Here is Bar#do_things:
class Bar
def do_things
Foo.some_method(x) do |x|
y = x.do_something
return y_is_bad if y.bad? # how do i tell it to stop and return do_things?
y.do_something_else
end
keep_doing_more_things
end
end
And here is Foo#some_method:
class Foo
def self.some_method(targets, &block)
targets.each do |target|
begin
r = yield(target)
rescue
failed << target
end
end
end
end
I thought about using raise, but I am trying to make it generic, so I don't want to put anything any specific in Foo.
Use the keyword next. If you do not want to continue to the next item, use break.
When next is used within a block, it causes the block to exit immediately, returning control to the iterator method, which may then begin a new iteration by invoking the block again:
f.each do |line| # Iterate over the lines in file f
next if line[0,1] == "#" # If this line is a comment, go to the next
puts eval(line)
end
When used in a block, break transfers control out of the block, out of the iterator that invoked the block, and to the first expression following the invocation of the iterator:
f.each do |line| # Iterate over the lines in file f
break if line == "quit\n" # If this break statement is executed...
puts eval(line)
end
puts "Good bye" # ...then control is transferred here
And finally, the usage of return in a block:
return always causes the enclosing method to return, regardless of how deeply nested within blocks it is (except in the case of lambdas):
def find(array, target)
array.each_with_index do |element,index|
return index if (element == target) # return from find
end
nil # If we didn't find the element, return nil
end
I wanted to just be able to break out of a block - sort of like a forward goto, not really related to a loop. In fact, I want to break of of a block that is in a loop without terminating the loop. To do that, I made the block a one-iteration loop:
for b in 1..2 do
puts b
begin
puts 'want this to run'
break
puts 'but not this'
end while false
puts 'also want this to run'
end
Hope this helps the next googler that lands here based on the subject line.
If you want your block to return a useful value (e.g. when using #map, #inject, etc.), next and break also accept an argument.
Consider the following:
def contrived_example(numbers)
numbers.inject(0) do |count, x|
if x % 3 == 0
count + 2
elsif x.odd?
count + 1
else
count
end
end
end
The equivalent using next:
def contrived_example(numbers)
numbers.inject(0) do |count, x|
next count if x.even?
next (count + 2) if x % 3 == 0
count + 1
end
end
Of course, you could always extract the logic needed into a method and call that from inside your block:
def contrived_example(numbers)
numbers.inject(0) { |count, x| count + extracted_logic(x) }
end
def extracted_logic(x)
return 0 if x.even?
return 2 if x % 3 == 0
1
end
use the keyword break instead of return
Perhaps you can use the built-in methods for finding particular items in an Array, instead of each-ing targets and doing everything by hand. A few examples:
class Array
def first_frog
detect {|i| i =~ /frog/ }
end
def last_frog
select {|i| i =~ /frog/ }.last
end
end
p ["dog", "cat", "godzilla", "dogfrog", "woot", "catfrog"].first_frog
# => "dogfrog"
p ["hats", "coats"].first_frog
# => nil
p ["houses", "frogcars", "bottles", "superfrogs"].last_frog
# => "superfrogs"
One example would be doing something like this:
class Bar
def do_things
Foo.some_method(x) do |i|
# only valid `targets` here, yay.
end
end
end
class Foo
def self.failed
#failed ||= []
end
def self.some_method(targets, &block)
targets.reject {|t| t.do_something.bad? }.each(&block)
end
end
next and break seem to do the correct thing in this simplified example!
class Bar
def self.do_things
Foo.some_method(1..10) do |x|
next if x == 2
break if x == 9
print "#{x} "
end
end
end
class Foo
def self.some_method(targets, &block)
targets.each do |target|
begin
r = yield(target)
rescue => x
puts "rescue #{x}"
end
end
end
end
Bar.do_things
output: 1 3 4 5 6 7 8
You have four ways to unwind the stack in 'non-exceptional' ways: next, break, return, and throw.
next will cause the block to return.
break will cause the method that yielded to the block to return.
return will cause the method where the block is defined to return.
throw will traverse up the stack until it finds a catch with a matching symbol, and cause that to return. This is much like a 'lightweight' exception for non-exceptional situations.
All of them can take a return value that will be returned by whatever they caused to return instead of the value their last expression which they would return normally.
