I have two conditions, a and b. One block of code should execute if a is true, then another if either a or b is true. Is there a better way to structure the code than the following - can I avoid the repeated test of a?
if a or b:
if a:
<block 1>
<block 2>
It is not possible without the repeated test. To verify this, just write down the truth table.
However you could avoid the nesting of your if statements:
if a:
<block 1>
if a or b:
<block 2>
This could make the code more readable.
I assume here that a is a boolean value and not a placeholder for a more complex expression that might be expensive to evaluate. In the latter case you would of course evaluate the expression only once and assign the result to a boolean value.
Related
I am trying to write a compiler for a domain-specific language, targeting a stack-machine based VM that is NOT a JVM.
I have already generated a parser for my language, and can readily produce an AST which I can easily walk. I also have had no problem converting many of the statements of my language into the appropriate instructions for this VM, but am facing an obstacle when it comes to the matter of handling the generation of appropriate branching instructions when complex conditionals are encountered, especially when they are combined with (possibly nested) 'and'-like or 'or' like operations which should use short-circuiting branching as applicable.
I am not asking anyone to write this for me. I know that I have not begun to describe my problem in sufficient detail for that. What I am asking for is pointers to useful material that can get me past this hurdle I am facing. As I said, I am already past the point of converting about 90% of the statements in my language into applicable instructions, but it is the handling of conditionals and generating the appropriate flow control instructions that has got me stumped. Much of the info that I have been able to find so far on generating code from an AST only seems to deal with the generation of code corresponding to simple imperative-like statements, but the handing of conditionals and flow control appears to be much more scarce.
Other than the short-circuiting/lazy-evaluation mechanism for 'and' and 'or' like constructs that I have described, I am not concerned with handling any other optimizations.
Every conditional control flow can be modelled as a flow chart (or flow graph) in which the two branches of the conditional have different targets. Given that boolean operators short-circuit, they are control flow elements rather than simple expressions, and they need to be modelled as such.
One way to think about this is to rephrase boolean operators as instances of the ternary conditional operator. So, for example, A and B becomes A ? B : false and A or B becomes A ? true : B [Note 1]. Note that every control flow diagram has precisely two output points.
To combine boolean expressions, just substitute into the diagram. For example, here's A AND (B OR C)
You implement NOT by simply swapping the meaning of the two out-flows.
If the eventual use of the boolean expression is some kind of conditional, such as an if statement or a conditional loop, you can use the control flow as is. If the boolean expression is to be saved into a variable or otherwise used as a value, you need to fill in the two outflows with code to create the relevant constant, usually a true or false boolean constant, or (in C-like languages) a 1 or 0.
Notes:
Another way to write this equivalence is A and B ⇒ A ? B : A; A or B ⇒ A ? A : B, but that is less useful for a control flow view, and also clouds the fact that the intent is to only evaluate each expression once. This form (modified to reuse the initial computation of A) is commonly used in languages with multiple "falsey" values (like Python).
Taken from Introduction to Ada—If expressions:
Ada's if expressions are similar to if statements. However, there are a few differences that stem from the fact that it is an expression:
All branches' expressions must be of the same type
It must be surrounded by parentheses if the surrounding expression does not already contain them
An else branch is mandatory unless the expression following then has a Boolean value. In that case an else branch is optional and, if not present, defaults to else True.
I do not understand the need to have two different ways of constructing code with the if keyword. What is the reasoning behind this?
Also there case expressions and case statements. Why is this?
I think this is best answered by quoting the Ada 2012 Rationale Chapter 3.1:
One of the key areas identified by the WG9 guidance document [1] as
needing attention was improving the ability to write and enforce
contracts. These were discussed in detail in the previous chapter.
When defining the new aspects for preconditions, postconditions, type
invariants and subtype predicates it became clear that without more
flexible forms of expressions, many functions would need to be
introduced because in all cases the aspect was given by an expression.
