I have a module with functions where I need to ensure that all functions use the same random values. I currently have two solutions, resetting the seed at each call:
using Distributions
function random_values(n)
Random.seed!(1)
rand(Normal(), n)
end
or similarly, instantiating it directly:
using Distributions
function random_values(n)
rand(MersenneTwister(1), Normal(), n)
end
This works but I have several functions and the code becomes a bit verbose. I would rather set a seed at the module level so that all functions use the same. How can I best achieve this?
I believe that what you want is to have two or more functions (f1, f2, ...) that each call rand(). You want them in "sync" so that with a series of function calls each function gets the same series of values for each sequential call of each function (so that the third call to f2 uses the same rand() value as the third call to f1).
For this, you are best off explicitly passing a duplicated RNG object to each of your functions after initializing each to the same seed:
init = 1234
rngvector(n) = [MersenneTwister(init) for _ in 1:n]
RNG = rngvector(3)
function f1(rng, ...)
x = rand(rng, ...)
...
end
function f2(rng, ...)
x = rand(rng, ...)
...
end
f1(RNG[1], ...)
will then be in sync with
f2(RNG[2], ...).
This does mean more coding since you add one more argument to each function, but the flexibility and reproducibility that initial extra coding gives you with your Monte Carlo methods when coding the rest of what you do may make it very worthwhile.
What are the scenarios , where = is preferred over := ?
What are the scenarios, where := is preferred over = ?
I read from gnu site, that usage of = makes make run slower. I just wonder, when do we use = in makefile?
To answer your question, = is used when you want to delay expansion of the right side until the variable is used.
This allows you to define variables in any order. It also allows you to create variables with values that refer to automatic variables (remember automatic variables have no value until the rule is being run). So for example:
my_FLAGS = -a
your_FLAGS = -b
FLAGS = $($#_FLAGS)
my your : ; #echo $(FLAGS)
this cannot work if you use := because when the FLAGS variable is defined, $# has no value.
They are also useful when defining user-defined functions that can later be invoked with $(call ...); you don't want those to be expanded until they are called.
With simply defined variables you can do things like recursively use the variable:
ITEMS := one two three
ITEMS := $(addsuffix $(ITEMS))
This is because the simple assignment (:=) happens in the order you read them
Non simple assignment (=) is recursively expanded so if you assign it to other variables they are in turn expanded until you end up with the final result that contains all the expanded parts. Note that the makefile first parses the file so that the order you do the assignment is not so important, examples to follow:
i.e. this is not allowed:
ITEMS = one two three
ITEMS = $(addsuffix $(ITEMS))
So this affects when you want to use each type. With non simple you can do:
ITEMS1 = a b c
ITEMS_all = $(ITEMS1) $(ITEMS2)
ITEMS2 = d e f
And now ITEMS_ALL will contain a b c d e f - even though they are not defined in order, this can be very useful. So if you just want to assign a simply value - stick with := if you want to keep adding things to a variable you might want to use =...
I wrote a function that acts on each combination of columns in an input matrix. It uses multiple for loops and is very slow, so I am trying to parallelize it to use the maximum number of threads on my computer.
I am having difficulty finding the correct syntax to set this up. I'm using the Parallel package in octave, and have tried several ways to set up the calls. Here are two of them, in a simplified form, as well as a non-parallel version that I believe works:
function A = parallelExample(M)
pkg load parallel;
# Get total count of columns
ct = columns(M);
# Generate column pairs
I = nchoosek([1:ct],2);
ops = rows(I);
slice = ones(1, ops);
Ic = mat2cell(I, slice, 2);
## # Non-parallel
## A = zeros(1, ops);
## for i = 1:ops
## A(i) = cmbtest(Ic{i}, M);
## endfor
# Parallelized call v1
A = parcellfun(nproc, #cmbtest, Ic, {M});
## # Parallelized call v2
## afun = #(x) cmbtest(x, M);
## A = parcellfun(nproc, afun, Ic);
endfunction
# function to apply
function P = cmbtest(indices, matrix)
colset = matrix(:,indices);
product = colset(:,1) .* colset(:,2);
P = sum(product);
endfunction
For both of these examples I generate every combination of two columns and convert those pairs into a cell array that the parcellfun function should split up. In the first, I attempt to convert the input matrix M into a 1x1 cell array so it goes to each parallel instance in the same form. I get the error 'C must be a cell array' but this must be internal to the parcellfun function. In the second, I attempt to define an anonymous function that includes the matrix. The error I get here specifies that 'cmbtest' is undefined.
