Blocks and Exceptions

Block 代码块

1. Block是一个参数
2. 匿名参数
3. Callback
4. 使用do/en或者{}来定义

{puts 'hello'}

Demo:

# block usage
def hello
    puts 'hello method start'
    yield
    yield
    puts 'hello method end'
end

hello {puts 'i am in block'}

#output 
hello method start
i am in block
i am in block
hello method end

# yield with parameter
def hello
    puts 'hello method start'
    yield('hello','world')
    puts 'hello method end'
end

hello {|x,y| puts "i am in block,#{x} #{y}"}

#output
hello method start
i am in block,hello world
hello method end

# yield with paramter
def hello name
    puts 'hello method start'
    result = "hello " + name
    yield(result)
    puts 'hello method end'
end

hello('world'){|x| puts "i am in block,i got #{x}"}

#output
hello method start
i am in block,i got hello world
hello method end

# build in methods
['cat', 'dog','frog'].each do |animal|
    puts animal
end
puts '-' * 30
['cat','dog','frog'].each{|animal| puts animal}
#output
cat
dog
frog
------------------------------
cat
dog
frog

# build in methods
10.times do |t|
    puts t
end

puts '-' * 30
('a'..'d').each { |char| puts char}

#output
0
1
2
3
4
5
6
7
8
9
------------------------------
a
b
c
d

# varibale scope

# before ruby2.0
x = 1
[1, 2, 3].each { |x| puts x}
puts x # => x will be 3,which is incorrect

#output
1
2
3
1

如果是在ruby2之前的版本 那么外部的变量x会被改变

# varibale scope
# 如果是2.0版本之后 puts x会报错
sum = 0
[1, 2, 3].each { |x| sum += x}
puts sum
# puts x  

#output
6

# block return value
class Array
    def find
        each do |value|
            return value if yield (value)
        end
        nil
    end
end

puts [1, 2, 3].find { |x| x == 2 }

#output
2

# block as named parameter
def hello name, &block
    puts "hello #{name}, from method"
    block.call(name)
end

hello('world') {|x| puts "hello #{x} form block"}

#output
hello world, from method
hello world form block

# yield with parameter

def hello
    puts 'hello method start'
    yield('hello','world')
    puts 'hello method end'
end

hello {|x,y| puts "i am block ,#{x},#{y}"}

#output
hello method start
i am block ,hello,world
hello method end

# block_given?

def hello
    if block_given?
        yield
    else
        puts 'hello from method'
    end
end

hello
puts '-' * 30
hello {puts 'hello from block'}

#output
hello from method
------------------------------
hello from block

# block can be closure

def counter
    sum = 0
    # 代码库接收了一个参数x 如果x没有定义那么x为1 然后 sum +=x
    proc {|x| x = 1 unless x; sum +=x }
end

c2 = counter
puts c2.call(1) #1
puts c2.call(2) 
puts c2.call(3)

# 这里 closure 为闭包 
# 
#output
1
3
6

# new method to create block

# name is required 
hello = -> (name){"hello #{name}"}
puts hello.call('world')

puts '-' * 30

# name is required
hello3 = lambda {|name| "hello #{name}"}
puts hello3.call('world')

puts '-' * 30

hello2 = proc {|name| "hello #{name}"}
puts hello2.call
puts hello2.call('world') 

# lambda和proc区别 proc可以不传参数 lambda 更像是一个方法，必须传递参数
#output
hello world
------------------------------
hello world
------------------------------
hello
hello world

Exceptions 异常

All Exception inherited from Exception Class

所有异常都继承自Exception类

常见Exception

1. StandardError
2. SyntaxError
3. RuntimeError
4. ArgumentError
5. NameError
6. etc.

ruby抓取Exception

# exception

def hello name
    raise name #抛出异常 
end

hello # =>ArgumentError
hello('world') # =>RuntimeError

# exception catch

def hello
    raise
end

begin
    hello
rescue RuntimeError
    puts 'got it'
end

# exception catch

def hello
    raise
end

begin
    hello
rescue => e #出现异常捕获给e
    puts "catch exception with name :#{e.class}"
else #没有发生异常
    # ...
ensure #确保不论有没有发生异常
    # ...
end