Here are some examples:
def doSomething
puts "> doSomething"
yield
puts "< doSomething"
end
def withNext
puts "> withNext"
doSomething do
puts "> block"
puts "* NEXT! causes the block to return to doSomething"
next
puts "< block"
end
puts "< withNext"
end
def withBreak
puts "> withBreak"
doSomething do
puts "> block"
puts "* BREAK! causes doSomething to return to withBreak"
break
puts "< block"
end
puts "< withBreak"
end
def withReturn
puts "> withReturn"
doSomething do
puts "> block"
puts "* RETURN! causes withReturn to return"
return
puts "< block"
end
puts "< withReturn"
end
def withThrow
puts "> withThrow"
catch :label do
puts "> catch :label"
doSomething do
puts "> block 1"
doSomething do
puts "> block 2"
puts "* THROW! causes catch :label to return to withThrow"
throw :label
puts "< block 2"
end
puts "< block 1"
end
puts "< catch :label"
end
puts "< withThrow"
end
withNext
puts "* Done"
puts
withBreak
puts "* Done"
puts
withReturn
puts "* Done"
puts
withThrow
puts "* Done"
output:
> withNext
> doSomething
> block
* NEXT! causes the block to return to doSomething
< doSomething
< withNext
* Done
> withBreak
> doSomething
> block
* BREAK! causes doSomething to return to withBreak
< withBreak
* Done
> withReturn
> doSomething
> block
* RETURN! causes withReturn to return
* Done
> withThrow
> catch :label
> doSomething
> block 1
> doSomething
> block 2
* THROW! causes catch :label to return to withThrow
< withThrow
* Done
To break out from a ruby block simply use return keyword return if value.nil? next.
next terminates lambda, block, or proc it is in.
break terminates the method that yielded to the block or invoked the proc or lambda it is in.
Credit to: Ruby block return, break, next
Related
I can't figure out how to exit the game loop. I'm having a hard tried making a lose? function, I tried doing like lose?(x) that would return true when x==1 but that didn't get it to exit the run method. Here's my code for the Game class.
class Minesweeper
attr_accessor :board
def initialize
#board = Board.new
end
def run
puts "Welcome to minesweeper!"
x = nil
play_turn until win? || lose?(x)
end
def play_turn
board.render
pos, command = get_input
debugger
if !explode?(pos, command)
board.set_input(pos,command)
else
puts "You lose!"
lose?(1)
end
end
def explode?(pos, cmd)
board.grid[pos[0]][pos[1]].bomb && cmd == "reveal"
end
def get_input
pos = nil
command = nil
until pos && check_pos(pos)
puts "What position?"
pos = parse_pos(gets.chomp)
end
until command && check_command(command)
puts
puts "What would you like to do (e.g. reveal, flag... ~ else?)"
command = gets.chomp
puts
end
[pos, command]
end
#Some code here (check_pos, etc)
def lose?(x)
return true if x == 1
false
end
def win?
# board.all? {}
end
end
I had explode? in the Board class before, but for the sake of being able to end the game, moved it here. Any help is greatly appreciated!
If we expand your run method, a little, it becomes more obvious what the issue is:
def run
puts "Welcome to minesweeper!"
x = nil
until(win? || lose(x)) do
play_turn
end
end
In this method, you're creating a new variable x which is scoped to this method and has a value of nil. This variable is never set in the scope of that method, so is always nil, meaning that the check win? || lose(x) could also be written win? || lose(nil), and lose will never return true.
If you return a value from your play_turn method, you can then use that. Note that the result of the last thing executed in the method is what is returned:
def play_turn
board.render
pos, command = get_input
debugger
if !explode?(pos, command)
board.set_input(pos,command)
true
else
puts "You lose!"
false
end
end
which means your run method can then check the result:
def run
puts "Welcome to minesweeper!"
# we now know that play_turn returns true if the turn was not a loser
# (i.e. it should continue to the next loop) and false if the player
# lost, so we can use that returned value in our loop.
while(play_turn) do
if win?
puts "looks like you won!"
