ ) -> Array (Synonym)
Returns a new array consisting of values for the given key(s).
– Will insert the default value for keys that are not found.
|
indexes
h = { "a" => 100, "b" => 200, "c" => 300 }
h.indexes("a", "c") #->[100, 300]
h.indexes("a", "c", "z") #->[100, 300, nil]
|
hsh.invert -> Hash
Returns a new hash created by using hsh's values as keys, and the keys as values.
|
invert
h = { "n" => 100, "m" => 100, "y" => 300, "d" => 200, "a" => 0 }
h.invert #->{0=>"a", 100=>"n", 200=>"d", 300=>"y"}
|
hsh.keys -> Array
Returns a new array populated with the keys from this hash.
|
keys
h = { "a" => 100, "b" => 200, "c" => 300, "d" => 400 }
h.keys #->["a", "b", "c", "d"]
|
hsh.values -> Array
Returns a new array populated with the values from hsh.
|
values
h = { "a" => 100, "b" => 200, "c" => 300 }
h.values #->[100, 200, 300]
|
hsh.length -> Fixnum
hsh.size -> Fixnum (Synonym)
Returns the number of key-value pairs in the hash.
|
length
h = { "d" => 100, "a" => 200, "v" => 300, "e" => 400 }
h.length #->4
h.delete("a") #->200
h.length #->3
|
hsh.rehash -> hsh
Rebuilds the hash based on the current hash values for each key.
– If values of key objects have changed since they were inserted, this method will reindex hsh.
– If Hash.rehash is called while an iterator is traversing the hash, an IndexError will be raised in the iterator.
|
rehash
a = [ "a", "b" ]
c = [ "c", "d" ]
h = { a => 100, c => 300 }
h[a] #->100
a[0] = "z"
h[a] #->nil
h.rehash #->{["z", "b"]=>100, ["c", "d"]=>300}
h[a] #->100
|
hsh.reject {| key, value | block } -> Hash
Same as Hash.delete_if, but works on (and returns) a copy of the hsh.
– Equivalent to hsh.dup.delete_if hsh.reject! {| key, value | block } -> hsh or nil
Equivalent to Hash.delete_if, but returns nil if no changes were made.
| |
hsh.replace( anOtherHash ) -> hsh
Replaces the contents of hsh with the contents of anOtherHash.
|
replace
h = { "a" => 100, "b" => 200 }
h.replace({ "c" => 300, "d" => 400 }) #->{"c"=>300, "d"=>400}
|
hsh.shift -> Array or nil
Removes a key-value pair from hsh and returns it as the two-item array [ key, value ], or nil if the hash is empty.
|
shift
h = { 1 => "a", 2 => "b", 3 => "c" }
h.shift #->[1, "a"]
h #->{2=>"b", 3=>"c"}
|
hsh.to_a -> Array
Converts hsh to a nested array of [ key, value ] arrays.
|
to_a
h = { "c" => 300, "a" => 100, "d" => 400, "c" => 300 }
h.to_a #->[["a", 100], ["c", 300], ["d", 400]]
|
hsh.to_s -> String
Converts hsh to a string by converting the hash to an array of [ key, value ] pairs and then converting that array to a string using Array.join with the default separator.
|
to_s
h = { "c" => 300, "a" => 100, "d" => 400, "c" => 300 }
h.to_s #->"a100c300d400"
|
hsh.update( anOtherHash ) -> hsh
Adds the contents of anOtherHash to hsh, overwriting entries with duplicate keys with those from anOtherHash.
|
update
h1 = { "a" => 100, "b" => 200 }
h2 = { "b" => 254, "c" => 300 }
h1.update(h2) #->{"a"=>100, "b"=>254, "c"=>300}
inline
h1 = {:a => 100, :b => 200}
h1.update(:b => 254, :c => 300) #-> {:b=>254, :c=>300, :a=>100}
|
| |
Scope
The while, until, and for loops are built into the language and do not introduce new scope; previously existing locals can be used in the loop, and any new locals created will be available afterward.
The blocks used by iterators (such as loop and each) are a little different. Normally, the local variables created in these blocks are not accessible outside the block. However, if at the time the block executes a local variable already exists with the same name as that of a variable in the block, the existing local variable will be used in the block. Its value will therefore be available after the block finishes.
Note that the variable need not have been given a value in the outer scope: the Ruby interpreter just needs to have seen it.
A couple of suggestions to minimize the problems with local and block variables interfering:
• Keep your methods and blocks short. The fewer variables, the smaller the chance that they’ll clobber each other. It’s also easier to eyeball the code and check that you don’t have conflicting names.