However, declaring a function and thus giving the detail of the
condition, invariant or predicate in the function body makes the
detail of the contract rather remote for the human reader. Information
hiding is usually a good thing but in this case, it just introduces
obscurity. Four forms are introduced, namely, if expressions, case
expressions, quantified expressions and expression functions. Together
they give Ada some of the flexible feel of a functional language.
In addition, if statements and case statements often assigns different values to the same variable in all branches, and nothing else:
if Foo > 10 then
Bar := 1;
else
Bar := 2;
end if;
In this case, an if expression may increase readability and more clearly state in the code what's going on:
Bar := (if Foo > 10 then 1 else 2);
We can now see that there's no longer a need for the maintainer of the code to read a whole if statement in order to see that only a single variable is updated.
Same goes for case expressions, which can also reduce the need for nesting if expressions.
Also, I can throw the question back to you: Why does C-based languages have the ternary operator ?: in addition to if statements?
Egilhh already covered the main reason, but there are sometimes other useful reasons to implement expressions. Sometimes you make packages where only one or two methods are needed and they are the only reason to make a package body. You can use expressions to make expression functions which allow you to define the operations in the spec file.
Additionally, if you ever end up with some complex variant record combinations, sometimes expressions can be used to setup default values for them in instances where you normally would not be able to as cleanly. Consider the following example:
with Ada.Text_IO; use Ada.Text_IO;
procedure Hello is
type Binary_Type is (On, Off);
type Inner(Binary : Binary_Type := Off) is record
case Binary is
when On =>
Value : Integer := 0;
when Off =>
null;
end case;
end record;
type Outer(Some_Flag : Boolean) is record
Other : Integer := 32;
Thing : Inner := (if Some_Flag then
(Binary => Off)
else
(Binary => On, Value => 23));
end record;
begin
Put_Line("Hello, world!");
end Hello;
I had something come up with a more complex setup that was meant to map to a complex messaging interface at the hardware level. It's nice to have defaults whenever possible. Now I cold have used a case inside of Outer, but then I would have had to come up with two separately named versions of the message field for each case, which really isn't optimal when you want your code to map to an ICD. Again, I could have used a function to initialize it as well, but as noted in the other posters answer, that isn't always a good way to go.
Another place that outlines the motivation for adding conditional expressions to Ada can be found in the ARG document, AI05-0147-1, which explains the motivation and gives some examples of use.
An example of a place where I find them quite useful is in processing command line parameters, for the case when a default value is used if the parameter is not specified on the command line. Generally, you'd want to declare such values as constants in one's program. Conditional expressions makes it easier to do that.
with Ada.Command_Line; use Ada;
procedure Main
is
N : constant Positive :=
(if Command_Line.Argument_Count = 0 then 2_000_000
else Positive'Value (Command_Line.Argument (1)));
...
Otherwise, without conditional expressions, in order to achieve the same effect you'd need to declare a function, which I find to be more difficult to read;
with Ada.Command_Line; use Ada;
procedure Main
is
function Get_N return Positive is
begin
if Command_Line.Argument_Count = 0 then
return 2_000_000;
else
return Positive'Value (Command_Line.Argument (1));
end if;
end Get_N;
N : constant Positive := Get_N;
...
The if expression in Ada feels and works a lot like a statement using the ternary operator in the C-based languages. I took the liberty of copying some code from learn.adacore.com that introduces the if expression:
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
procedure Check_Positive is
N : Integer;
begin
Put ("Enter an integer value: ");
Get (N);
Put (N,0);
declare
S : constant String :=
(if N > 0 then " is a positive number"
else " is not a positive number");
begin
Put_Line (S);
end;
end Check_Positive;
And I translated it to a C-based language - in this case, Java. I believe the main point to notice is that both languages, although syntactically different, are effectively doing the same thing: testing a condition and assigning one of two values to a variable all within one statement. Although I realize this is an oversimplification for most here on stackoverlfow. My goal is to help the beginner to understand the basic concept with introductory examples. Cheers.
