I am a Java programmer, and I am learning Ruby...
But I don't get where those blocks of codes can give me gain... like what's the purpose of passing a block as an argument ? why not have 2 specialized methods instead that can be reused ?
Why have some code in a block that cannot be reused ?
I would love some code examples...
Thanks for the help !
Consider some of the things you would use anonymous classes for in Java. e.g. often they are used for pluggable behaviour such as event listeners or to parametrize a method that has a general layout.
Imagine we want to write a method that takes a list and returns a new list containing the items from the given list for which a specified condition is true. In Java we would write an interface:
interface Condition {
boolean f(Object o);
}
and then we could write:
public List select(List list, Condition c) {
List result = new ArrayList();
for (Object item : list) {
if (c.f(item)) {
result.add(item);
}
}
return result;
}
and then if we wanted to select the even numbers from a list we could write:
List even = select(mylist, new Condition() {
public boolean f(Object o) {
return ((Integer) o) % 2 == 0;
}
});
To write the equivalent in Ruby it could be:
def select(list)
new_list = []
# note: I'm avoid using 'each' so as to not illustrate blocks
# using a method that needs a block
for item in list
# yield calls the block with the given parameters
new_list << item if yield(item)
end
return new_list
end
and then we could select the even numbers with simply
even = select(list) { |i| i % 2 == 0 }
Of course, this functionality is already built into Ruby so in practice you would just do
even = list.select { |i| i % 2 == 0 }
As another example, consider code to open a file. You could do:
f = open(somefile)
# work with the file
f.close
but you then need to think about putting your close in an ensure block in case an exception occurs whilst working with the file. Instead, you can do
open(somefile) do |f|
# work with the file here
# ruby will close it for us when the block terminates
end
The idea behind blocks is that it is a highly localized code where it is useful to have the definition at the call site. You can use an existing function as a block argument. Just pass it as an additional argument, and prefix it with an &
Related
UserList is a list of dictionaries, like:
[
{Name:"Alex",Age:25},
{Name:"Peter",Age:35},
{Name:"Muhammad",Age:28},
{Name:"Raul",Age:29}
]
RowColorList is a list of colors: [#bcf,#fc0]
The new UserList should contain one RowColor for every name, taken in sequence from RowColorList:
[
{Name:"Alex",Age:25,RowColor:#bcf},
{Name:"Peter",Age:35,RowColor:#fc0},
{Name:"Muhammad",Age:28,RowColor:#bcf},
{Name:"Raul",Age:29,RowColor:#fc0}
]
I tried the following code:
UserList.Zip(RowColorList,(user,color) => user.Add("RowColor",color))
With this code, the new UserList will only contain as many entries as are in RowColorList. I would like him to start from the beginning of RowColorList again, whenever the available colors are used up. How?
You can create a function to return an infinite enumerable of Color / string (or whatever the type of RowColor is), by using yield return as a lazy generator:
public IEnumerable<Color> InfiniteColors()
{
while (true)
{
foreach (var color in RowColors)
{
yield return color;
}
}
}
This can then be used with any of the Linq IEnumerable extension methods such as Zip.
UserList.Zip(InfiniteColors(),(user,color) => user.Add("RowColor",color))
Edit - Explanation
The reason why InfiniteColors doesn't hang is because the state machine will yield back to the caller after each result, and Zip will terminate on the first enumerable to complete, which is because the other collection being zipped is finite (i.e. UserList)
Obviously you shouldn't try and Zip the InfiniteColors enumerable with itself, nor should you try and materialize InfiniteColors, i.e. don't call InfiniteColors.ToList() or such :-):
Something like this should do the trick:
var i = 0;
var l = RowColorList.Count;
UserList.ForEach(user => user.Add("RowColor", RowColorList[(i++) % l]));
The % operator will guarantee "cyclic" access to the RowColorList.
So I found this quiz on a website that I was excited to solve with my newly acquired Ruby skills (CodeAcademy, not quite finished yet).
What I want to do is make an array with 100 entries, all set to "open". Then, I planned to create a method containing a for loop that iterates through every nth entry of the array and changes it to either "open" or "closed", based on what it was before. In the for loop, n should be increased from 1 to 100.
