I have been working on code where I have to generate all possible ways to the target string. I am using the below-mentioned code.
Print Statement:
println("---------- How Construct -------")
println("${
window.howConstruct("purple", listOf(
"purp",
"p",
"ur",
"le",
"purpl"
))
}")
Function Call:
fun howConstruct(
target: String,
wordBank: List<String>,
): List<List<String>> {
if (target.isEmpty()) return emptyList()
var result = emptyList<List<String>>()
for (word in wordBank) {
if (target.indexOf(word) == 0) { // Starting with prefix
val substring = target.substring(word.length)
val suffixWays = howConstruct(substring, wordBank)
val targetWays = suffixWays.map { way ->
val a = way.toMutableList().apply {
add(word)
}
a.toList()
}
result = targetWays
}
}
return result
}
Expected Output:-
[['purp','le'],['p','ur','p','le']]
Current Output:-
[]
Your code is almost working; only a couple of small changes are needed to get the required output:
If the target is empty, return listOf(emptyList()) instead of emptyList().
Use add(0, word) instead of add(word).
The first of those changes is the important one. Your function returns a list of matches; and since each match is itself a list of strings, it returns a list of lists of strings. Once your code has matched the entire target and calls itself one last time, it returned an empty list — i.e. no matches — instead of a list containing an empty list — meaning one match with no remaining strings.
The second change simply fixes the order of strings within each match, which was reversed (because it appended the prefix after the returned suffix match).
However, there are many others ways that code could be improved. Rather than list them all individually, it's probably easier to give an alternative version:
fun howConstruct(target: String, wordBank: List<String>
): List<List<String>>
= if (target == "") listOf(emptyList())
else wordBank.filter{ target.endsWith(it) } // Look for suffixes of the target in the word bank
.flatMap { suffix: String ->
howConstruct(target.removeSuffix(suffix), wordBank) // For each, recurse to search the rest
.map{ it + suffix } } // And append the suffix to each match.
That does almost exactly the same as your code, except that it searches from the end of the string — matching suffixes — instead of from the beginning. The result is the same; the main benefit is that it's simpler to append a suffix string to a partial match list (using +) than to prepend a prefix (which is quite messy, as you found).
However, it's a lot more concise, mainly because it uses a functional style — in particular, it uses filter() to determine which words are valid suffixes, and flatMap() to collate the list of matches corresponding to each one recursively, as well as map() to append the suffix to each one (like your code does). That avoids all the business of looping over lists, creating lists, and adding to them. As a result, it doesn't need to deal with mutable lists or variables, avoiding some sources of confusion and error.
I've written it as an expression body (with = instead of { … }) for simplicity. I find that's simpler and clearer for short functions — this one is about the limit, though. It might fit as it an extension function on String, since it's effectively returning a transformation of the string, without any side-effects — though again, that tends to work best on short functions.
There are also several small tweaks. It's a bit simpler — and more efficient — to use startsWith() or endsWith() instead of indexOf(); removePrefix() or removeSuffix() is arguably slightly clearer than substring(); and I find == "" clearer than isEmpty().
(Also, the name howConstruct() doesn't really describe the result very well, but I haven't come up with anything better so far…)
Many of these changes are of course a matter of personal preference, and I'm sure other developers would write it in many other ways! But I hope this has given some ideas.
I've built an Xpath expression by concatenating strings in VB6:
strXPath = "xDOC.selectNodes(" & """/GroupType1""" & ").item(" & CStr(i) & ").selectNodes(" & """/OperationStageCollection/OperationStage""" & ").length"
"i" is an integer used to index into
I want to evaluate strXPath to get a loop counter, for example:
n = CInt(strXPath)
n is declared as Integer; strXPath is declared as string. VB6 throws a Type Mismatch error on the above evaluation expression. I must be missing something obvious. How can I evaluate strXPath?
I realize that there may be errors in the XPath expression itself, but I'd like to get the evaluation working in order to debug such possible errors.
Try removing some of the double-quotes:
iLength = xDOC.selectNodes("/GroupType1").item(i).selectNodes("/OperationStageCollection/OperationStage").length
This should return the length property you want, as an Integer.
Then you can use iLength in your loop.
#BRW: both of your questions are very specific, i.e. how to achieve certain results using XPath. But I have the suspicion that if you would explain what (data) you try to retrieve form the XML, commenters might show you ways you didn't think of, e.g. say you want to iterate through all <OperationEvent>s within a <OperationEventCollection>, a single <OperationEvent> can be retrieved by //GroupType1/OperationStageCollection/OperationStage/OperationEventCollection/OperationEvent[1-based-index], e.g. //GroupType1/OperationStageCollection/OperationStage/OperationEventCollection/OperationEvent[1], which results in a single XML node:
<OperationEvent>
<OperationEventDate1>2018-12-16</OperationEventDate1>
<OperationEventCode>5</OperationEventCode>
<OperationEventDate2>2018-05-16</OperationEventDate2>
</OperationEvent>
So instead multiple selectNodes methods, one proper XPath query might yield the desired outcome right away.