import java.util.Scanner;
public class IfExpression {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
System.out.print("Enter an integer value: ");
var N = in.nextInt();
System.out.print(N);
var S = N > 0 ? " is a positive number" : " is not a positive number";
System.out.println(S);
in.close();
}
}
I'm trying to replace some old unit tests with property based testing (PBT), concreteley with scala and scalatest - scalacheck but I think the problem is more general. The simplified situation is , if I have a method I want to test:
def upcaseReverse(s:String) = s.toUpperCase.reverse
Normally, I would have written unit tests like:
assertEquals("GNIRTS", upcaseReverse("string"))
assertEquals("", upcaseReverse(""))
// ... corner cases I could think of
So, for each test, I write the output I expect, no problem. Now, with PBT, it'd be like :
property("strings are reversed and upper-cased") {
forAll { (s: String) =>
assert ( upcaseReverse(s) == ???) //this is the problem right here!
}
}
As I try to write a test that will be true for all String inputs, I find my self having to write the logic of the method again in the tests. In this case the test would look like :
assert ( upcaseReverse(s) == s.toUpperCase.reverse)
That is, I had to write the implementation in the test to make sure the output is correct.
Is there a way out of this? Am I misunderstanding PBT, and should I be testing other properties instead, like :
"strings should have the same length as the original"
"strings should contain all the characters of the original"
"strings should not contain lower case characters"
...
That is also plausible but sounds like much contrived and less clear. Can anybody with more experience in PBT shed some light here?
EDIT : following #Eric's sources I got to this post, and there's exactly an example of what I mean (at Applying the categories one more time): to test the method times in (F#):
type Dollar(amount:int) =
member val Amount = amount
member this.Add add =
Dollar (amount + add)
member this.Times multiplier =
Dollar (amount * multiplier)
static member Create amount =
Dollar amount
the author ends up writing a test that goes like:
let ``create then times should be same as times then create`` start multiplier =
let d0 = Dollar.Create start
let d1 = d0.Times(multiplier)
let d2 = Dollar.Create (start * multiplier) // This ones duplicates the code of Times!
d1 = d2
So, in order to test that a method, the code of the method is duplicated in the test. In this case something as trivial as multiplying, but I think it extrapolates to more complex cases.
This presentation gives some clues about the kind of properties you can write for your code without duplicating it.
In general it is useful to think about what happens when you compose the method you want to test with other methods on that class:
size
++
reverse
toUpperCase
contains
For example:
upcaseReverse(y) ++ upcaseReverse(x) == upcaseReverse(x ++ y)
Then think about what would break if the implementation was broken. Would the property fail if:
size was not preserved?
not all characters were uppercased?
the string was not properly reversed?
1. is actually implied by 3. and I think that the property above would break for 3. However it would not break for 2 (if there was no uppercasing at all for example). Can we enhance it? What about:
upcaseReverse(y) ++ x.reverse.toUpper == upcaseReverse(x ++ y)
I think this one is ok but don't believe me and run the tests!
Anyway I hope you get the idea:
compose with other methods
see if there are equalities which seem to hold (things like "round-tripping" or "idempotency" or "model-checking" in the presentation)
check if your property will break when the code is wrong
Note that 1. and 2. are implemented by a library named QuickSpec and 3. is "mutation testing".
Addendum
About your Edit: the Times operation is just a wrapper around * so there's not much to test. However in a more complex case you might want to check that the operation:
has a unit element
is associative
is commutative
is distributive with the addition
If any of these properties fails, this would be a big surprise. If you encode those properties as generic properties for any binary relation T x T -> T you should be able to reuse them very easily in all sorts of contexts (see the Scalaz Monoid "laws").