(Naturally, the actual function I'm trying to apply is far more complex than cmbtest here)
Other things I have tried:
Put M into a global variable so it doesn't need to be passed. Seemed to be impossible to put a global variable in a function file, though I may just be having syntax issues.
Make cmbtest a nested function so it can access M (parcellfun doesn't support that)
I'm out of ideas at this point and could use help figuring out how to get this to work.
Converting my comments above to an answer.
When performing parallel operations, it is useful to think of each parallel worker that will result as separate and independent octave instances, which need to have appropriate access to all functions and variables they will require in order to do their independent work.
Therefore, do not rely on subfunctions when calling parcellfun from a main function, since this might lead to errors if the worker is unable to access the subfunction directly under the hood.
In this case, separating the subfunction into its own file fixed the problem.
Is there a method to overwrite variable without copying its name? For example, when I want to change my_var = '3' to an integer, I must do something like this:
my_var = my_var.to_i
Is there way to do this without copying variable's name? I want to do something like this:
my_var = something_const.to_i
For numbers there exists +=, -= etc, but is there universal way to do this for all methods ?
There is no way to covert a string to an integer like that, without repeating the variable name. Methods such as String#upcase! and Array#flatten! work by mutating the object; however, it is not possible to define such a method like String#to_i! because we are converting the object to an instance of a different class.
For example, here is a (failed) attempt to define such a method:
# What I want to be able to do:
# my_var = "123"
# my_var.to_i! # => my_var == 123
class String
def to_i!
replace(Integer(self))
end
end
my_var = "123"
my_var.to_i! # TypeError: no implicit conversion of Fixnum into String
...And even if this code were valid, it would still offer no performance gain since a new object is still being created.
As for your examples of += and -=, these are in fact simply shorthand for:
x += 1
# Is equivalent to:
x = x + 1
So again, there is no performance gain here either; just slightly nicer syntax. A good question to ask is, why doesn't ruby support a ++ operator? If such an operator existed then it would offer performance gain... But I'll let you research for yourself why this is missing from the language.
So to summarise,
is there universal way to do this for all methods?
No. The special operators like +=, -=, |= and &= are all predefined; there is no "generalised" version such as method_name=.
You can also define methods that mutate the object, but only when appropriate. Such methods are usually named with a !, are called "bang-methods", and have a "non-bang" counterpart. On String objects, for example, there is String#capitalize! (and String#capitalize), String#delete! (and String#delete), String#encode! (and String#encode), .... but no String#to_i! for the reasons discussed above.
What are the precise rules for when you can omit (omit) parentheses, dots, braces, = (functions), etc.?
For example,
(service.findAllPresentations.get.first.votes.size) must be equalTo(2).
service is my object
def findAllPresentations: Option[List[Presentation]]
votes returns List[Vote]
must and be are both functions of specs
Why can't I go:
(service findAllPresentations get first votes size) must be equalTo(2)
?
The compiler error is:
"RestServicesSpecTest.this.service.findAllPresentations
of type
Option[List[com.sharca.Presentation]]
does not take parameters"
Why does it think I'm trying to pass in a parameter? Why must I use dots for every method call?
Why must (service.findAllPresentations get first votes size) be equalTo(2) result in:
"not found: value first"
Yet, the "must be equalTo 2" of
(service.findAllPresentations.get.first.votes.size) must be equalTo 2, that is, method chaining works fine? - object chain chain chain param.
I've looked through the Scala book and website and can't really find a comprehensive explanation.
Is it in fact, as Rob H explains in Stack Overflow question Which characters can I omit in Scala?, that the only valid use-case for omitting the '.' is for "operand operator operand" style operations, and not for method chaining?
You seem to have stumbled upon the answer. Anyway, I'll try to make it clear.
You can omit dot when using the prefix, infix and postfix notations -- the so called operator notation. While using the operator notation, and only then, you can omit the parenthesis if there is less than two parameters passed to the method.
Now, the operator notation is a notation for method-call, which means it can't be used in the absence of the object which is being called.
I'll briefly detail the notations.
Prefix:
Only ~, !, + and - can be used in prefix notation. This is the notation you are using when you write !flag or val liability = -debt.