Most style guides, such as Google's, for C++ recommend a maximum line length of 80 characters as well as relevant guidelines for function calls such that the function call is formatted correctly.
For example, here is what Google's style guide has to say on the matter:
Function calls have the following format:
bool retval = DoSomething(argument1, argument2, argument3);
If the arguments do not all fit on one line, they should be broken up
onto multiple lines, with each subsequent line aligned with the first
argument. Do not add spaces after the open paren or before the close
paren:
bool retval = DoSomething(averyveryveryverylongargument1,
argument2, argument3);
If the function has many arguments, consider having one per line if
this makes the code more readable:
bool retval = DoSomething(argument1,
argument2,
argument3,
argument4);
Arguments may optionally all be placed on subsequent lines, with one
line per argument:
if (...) {
...
...
if (...) {
DoSomething(
argument1, // 4 space indent
argument2,
argument3,
argument4);
}
In particular, this should be done if the function signature is so
long that it cannot fit within the maximum line length.
What isn't discussed is what is preferable for binary operators. This is a matter of style, so there's not a particularly correct answer, but I was hoping to get some opinions on what people prefer in the case of binary operators with long variable names or expressions.
For example, consider the simple case
result = reallyReallyReallyReallyLongVariableName * otherReallyReallyLongVariableName;
There are three formats that jump out at me:
// #1: The assignment operator is treated as a call for the sake of formatting.
result =
reallyReallyReallyReallyLongVariableName * otherReallyReallyLongVariableName;
// #2: The binary operator is positioned under the equal sign, my preference.
result = reallyReallyReallyReallyLongVariableName
* otherReallyReallyLongVariableName;
// #3: There is basic indentation of 4 spaces.
result = reallyReallyReallyReallyLongVariableName
* otherReallyReallyLongVariableName;
The first two are a bit more intuitive in my opinion, and Zend Framework Style Guide recommends the format of #2 for repeated concatenation of strings in PHP.
As for the third, Pear's PHP Style Guide recommends starting new lines with the -> operator in the case of repeated function calls, which is somewhat loosely analogous to a binary operator.
I personally prefer the second, but I was just wondering what others' opinions are.
results.Where(x=>x.Members.Any(y=>members.Contains(y.Name.ToLower())
I happened to see this query in internet. Can anyone explain this query please.
suggest me a good LINQ tutorial for this newbie.
thank you all.
Edited:
what is this x and y stands for?
x is a single result, of the type of the elements in the results sequence.
y is a single member, of the type of the elements in the x.Members sequence.
These are lambda expressions (x => x.whatever) that were introduced into the language with C# 3, where x is the input, and the right side (x.whatever) is the output (in this particular usage scenario).
An easier example
var list = new List<int> { 1, 2, 3 };
var oddNumbers = list.Where(i => i % 2 != 0);
Here, i is a single int item that is an input into the expression. i % 2 != 0 is a boolean expression evaluating whether the input is even or odd. The entire expression (i => i % 2 != 0) is a predicate, a Func<int, bool>, where the input is an integer and the output is a boolean. Follow? As you iterate over the query oddNumbers, each element in the list sequence is evaluated against the predicate. Those that pass then become part of your output.
foreach (var item in oddNumbers)
Console.WriteLine(item);
// writes 1, 3
Its a lambda expression. Here is a great LINQ tutorial
Interesting query, but I don't like it.
I'll answer your second question first. x and y are parameters to the lambda methods that are defined in the calls to Where() and Any(). You could easy change the names to be more meaningful:
results.Where(result =>
result.Members.Any(member => members.Contains(member.Name.ToLower());
And to answer your first question, this query will return each item in results where the Members collection has at least one item that is also contained in the Members collection as a lower case string.
The logic there doesn't make a whole lot of sense to me with knowing what the Members collection is or what it holds.
x will be every instance of the results collection. The query uses lambda syntax, so x=>x.somemember means "invoke somemember on each x passed in. Where is an extension method for IEnumerables that expects a function that will take an argument and return a boolean. Lambda syntax creates delegates under the covers, but is far more expressive for carrying out certain types of operation (and saves a lot of typing).
Without knowing the type of objects held in the results collection (results will be something that implements IEnumerable), it is hard to know exactly what the code above will do. But an educated guess is that it will check all the members of all the x's in the above collection, and return you an IEnumerable of only those that have members with all lower-case names.
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.