Function inputs and outputs in PROLOG - prolog

I'm building some relatively simple functions in PROLOG that take one input and one output. For simplicity, something like
func(List, Item, [Item | List]).
Now, I've got code that will call several of these functions in a row and pass the result on. The issue is that I have to keep creating new variable names for all of the outputs.
someOtherFunc(List, Item) :-
func(List, Item, Output1),
doSomething(Output1).
The issue here is that I actually have several func and several doSomething and would really appreciate not having to bind an Output1 variable explicitly. Is there any way to achieve this?

I'm not sure about what you're asking, but first of all please note that those are not functions, but predicates. This is a totally different programming paradigm. Variables are not "boxes" where you put in and out some data: they're closer to the mathematical meaning of variable, since once you bind them to some constraints on their values it's forever.
To go back to your question, the answer is no, you can't avoid binding some Output1 like that. Sometimes you can put in an underscore to tell prolog you just don't care about that value, but doing so you lose the ability to make use of that particular value. In your example you would like to do something like this (in a imperative pseudocode):
var list = ..., item = ...;
doSomething(func(list, item));
There's no other way in prolog as far as I know, you just have to use intermediate variables as you did. The only improvement I can suggest, is to choose very carefully predicates and variables names.

func1(Input1, Input2) :-
func2(Input1, Input2, Output1),
useFun(Output1, Output2).
/* Output2 the result I obtain from the function useFun */

Related

Extract struct contents in Prolog

Say I have a struct such as this:
flight( bos, nyc, time( 7,30 ) )
in the form of a variable: My_Flight.
If this variable was passed into a function like this:
my_func(Flight) :-
write(Flight[0])
How would I be able to extract the contents of the struct and do something with each part like the second line of the above function?
Sorry if my terminology is off, I'm pretty new to this language!
First, I recommend you think declaratively:
What must be the properties of such a struct in order for my_func/1 to hold?
Answer: For my_func/1 to hold, first, Flight must be of the form
flight(From, To, Time)
Using (=)/2 or, even better, direct pattern matching, we can already formulate this as a constraint on the argument:
my_func(flight(From,To,Time)) :-
...
You then state what else must hold about the arguments for my_func/1 to hold.
Do not think "how do I extract...", because the argument may not even be instantiated. Instead, ask: "What must hold about the argument?" This paves the way for many more usage modes: To extract, to construct, to check etc.

Refactoring Business Rule, Function Naming, Width, Height, Position X & Y

I am refactoring some business rule functions to provide a more generic version of the function.
The functions I am refactoring are:
DetermineWindowWidth
DetermineWindowHeight
DetermineWindowPositionX
DetermineWindowPositionY
All of them do string parsing, as it is a string parsing business rules engine.
My question is what would be a good name for the newly refactored function?
Obviously I want to shy away from a function name like:
DetermineWindowWidthHeightPositionXPositionY
I mean that would work, but it seems unnecessarily long when it could be something like:
DetermineWindowMoniker or something to that effect.
Function objective: Parse an input string like 1280x1024 or 200,100 and return either the first or second number. The use case is for data-driving test automation of a web browser window, but this should be irrelevant to the answer.
Question objective: I have the code to do this, so my question is not about code, but just the function name. Any ideas?
There are too little details, you should have specified at least the parameters and returns of the functions.
Have I understood correctly that you use strings of the format NxN for sizes and N,N for positions?
And that this generic function will have to parse both (and nothing else), and will return either the first or second part depending on a parameter of the function?
And that you'll then keep the various DetermineWindow* functions but make them all call this generic function?
If so:
Without knowing what parameters the generic function has it's even harder to help, but it's most likely impossible to give it a simple name.
Not all batches of code can be described by a simple name.
You'll most likely need to use a different construction if you want to have clear names. Here's an idea, in pseudo code:
ParseSize(string, outWidth, outHeight) {
ParsePair(string, "x", outWidht, outHeight)
}
ParsePosition(string, outX, outY) {
ParsePair(string, ",", outX, outY)
}
ParsePair(string, separator, outFirstItem, outSecondItem) {
...
}
And the various DetermineWindow would call ParseSize or ParsePosition.
You could also use just ParsePair, directly, but I thinks it's cleaner to have the two other functions in the middle.
Objects
Note that you'd probably get cleaner code by using objects rather than strings (a Size and a Position one, and probably a Pair one too).
The ParsePair code (adapted appropriately) would be included in a constructor or factory method that gives you a Pair out of a string.
---
Of course you can give other names to the various functions, objects and parameters, here I used the first that came to my mind.
It seems this question-answer provides a good starting point to answer this question:
Appropriate name for container of position, size, angle
A search on www.thesaurus.com for "Property" gives some interesting possible answers that provide enough meaningful context to the usage:
Aspect
Character
Characteristic
Trait
Virtue
Property
Quality
Attribute
Differentia
Frame
Constituent
I think ConstituentProperty is probably the most apt.