Infix:
That's the notation where the method appears between an object and it's parameters. The arithmetic operators all fit here.
Postfix (also suffix):
That notation is used when the method follows an object and receives no parameters. For example, you can write list tail, and that's postfix notation.
You can chain infix notation calls without problem, as long as no method is curried. For example, I like to use the following style:
(list
filter (...)
map (...)
mkString ", "
)
That's the same thing as:
list filter (...) map (...) mkString ", "
Now, why am I using parenthesis here, if filter and map take a single parameter? It's because I'm passing anonymous functions to them. I can't mix anonymous functions definitions with infix style because I need a boundary for the end of my anonymous function. Also, the parameter definition of the anonymous function might be interpreted as the last parameter to the infix method.
You can use infix with multiple parameters:
string substring (start, end) map (_ toInt) mkString ("<", ", ", ">")
Curried functions are hard to use with infix notation. The folding functions are a clear example of that:
(0 /: list) ((cnt, string) => cnt + string.size)
(list foldLeft 0) ((cnt, string) => cnt + string.size)
You need to use parenthesis outside the infix call. I'm not sure the exact rules at play here.
Now, let's talk about postfix. Postfix can be hard to use, because it can never be used anywhere except the end of an expression. For example, you can't do the following:
list tail map (...)
Because tail does not appear at the end of the expression. You can't do this either:
list tail length
You could use infix notation by using parenthesis to mark end of expressions:
(list tail) map (...)
(list tail) length
Note that postfix notation is discouraged because it may be unsafe.
I hope this has cleared all the doubts. If not, just drop a comment and I'll see what I can do to improve it.
Class definitions:
val or var can be omitted from class parameters which will make the parameter private.
Adding var or val will cause it to be public (that is, method accessors and mutators are generated).
{} can be omitted if the class has no body, that is,
class EmptyClass
Class instantiation:
Generic parameters can be omitted if they can be inferred by the compiler. However note, if your types don't match, then the type parameter is always infered so that it matches. So without specifying the type, you may not get what you expect - that is, given
class D[T](val x:T, val y:T);
This will give you a type error (Int found, expected String)
var zz = new D[String]("Hi1", 1) // type error
Whereas this works fine:
var z = new D("Hi1", 1)
== D{def x: Any; def y: Any}
Because the type parameter, T, is inferred as the least common supertype of the two - Any.
Function definitions:
= can be dropped if the function returns Unit (nothing).
{} for the function body can be dropped if the function is a single statement, but only if the statement returns a value (you need the = sign), that is,
def returnAString = "Hi!"
but this doesn't work:
def returnAString "Hi!" // Compile error - '=' expected but string literal found."
The return type of the function can be omitted if it can be inferred (a recursive method must have its return type specified).
() can be dropped if the function doesn't take any arguments, that is,
def endOfString {
return "myDog".substring(2,1)
}
which by convention is reserved for methods which have no side effects - more on that later.
() isn't actually dropped per se when defining a pass by name paramenter, but it is actually a quite semantically different notation, that is,
def myOp(passByNameString: => String)
Says myOp takes a pass-by-name parameter, which results in a String (that is, it can be a code block which returns a string) as opposed to function parameters,
def myOp(functionParam: () => String)
which says myOp takes a function which has zero parameters and returns a String.
(Mind you, pass-by-name parameters get compiled into functions; it just makes the syntax nicer.)
() can be dropped in the function parameter definition if the function only takes one argument, for example:
def myOp2(passByNameString:(Int) => String) { .. } // - You can drop the ()
def myOp2(passByNameString:Int => String) { .. }
But if it takes more than one argument, you must include the ():
def myOp2(passByNameString:(Int, String) => String) { .. }
Statements:
. can be dropped to use operator notation, which can only be used for infix operators (operators of methods that take arguments). See Daniel's answer for more information.
. can also be dropped for postfix functions
list tail
() can be dropped for postfix operators
list.tail
() cannot be used with methods defined as:
def aMethod = "hi!" // Missing () on method definition
aMethod // Works
aMethod() // Compile error when calling method
Because this notation is reserved by convention for methods that have no side effects, like List#tail (that is, the invocation of a function with no side effects means that the function has no observable effect, except for its return value).
