I'm getting to grips with rails and whilst I feel I am progressing there is one thing that I am struggling to get to grips with and it's very basic. I am trying to understand the different usage of [] {} and () Are there any good sources of their usage and are there any tips you can give to a beginner in recognizing when to use one or the other, or as I seem to see in some cases when they are not required at all?
I know this is extremely basic but I have struggled to find literature which explains concisely the interplay between them and Ruby or specifically RoR
0
votes
2 Answers
4
votes
It has nothing to do with RoR; the various brackets are Ruby language constructs.
[] is the array operator, for arrays and other classes that implement it (like a string taking a range to get substrings, or hashes to look up a key's value):
a = [1, 2, 3]
a.each { |n| puts n }
s = "ohai"
puts s[1..-1]
h = { foo: "bar", baz: "plugh" }
puts h[:foo]
{} is for hashes, and one of two ways of delimiting blocks (the other being begin/end). (And used with # for string interpolation.)
h = { foo: "bar", baz: "plugh" }
h.each { |k, v| puts "#{k} == #{v}" }
() is for method parameters, or for enforcing evaluation order in an expression.
> puts 5 * 3 + 5 # Normal precedence has * ahead of +
=> 20
> puts 5 * (3 + 5) # Force 3+5 to be evaluated first
=> 40
def foo(s)
puts(s)
end
They're sometimes optional if the statement has no ambiguity:
def foo s
puts s
end
(They're not always optional, and putting a space between the method call and its parenthetical parameter list can cause issues--best not to, IMO.)
(I probably missed something, too, but there's the nutshell.)
1
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[] are used to access objects within a hash (via a key) or within an array (via an index).
hash[:key] # returns a value
array[0] # returns the first array element
[] is used to describe an array.
array = ['a', 'b', 'c']
Of course this can be nested.
nested = [['a','b','c'], [1,2,3]]
[] can be used to declare a hash, but that's because the Hash class can accept an array.
hash = Hash[['a',1], ['b',2]] # { 'a' => 1, 'b', => 2 }
{} is used to declare a hash.
hash = { 'a' => 1, 'b' => 2 }
This too can be nested.
hash = { 'a' => { 'c' => 3 }, 'b' => { 'd' => 4 } }
{} is also used to delimit blocks. The .each method is a common one. The following two blocks of code are equivalent.
array.each do |n|
puts n
end
array.each { |n| puts n }
The () is just used for grouping in cases where ambiguity needs clarification. This is especially true in methods that take many arguments, some of which may be nil, some of which may be obejcts, etc. You'll see a lot of code that omit them entirely as no grouping is needed for clarity.
puts(string)
puts string
I recommend firing up the rails console and start declaring variables and accessing them.