Mathematica - can I define a block of code using a single variable?

It has been a while since I've used Mathematica, and I looked all throughout the help menu. I think one problem I'm having is that I do not know what exactly to look up. I have a block of code, with things like appending lists and doing basic math, that I want to define as a single variable.
My goal is to loop through a sequence and when needed I wanted to call a block of code that I will be using several times throughout the loop. I am guessing I should just put it all in a loop anyway, but I would like to be able to define it all as one function.
It seems like this should be an easy and straightforward procedure. Am I missing something simple?
This is the basic format for a function definition in Mathematica.
myFunc[par1_,par2_]:=Module[{localVar1,localVar2},
statement1; statement2; returnStatement ]
Your question is not entirely clear, but I interpret that you want something like this:
facRand[] :=
({b, x} = Last#FactorInteger[RandomInteger[1*^12]]; Print[b])
Now every time facRand[] is called a new random integer is factored, global variables b and x are assigned, and the value of b is printed. This could also be done with Function:
Clear[facRand]
facRand =
({b, x} = Last#FactorInteger[RandomInteger[1*^12]]; Print[b]) &
This is also called with facRand[]. This form is standard, and allows addressing or passing the symbol facRand without triggering evaluation.

Correct use of findall/3, especially the first template argument

i know there is a build-in function findall/3 in prolog,
and im trying to find the total numbers of hours(Thrs) and store them in a list, then sum the list up. but it doesnt work for me. here is my code:
totalLecHrs(LN,THrs) :-
lecturer(LN,LId),
findall(Thrs, lectureSegmentHrs(CC,LId,B,E,THrs),L),
sumList(L,Thrs).
could you tell me what's wrong with it? thanks a lot.
You need to use a "dummy" variable for Hours in the findall/3 subgoal. What you wrote uses THrs both as the return value for sumList/2 and as the variable to be listed in L by findall/3. Use X as the first argument of findall and in the corresponding subgoal lectureSegmentHrs/5 as the last argument.
It looks like the problem is that you're using the same variable (Thrs) twice for different things. However it's hard to tell as you've also used different capitalisation in different places. Change the findall line so that the initial variable has the same capitalisation in the lectureSegmentHrs call. Then use a different variable completely to get the final output value (ie the one that appears in sumList and in the return slot of the entire predicate).
You need to use a different variable because Prolog does not support variable reassignment. In a logical language, the notion of reassigning a variable is inherently impossible. Something like the following may seem sensible...
...
X = 10,
X = 11,
...
But you have to remember that , in Prolog is the conjunction operator. You're effectively telling Prolog to find a solution to your problem where X is both 10 and 11 at the same time. So it's obviously going to tell you that that can't be done.
Instead you have to just make up new variable names as you go along. Sometimes this does get a bit annoying but it's just goes with the territory of a logical languages.

Are there alternative ways to display a list other than by using loop?

I know on how to display a list by using loop.
For example,
choice(a):-write('This is the top 15 countries list:'),nl,
loop(X).
loop(X):-country(X),write(X),nl,fail.
Unfortunately, I don't know on how to display list by using list. Anyone can guide me?
it's not very clear what it is that you're trying to achieve.
I'm not sure from your description whether you have quite got to grips with the declarative style of Prolog. When you wrote your rule for loop you were providing a set of conditions under which Prolog would match the rule. This is different from a set of procedural instructions.
If you want to collect all the countries into a list you can use the setof rule like follows
top_countries(Cs):-
setof(C, country(C), Cs).
This will return a list [] of the countries matched by the rule.
If you wanted to output each element of this list on a new line you could do something like the following recursive function.
write_list([]).
write_list([H|T]):-
write(H),nl,
write_list(T).
The first rule matches the base case; this is when there are no elements left in the list. At this point we should match and stop. The second rule matches (unifies) the head of the list and writes it to screen with a newline after it. The final line unifies the tail (remainder) of the list against the write_list function again.
You could then string them together with something like the following
choice(a):-
write('This is the top 15 countries list:'),nl,
top_countries(X),
write_list(X).
Things to note
Try not to have singleton variables such as the X in your choice rule. Variables are there to unify (match) against something.
Look into good declarative programming style. When you use functions like write it can be misleading and tempting to treat Prolog in a procedural manner but this will just cause you problems.
Hope this helps
write/1 doesn't only write strings, it writes any Prolog term. So, though Oli has given a prettier write_list, the following would do the job:
choice(Countries):-write('This is the top 15 countries list:'),nl,write(Countries).

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