() can be dropped for operator notation when passing in a single argument
() may be required to use postfix operators which aren't at the end of a statement
() may be required to designate nested statements, ends of anonymous functions or for operators which take more than one parameter
When calling a function which takes a function, you cannot omit the () from the inner function definition, for example:
def myOp3(paramFunc0:() => String) {
println(paramFunc0)
}
myOp3(() => "myop3") // Works
myOp3(=> "myop3") // Doesn't work
When calling a function that takes a by-name parameter, you cannot specify the argument as a parameter-less anonymous function. For example, given:
def myOp2(passByNameString:Int => String) {
println(passByNameString)
}
You must call it as:
myOp("myop3")
or
myOp({
val source = sourceProvider.source
val p = myObject.findNameFromSource(source)
p
})
but not:
myOp(() => "myop3") // Doesn't work
IMO, overuse of dropping return types can be harmful for code to be re-used. Just look at specification for a good example of reduced readability due to lack of explicit information in the code. The number of levels of indirection to actually figure out what the type of a variable is can be nuts. Hopefully better tools can avert this problem and keep our code concise.
(OK, in the quest to compile a more complete, concise answer (if I've missed anything, or gotten something wrong/inaccurate please comment), I have added to the beginning of the answer. Please note this isn't a language specification, so I'm not trying to make it exactly academically correct - just more like a reference card.)
A collection of quotes giving insight into the various conditions...
Personally, I thought there'd be more in the specification. I'm sure there must be, I'm just not searching for the right words...
There are a couple of sources however, and I've collected them together, but nothing really complete / comprehensive / understandable / that explains the above problems to me...:
"If a method body has more than one
expression, you must surround it with
curly braces {…}. You can omit the
braces if the method body has just one
expression."
From chapter 2, "Type Less, Do More", of Programming Scala:
"The body of the upper method comes
after the equals sign ‘=’. Why an
equals sign? Why not just curly braces
{…}, like in Java? Because semicolons,
function return types, method
arguments lists, and even the curly
braces are sometimes omitted, using an
equals sign prevents several possible
parsing ambiguities. Using an equals
sign also reminds us that even
functions are values in Scala, which
is consistent with Scala’s support of
functional programming, described in
more detail in Chapter 8, Functional
Programming in Scala."
From chapter 1, "Zero to Sixty: Introducing Scala", of Programming Scala:
"A function with no parameters can be
declared without parentheses, in which
case it must be called with no
parentheses. This provides support for
the Uniform Access Principle, such
that the caller does not know if the
symbol is a variable or a function
with no parameters.
The function body is preceded by "="
if it returns a value (i.e. the return
type is something other than Unit),
but the return type and the "=" can be
omitted when the type is Unit (i.e. it
looks like a procedure as opposed to a
function).
Braces around the body are not
required (if the body is a single
expression); more precisely, the body
of a function is just an expression,
and any expression with multiple parts
must be enclosed in braces (an
expression with one part may
optionally be enclosed in braces)."
"Functions with zero or one argument
can be called without the dot and
parentheses. But any expression can
have parentheses around it, so you can
omit the dot and still use
parentheses.
And since you can use braces anywhere
you can use parentheses, you can omit
the dot and put in braces, which can
contain multiple statements.
Functions with no arguments can be
called without the parentheses. For
example, the length() function on
String can be invoked as "abc".length
rather than "abc".length(). If the
function is a Scala function defined
without parentheses, then the function
must be called without parentheses.
By convention, functions with no
arguments that have side effects, such
as println, are called with
parentheses; those without side
effects are called without
parentheses."
From blog post Scala Syntax Primer:
"A procedure definition is a function
definition where the result type and
the equals sign are omitted; its
defining expression must be a block.
E.g., def f (ps) {stats} is
equivalent to def f (ps): Unit =
{stats}.
Example 4.6.3 Here is a declaration
and a de?nition of a procedure named
write:
trait Writer {
def write(str: String)
}
object Terminal extends Writer {
def write(str: String) { System.out.println(str) }
}
The code above is implicitly completed
to the following code:
trait Writer {
def write(str: String): Unit
}
object Terminal extends Writer {
def write(str: String): Unit = { System.out.println(str) }
}"
From the language specification:
"With methods which only take a single
parameter, Scala allows the developer
to replace the . with a space and omit
the parentheses, enabling the operator
syntax shown in our insertion operator
example. This syntax is used in other
places in the Scala API, such as
constructing Range instances:
val firstTen:Range = 0 to 9
Here again, to(Int) is a vanilla
method declared inside a class
(there’s actually some more implicit
type conversions here, but you get the
drift)."