# if the player wins, we want to exit the loop as well. This is done
# using break
break
end
end
end
Note that this also means that you don't need a separate lose method
I am trying to learn blocks but I am not able to run the below code after verifying the syntax:
class Test
def arthmatic(a=5,b=6)
yield(a,b)
end
arthmatic do |a,b|
if a > b
puts "x is greater"
else
puts "y is greater"
end
end
end
test.new.arthmatic(6, 7)
You supply the block to the method when you call it, not in the class, so try this:
class Test
def arthmatic(a=5,b=6)
yield(a,b)
end
end
Test.new.arthmatic(6, 7) do |a, b|
if a > b
puts "x is greater"
else
puts "y is greater"
end
end
I got this error:
test.rb:7: undefined method `arthmatic' for Test:Class (NoMethodError)
Your syntax is ok, but you are trying to call arthmatic from a wrong scope (the class scope of Test)
You need to do this:
class Test
def arthmatic(a=5,b=6)
yield(a,b)
end
Test.new.arthmatic {|a,b|
if a > b
then
puts "x is greater"
else
puts "y is greater"
end
}
end
Test.new.arthmatic(6, 7) do |a,b|
puts a,b
end
Or this
class Test
def self.arthmatic(a=5,b=6) # class method
yield(a,b)
end
arthmatic {|a,b|
if a > b
then
puts "x is greater"
else
puts "y is greater"
end
}
end
Test.arthmatic(6, 7) do |a,b|
puts a, b
end
That was regarding method scope, also you forget to pass the block in the last method call, you need to fix by checking the block on the method:
def self.arthmatic(a=5,b=6) # class method
yield(a,b) if block_given?
end
Or by always passing it
Test.arthmatic(6, 7) do |a,b|
puts a, b
end
A few thoughts:
in Ruby, we don't normally use if..then... rather, just if ... else... end.
if
do something
else
do something different
end
Also, blocks of code (if more then one line) normally start with "do" and finish with "end":
do
..something..
end
blocks of code, if passed to a method, should be passed as part of the method call, usually after the arguments (this is design dependent, sometimes the block will be the last argument, but it will always be part of the method call)...
Also, Ruby is case sensitive. you called test.new when you wanted Test.new.
The final code will look like:
class Test
def arthmatic(a=5,b=6)
yield(a,b)
end
end
Test.new.arthmatic(6, 7) do |a,b|
if a > b
then
puts "x is greater"
else
puts "y is greater"
end
end
Good luck!
Ruby has the File class that can be initialized using the normal new() method, or using open() and passing a block. How would I write a class that behaved like this?
File.open("myfile.txt","r") do |f|
...
end
This is a simple example of passing a block to new/open method
class Foo
def initialize(args, &block)
if block_given?
p block.call(args) # or do_something
else
#do_something else
end
end
def self.open(args, &block)
if block_given?
a = new(args, &block) # or do_something
else
#do_something else
end
ensure
a.close
end
def close
puts "closing"
end
end
Foo.new("foo") { |x| "This is #{x} in new" }
# >> "This is foo in new"
Foo.open("foo") { |x| "This is #{x} in open" }
# >> "This is foo in open"
# >> closing
The general outline of File.open is something like this:
def open(foo, bar)
f = do_opening_stuff(foo, bar)
begin
yield f
ensure
do_closing_stuff(f)
end
end
It is yield that invokes the block passed by the caller. Putting do_closing_stuff within an ensure guarantees that the file gets closed even if the block raises an exception.
More nitty-gritty on the various ways of calling blocks here: http://innig.net/software/ruby/closures-in-ruby
You can create a class method which creates an instance, yields it, and then then performs cleanup after the yield.
class MyResource
def self.open(thing, otherthing)
r = self.new(thing, otherthing)
yield r
r.close
end
def initialize(thing, otherthing)
#thing = thing
#otherthing = otherthing
end
def do_stuff
puts "Doing stuff with #{#thing} and #{#otherthing}"
end
def close
end
end
Now, you can either use it with a constructor:
r = MyResource.new(1, 2)
r.do_stuff
r.close
or using a block, which automatically closes the object:
MyResource.open(1, 2) do |r|
r.do_stuff
end
def any?
if block_given?
method_missing(:any?) { |*block_args| yield(*block_args) }
else
!empty?
end
end
In this code from ActiveRecord, what is the purpose of a yield statement that exists within a block?