Coming back to your upperCaseReverse example I would actually write 2 separate properties:
"upperCaseReverse must uppercase the string" >> forAll { s: String =>
upperCaseReverse(s).forall(_.isUpper)
}
"upperCaseReverse reverses the string regardless of case" >> forAll { s: String =>
upperCaseReverse(s).toLowerCase === s.reverse.toLowerCase
}
This doesn't duplicate the code and states 2 different things which can break if your code is wrong.
In conclusion, I had the same question as you before and felt pretty frustrated about it but after a while I found more and more cases where I was not duplicating my code in properties, especially when I starting thinking about
combining the tested function with other functions (.isUpper in the first property)
comparing the tested function with a simpler "model" of computation ("reverse regardless of case" in the second property)
I have called this problem "convergent testing" but I can't figure out why or where there term comes from so take it with a grain of salt.
For any test you run the risk of the complexity of the test code approaching the complexity of the code under test.
In your case, the the code winds up being basically the same which is just writing the same code twice. Sometimes there is value in that. For example, if you are writing code to keep someone in intensive care alive, you could write it twice to be safe. I wouldn't fault you for the abundance of caution.
For other cases there comes a point where the likelihood of the test breaking invalidates the benefit of the test catching real issues. For that reason, even if it is against best practice in other ways (enumerating things that should be calculated, not writing DRY code) I try to write test code that is in some way simpler than the production code, so it is less likely to fail.
If I cannot find a way to write code simpler than the test code, that is also maintainable(read: "that I also like"), I move that test to a "higher" level(for example unit test -> functional test)
I just started playing with property based testing but from what I can tell it is hard to make it work with many unit tests. For complex units, it can work, but I find it more helpful at functional testing so far.
For functional testing you can often write the rule a function has to satisfy much more simply than you can write a function that satisfies the rule. This feels to me a lot like the P vs NP problem. Where you can write a program to VALIDATE a solution in linear time, but all known programs to FIND a solution take much longer. That seems like a wonderful case for property testing.
I'm using ScalaTest for my unit testing. I have a test result (JSON) that might look like below. The actual result is huge and complex. This is an example.
[{"name":"George", "when":143828333, ...}, {"name":"Fred", "when":14857777, ... }]
The 'when' field values are dynamic and will change from test-to-test (i.e. current timestamp), so I can't test against these. I could use some regex to mask these out, basically replacing them with some inert token.
Does ScalaTest have an more elegant way of handling dynamic bits of data like this?
You can make a custom Equality for the types you're comparing. Your custom Equality can ignore the dynamic fields for the equality comparison. Info on Equality is here:
http://doc.scalatest.org/2.2.0/index.html#org.scalactic.Equality
All you need to do is define the areEqual method and then make it implicit. So Equality[JsonType] or Equality[String], whatever the type is. This will then be picked up by the === operator and the equal matcher in your assertions.
What I've done many times in the past in this situation is exactly what you propose: use a regex to replace the dates with a constant so your comparisons will work.
Is there any way not to leave <choose> after first <when> match but continue check else conditions?
I believe it's a no. As the spec says:
The content of the first, and only the first, xsl:when element whose
test is true is instantiated. If no xsl:when is true, the content of
the xsl:otherwise element is instantiated. If no xsl:when element is
true, and no xsl:otherwise element is present, nothing is created.
from: http://www.w3.org/TR/xslt#section-Conditional-Processing-with-xsl:choose
you can't make it fall through other conditions like that. just convert it into a set of <xsl:if> following one another if you need a fall through
UPDATE. Here's a quote from the O'Reilly's XSLT book ( http://docstore.mik.ua/orelly/xml/xslt/ch04_02.htm ):
The C, C++, and Java switch statement is roughly equivalent to the
element. The one exception is that procedural languages
tend to use fallthrough processing. In other words, if a branch of the
switch statement evaluates to true, the runtime executes everything
until it encounters a break statement, even if some of that code is
part of other branches. The element doesn't work that
way. If a given evaluates to true, only the statements
inside that are evaluated