From Scala for Java Refugees Part 6: Getting Over Java:
"Now, when you try "m 0", Scala
discards it being a unary operator, on
the grounds of not being a valid one
(~, !, - and +). It finds that "m" is
a valid object -- it is a function,
not a method, and all functions are
objects.
As "0" is not a valid Scala
identifier, it cannot be neither an
infix nor a postfix operator.
Therefore, Scala complains that it
expected ";" -- which would separate
two (almost) valid expressions: "m"
and "0". If you inserted it, then it
would complain that m requires either
an argument, or, failing that, a "_"
to turn it into a partially applied
function."
"I believe the operator syntax style
works only when you've got an explicit
object on the left-hand side. The
syntax is intended to let you express
"operand operator operand" style
operations in a natural way."
Which characters can I omit in Scala?
But what also confuses me is this quote:
"There needs to be an object to
receive a method call. For instance,
you cannot do “println “Hello World!”"
as the println needs an object
recipient. You can do “Console
println “Hello World!”" which
satisfies the need."
Because as far as I can see, there is an object to receive the call...
I find it easier to follow this rule of thumb: in expressions spaces alternate between methods and parameters. In your example, (service.findAllPresentations.get.first.votes.size) must be equalTo(2) parses as (service.findAllPresentations.get.first.votes.size).must(be)(equalTo(2)). Note that the parentheses around the 2 have a higher associativity than the spaces. Dots also have higher associativity, so (service.findAllPresentations.get.first.votes.size) must be.equalTo(2)would parse as (service.findAllPresentations.get.first.votes.size).must(be.equalTo(2)).
service findAllPresentations get first votes size must be equalTo 2 parses as service.findAllPresentations(get).first(votes).size(must).be(equalTo).2.
Actually, on second reading, maybe this is the key:
With methods which only take a single
parameter, Scala allows the developer
to replace the . with a space and omit
the parentheses
As mentioned on the blog post: http://www.codecommit.com/blog/scala/scala-for-java-refugees-part-6 .
So perhaps this is actually a very strict "syntax sugar" which only works where you are effectively calling a method, on an object, which takes one parameter. e.g.
1 + 2
1.+(2)
And nothing else.
This would explain my examples in the question.
But as I said, if someone could point out to be exactly where in the language spec this is specified, would be great appreciated.
Ok, some nice fellow (paulp_ from #scala) has pointed out where in the language spec this information is:
6.12.3:
Precedence and associativity of
operators determine the grouping of
parts of an expression as follows.
If there are several infix operations in an expression, then
operators with higher precedence bind
more closely than operators with lower
precedence.
If there are consecutive infix operations e0 op1 e1 op2 . . .opn en
with operators op1, . . . , opn of the
same precedence, then all these
operators must have the same
associativity. If all operators are
left-associative, the sequence is
interpreted as (. . . (e0 op1 e1) op2
. . .) opn en. Otherwise, if all
operators are rightassociative, the
sequence is interpreted as e0 op1 (e1
op2 (. . .opn en) . . .).
Postfix operators always have lower precedence than infix operators. E.g.
e1 op1 e2 op2 is always equivalent to
(e1 op1 e2) op2.
The right-hand operand of a
left-associative operator may consist
of several arguments enclosed in
parentheses, e.g. e op (e1, . . .
,en). This expression is then
interpreted as e.op(e1, . . . ,en).
A left-associative binary operation e1
op e2 is interpreted as e1.op(e2). If
op is rightassociative, the same
operation is interpreted as { val
x=e1; e2.op(x ) }, where x is a fresh
name.
Hmm - to me it doesn't mesh with what I'm seeing or I just don't understand it ;)
There aren't any. You will likely receive advice around whether or not the function has side-effects. This is bogus. The correction is to not use side-effects to the reasonable extent permitted by Scala. To the extent that it cannot, then all bets are off. All bets. Using parentheses is an element of the set "all" and is superfluous. It does not provide any value once all bets are off.
This advice is essentially an attempt at an effect system that fails (not to be confused with: is less useful than other effect systems).
Try not to side-effect. After that, accept that all bets are off. Hiding behind a de facto syntactic notation for an effect system can and does, only cause harm.