Basically if the current method has been given a code-block (by the caller, when it was invoked), the yield executes the code block passing in the specified parameters.
[1,2,3,4,5].each { |x| puts x }
Now { |x| puts x} is the code-block (x is a parameter) passed to the each method of Array. The Array#each implementation would iterate over itself and call your block multiple times with x = each_element
pseudocode
def each
#iterate over yourself
yield( current_element )
end
Hence it results
1
2
3
4
5
The *block_args is a Ruby way to accept an unknown number of parameters as an array. The caller can pass in blocks with different number of arguments.
Finally let's see what yield within a block does.
class MyClass
def print_table(array, &block)
array.each{|x| yield x}
end
end
MyClass.new.print_table( [1,2,3,4,5] ) { |array_element|
10.times{|i| puts "#{i} x #{array_element} = #{i*array_element}" }
puts "-----END OF TABLE----"
}
Here Array#each yields each element to the block given to MyClass#print_table...
It does not mean anything special. It's just a yield like any other yield.
def test_method
["a", "b", "c"].map {|i| yield(i) }
end
p test_method {|i| i.upcase }
# => ["A", "B", "C"]
In the code snippet from active record, the purpose is to yield every time the block of any? is called.
This helped me understand: yield is a way to insert blocks into a method you already wrote, which means "execute something here". For instance,
Without parameter
def original_method
puts "a"
puts "b"
end
What if you want to execute some unknown block between those two p?
def updated_method
puts "a"
yield
puts "b"
end
# these two are the same:
updated_method { puts "execute something here" }
updated_method do puts "execute something here" end
The result would be
a
execute something here
b
You can have as many yields in a method as you like.
With parameter
def updated_method
puts "a"
yield
puts "b"
end
What if you want to execute some unknown block on the string "Execute something on me!"?
def updated_method_with_argument
puts "a"
yield("Execute something on me!")
puts "b"
end
updated_method_with_argument do |argument|
puts "This argument gets put out: " << argument
end
The result would be
a
This argument gets put out: Execute something on me!
b
def any?
if block_given?
method_missing(:any?) { |*block_args| yield(*block_args) }
else
!empty?
end
end
In this code from ActiveRecord, what is the purpose of a yield statement that exists within a block?
Basically if the current method has been given a code-block (by the caller, when it was invoked), the yield executes the code block passing in the specified parameters.
[1,2,3,4,5].each { |x| puts x }
Now { |x| puts x} is the code-block (x is a parameter) passed to the each method of Array. The Array#each implementation would iterate over itself and call your block multiple times with x = each_element
pseudocode
def each
#iterate over yourself
yield( current_element )
end
Hence it results
1
2
3
4
5
The *block_args is a Ruby way to accept an unknown number of parameters as an array. The caller can pass in blocks with different number of arguments.
Finally let's see what yield within a block does.
class MyClass
def print_table(array, &block)
array.each{|x| yield x}
end
end
MyClass.new.print_table( [1,2,3,4,5] ) { |array_element|
10.times{|i| puts "#{i} x #{array_element} = #{i*array_element}" }
puts "-----END OF TABLE----"
}
Here Array#each yields each element to the block given to MyClass#print_table...
It does not mean anything special. It's just a yield like any other yield.
def test_method
["a", "b", "c"].map {|i| yield(i) }
end
p test_method {|i| i.upcase }
# => ["A", "B", "C"]
In the code snippet from active record, the purpose is to yield every time the block of any? is called.
This helped me understand: yield is a way to insert blocks into a method you already wrote, which means "execute something here". For instance,
Without parameter
def original_method
puts "a"
puts "b"
end
What if you want to execute some unknown block between those two p?
def updated_method
puts "a"
yield
puts "b"
end
# these two are the same:
updated_method { puts "execute something here" }
updated_method do puts "execute something here" end
The result would be
a
execute something here
b
You can have as many yields in a method as you like.
With parameter
def updated_method
puts "a"
yield
puts "b"
end
What if you want to execute some unknown block on the string "Execute something on me!"?
def updated_method_with_argument
puts "a"
yield("Execute something on me!")
puts "b"
end
updated_method_with_argument do |argument|
puts "This argument gets put out: " << argument
end
The result would be
a
This argument gets put out: Execute something on me!
b