What I have so far is this:
change_state = Proc.new { |element| element == "open" ? element = "closed" : element = "open" }
def janitor(array,n)
for i in 1..n
array.each { |element| if array.index(element) % i == 0 then element.change_state end }
end
end
lockers = [*1..100]
lockers = lockers.map{ |element| element = "closed" }
result = janitor(lockers,100)
When trying to execute I receive an error saying:
undefined method `change_state' for "closed":String (NoMethodError)
Anybody an idea what is wrong here? I kinda think I'm calling the "change_state" proc incorrectly on the current array element.
If you know the quiz, no spoilers please!
As you have implemented change_state, it is not a method of any class, and definitely not one attached to any of the individual elements of the array, despite you using the same variable name element. So you cannot call it as element.change_state.
Instead, it is a variable pointing to a Proc object.
To call the code in a Proc object, you would use the call method, and syntax like proc_obj.call( params ) - in your case change_state.call( element )
If you just drop in that change, your error message will change to:
NameError: undefined local variable or method `change_state' for main:Object
That's because the change_state variable is not in scope inside the method, in order to be called. There are lots of ways to make it available. One option would be to pass it in as a parameter, so your definition for janitor becomes
def janitor(array,n,state_proc)
(use the variable name state_proc inside your routine instead of change_state - I am suggesting you change the name to avoid confusing yourself)
You could then call it like this:
result = janitor(lockers,100,change_state)
Although your example does not really need this structure, this is one way in which Ruby code can provide a generic "outer" function - working through the elements of an array, say - and have the user of that code provide a small internal custom part of it. A more common way to achieve the same result as your example is to use a Ruby block and the yield method, but Procs also have their uses, because you can treat them like data as well as code - so you can pass them around, put them into hashes or arrays to decide which one to call etc.
There may be other issues to address in your code, but this is the cause of the error message in the question.
I am confused about bindings.
def repl(input_stream, output_stream)
loop do
output_stream.print "> "
input = input_stream.gets()
result = binding.eval(input)
output_stream.puts(result)
end
end
repl($stdin, $stdout)
I am going to call repl with just $stdin and $stdout. I need a dumbed down version of what the line:
binding.eval(input) is doing.
Bindings are just where we currently are in the call stack right? They hold the current local variables? Anything else? What's a good way to think of them differently from the current scope?
Objects of class Binding encapsulate the execution context at some particular place in the code and retain this context for future use. The variables, methods, value of self, and possibly an iterator block that can be accessed in this context are all retained. Binding objects can be created using Kernel#binding, and are made available to the callback of Kernel#set_trace_func.
static VALUE
bind_local_variable_defined_p(VALUE bindval, VALUE sym)
{
ID lid = check_local_id(bindval, &sym);
const rb_binding_t *bind;
if (!lid) return Qfalse;
GetBindingPtr(bindval, bind);
return get_local_variable_ptr(bind->env, lid) ? Qtrue : Qfalse;
}
Here is a example of Public Instance Methods
def get_binding(param)
return binding
end
b = get_binding("hello")
b.eval("param") #=> "hello"
I'm writing some training material for the Groovy language and I'm preparing an example which would explain Closures.
The example is a simple caching closure for "expensive" methods, withCache
def expensiveMethod( Long a ) {
withCache (a) {
sleep(rnd())
a*5
}
}
So, now my question is: which of the two following implementations would be the fastest and more idiomatic in Groovy?
def withCache = {key, Closure operation ->
if (!cacheMap.containsKey(key)) {
cacheMap.put(key, operation())
}
cacheMap.get(key)
}
or
def withCache = {key, Closure operation ->
def cached = cacheMap.get(key)
if (cached) return cached
def res = operation()
cacheMap.put(key, res)
res
}
I prefer the first example, as it doesn't use any variable but I wonder if accessing the get method of the Map is slower than returning the variable containing the computed result.
Obviously the answer is "it depends on the size of the Map" but, out of curiosity, I would like to have the opinion of the community.
Thanks!
Firstly I agree with OverZealous, that worrying about two get operations is a premature optimization. The second exmaple is also not equal to the first. The first allows null for example, while the second on uses Groovy-Truth in the if, which means that null evals to false, as does for example an empty list/array/map. So if you want to show calling Closure I would go with the first one. If you want something more idiomatic I would do this instead for your case:
def expensiveMethod( Long a ) {
sleep(rnd())
a*5
}
def cache = [:].withDefault this.&expensiveMethod
I've never used nested functions, but have seen references to them in several languages (as well as nested classes, which I assume are related).
What is a nested function?
Why?!?
What can you do with a nested function that you cannot do any other way?
What can you do with a nested function this is difficult or inelegant without nested functions?