• Use different naming schemes for local variables and block parameters. For example, you probably don’t want a local variable called “i,” but that might be perfectly acceptable as a block parameter.
In reality, this problem doesn’t arise in practice as often as you may think.
|
Scope in block
[ 1, 2, 3 ].each do |x|
y = x + 1
end
[ x, y ]
produces:
prog.rb:4: undefined local variable or method `x' for
main:Object (NameError)
local variable already exists
x = nil
y = nil
[ 1, 2, 3 ].each do |x|
y = x + 1
end
[ x, y ] #-->[3, 4]
the Ruby interpreter just needs to have seen it
if false
a = 1
end
3.times {|i| a = i }
a #-->2
|
while [is true]
...
end
The while loop executes its body zero or more times as long as its condition is true.
You’ll come across a wrinkle when you use while and until as statement modifiers. If the statement they are modifying is a begin/end block, the code in the block will always execute at least one time, regardless of the value of the boolean expression. ... while [is true]
Shortcut until [becomes true]
...
end
The until loop is the opposite; it executes the body until the condition becomes true. ... until [becomes true]
Shortcut
|
while sample
while weight < 100 and num_pallets <= 30
pallet = next_pallet()
weight += pallet.weight
num_pallets += 1
end
while
file = File.open("ordinal")
while line = file.gets
puts(line) if line =~ /third/ .. line =~ /fifth/
end
#or slightly differently
while file.gets
print if ~/third/ .. ~/fifth/
end
produces:
third
fourth
fifth
while with block
print "Hello\n" while false
begin
print "Goodbye\n"
end while false
produces:
Goodbye
until
until play_list.duration > 60
play_list.add(song_list.pop)
end
|
n.times
Iterates n times, executing the block inside 3.upto(6)
6.downto(3)
Iterating up and down between two integers .step(from, step)
Iteration with step
|
samples
5.times { print "*" } #-->*****
3.upto(6) {|i| print i } #-->3456
99.downto(95) {|i| print i, " " } #-->99 98 97 96 95
50.step(80, 5) {|i| print i, " " } #-->50 55 60 65 70 75 80
|
loop do
...
end
The loop iterator calls the associated block forever
The loop control constructs break, redo, and next let you alter the normal flow through a loop or iterator. next
Skips to the end of the loop, effectively starting the next iteration. break
Terminates the immediately enclosing loop; control resumes at the statement following the block.
Break and next can be given arguments. When used in conventional loops, it probably makes sense only to do this with break (as any value given to next is effectively lost). If a conventional loop doesn’t execute a break, its value is nil. redo
Repeats the loop from the start, but without reevaluating the condition or fetching the next element (in an iterator). retry
Restarts any kind of iterator loop.
Retry will reevaluate any arguments to the iterator before restarting it.
|
loop sample
i=0
loop do
i += 1
next if i < 3
print i
break if i > 4
end
produces:
345
while sample
while line = gets
next if line =~ /^\s*#/ # skip comments
break if line =~ /^END/ # stop at end
# substitute stuff in backticks and try again
redo if line.gsub!(/`(.*?)`/) { eval($1) }
# process line ...
end
retry
for i in 1..100
print "Now at #{i}. Restart? "
retry if gets =~ /^y/i
end
retry will reevaluate any arguments to the iterator before restarting it
def do_until(cond)
break if cond
yield
retry
end
i = 0
do_until(i > 10) do
print i, " "
i += 1
end
produces:
0 1 2 3 4 5 6 7 8 9 10
break with argument
result = while line = gets
break(line) if line =~ /answer/
end
process_answer(result) if result
|
for i in 1..3
print i, " "
end
You can use for to iterate over any object that responds to the method each, such as an Array or a Range.
When you write so, Ruby translates it into something like
songlist.each do |song|
song.play
end
The only difference between the for loop and the each form is the scope of local variables that are defined in the body
|
for in sample
for i in ['fee', 'fi', 'fo', 'fum']
print i, " "
end
produces: fee fi fo fum
for in with region
for i in 1..3
print i, " "
end
produces: 1 2 3
with array
for i in File.open("ordinal").find_all {|line| line =~ /d$/}
print i.chomp, " "
end
produces: second third
As long as your class defines a sensible each method, you can use a for loop to traverseits objects.
class Periods
def each
yield "Classical"
yield "Jazz"
yield "Rock"
end
end
periods = Periods.new
for genre in periods
print genre, " "
end
produces:
Classical Jazz Rock
|
| |
It’s a Ruby manual
Just type the word or symbol you are looking for
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