I assume nested functions are simply an artifact of treating everything as an object, and if objects can contain other objects then it follows.
Do nested functions have scope (in general, I suppose languages differ on this) just as variables inside a function have scope?
Please add the language you are referencing if you're not certain that your answer is language agnostic.
-Adam
One popular use of nested functions is closures. In a lexically scoped language with first-class functions it's possible to use functions to store data. A simple example in Scheme is a counter:
(define (make-counter)
(let ((count 0)) ; used to store the count
(define (counter) ; this is the counter we're creating
(set! count (+ count 1)) ; increment the count
count) ; return the new count
counter)) ; return the new counter function
(define mycounter (make-counter)) ; create a counter called mycounter
(mycounter) ; returns 1
(mycounter) ; returns 2
In this example, we nest the function counter inside the function make-counter, and by returning this internal function we are able to access the data available to counter when it was defined. This information is private to this instance of mycounter - if we were to create another counter, it would use a different spot to store the internal count. Continuing from the previous example:
(define mycounter2 (make-counter))
(mycounter2) ; returns 1
(mycounter) ; returns 3
It's useful for recursion when there is only 1 method that will ever call it
string[] GetFiles(string path)
{
void NestedGetFiles(string path, List<string> result)
{
result.AddRange( files in the current path);
foreach(string subPath in FoldersInTheCurrentPath)
NestedGetFiles(subPath, result);
}
List<string> result = new List<string>();
NestedGetFiles(path, result);
return result.ToArray();
}
The above code is completely made up but is based on C# to give the idea of what I mean. The only method that can call NestedGetFiles is the GetFiles method.
Nested functions allow you to encapsulate code that is only relevant to the inner workings of one function within that function, while still allowing you to separate that code out for readability or generalization. In some implementations, they also allow access to outer scope. In D:
int doStuff() {
int result;
void cleanUpReturn() {
myResource1.release();
myResource2.release();
return result * 2 + 1;
}
auto myResource1 = getSomeResource();
auto myResource2 = getSomeOtherResource();
if(someCondition) {
return cleanUpReturn();
} else {
doSomeOtherStuff();
return cleanUpReturn();
}
}
Of course, in this case this could also be handled with RAII, but it's just a simple example.
A nested function is simply a function defined within the body of another function. Why? About the only reason I could think of off the top of my head is a helper or utility function.
This is a contrived example but bear with me. Let's say you had a function that had to act on the results two queries and fill an object with values from one of the queries. You could do something like the following.
function process(qryResult q1, qryResult q2) {
object o;
if (q1.someprop == "useme") {
o.prop1 = q1.prop1;
o.prop2 = q1.prop2;
o.prop3 = q1.prop3;
} else if (q2.someprop == "useme") {
o.prop1 = q2.prop1;
o.prop2 = q2.prop2;
o.prop3 = q2.prop3;
}
return o;
}
If you had 20 properties, you're duplicating the code to set the object over and over leading to a huge function. You could add a simple nested function to do the copy of the properties from the query to the object. Like this:
function process(qryResult q1, qryResult q2) {
object o;
if (q1.someprop == "useme") {
fillObject(o,q1);
} else if (q2.someprop == "useme") {
fillObject(o,q2);
}
return o;
function fillObject(object o, qryResult q) {
o.prop1 = q.prop1;
o.prop2 = q.prop2;
o.prop3 = q.prop3;
}
}
It keeps things a little cleaner. Does it have to be a nested function? No, but you may want to do it this way if the process function is the only one that would have to do this copy.
(C#) :
I use that to simplify the Object Browser view, and to structure my classes better.
As class Wheel nested in Truck class.
Don't forget this detail :
"Nested types can access private and protected members of the containing type, including any inherited private or protected members."
They can also be useful if you need to pass a function to another function as an argument. They can also be useful for making factory functions for factory functions (in Python):
>>> def GetIntMaker(x):
... def GetInt():
... return x
... return GetInt
...
>>> GetInt = GetIntMaker(1)
>>> GetInt()
1
A nested function is just a function inside another function.
Yes, it is a result of everything being an object. Since you can have variables only visible in the function's scope and variables can point to functions you can have a function that is referenced by a local variable.
I don't think there is anything that you can do with a nested function that you absolutely couldn't do without. A lot of the times it makes sense, though. Namely, whenever a function is a "sub-function" of some other function.
A common use-case for me is when a function performs a lot of complicated logic but what the function computes/returns is easy to abstract for all the cases dictated by the logic.