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Suppose we have a relation R(A,B,C,D,E) with candidate keys {A,B} , {C,D} and let E be a non prime attribute.
If there is a dependency C->E then it is partial dependency as E depends on a part of candidate key {C,D}
Is dependency BC->E partial?
A functional dependency is partial if, on removing one or more attributes from the determinant (left-hand side), you still have a dependency that holds. In your example therefore:
{C}→{E} is a full functional dependency
{C,D}→{E} is a partial functional dependency because {C}→{E}
{B,C}→{E} is a partial functional dependency because {C}→{E}
It is idiomatic in lisps such as Clojure to use simple data-structures and lots of functions. Still, there are many times when we must work with complex data-structures composed of many simpler ones.
My question is about a matter of good style/practice. In general, should we create functions that take the entire complex object, and within that extract what we need, or should they take exactly and only what they need?
For concreteness, I compare these two options in the following psuedo code
(defrecord Thing
[a b])
(defn one-option
[a]
.. a .. ))
(one-option (:a a-thing))
;; ==============
(defn another-option
[a-thing]
.. (:a a-thing) .. ))
The pros of one-option-f is that the function is simpler, and has fewer responsibilities. It is also more compositional - it can be used in more places.
The downside is that we may end up repeating ourselves. If, for example, we have many functions which transform one Thing into another Thing, many of which need use one-option, within each one we will find ourselves repeating the extraction code. Of course, another option is to create both, but this also adds a certain code overhead.
I think the answer is "It depends" and "It doesn't matter as much in Clojure as it does in object systems". Clojure has a number of largely orthogonal mechanisms designed to express modes of composition. The granularity of function arguments will tend to fall out of how the structure of the program is conceived of in the large.
So much for the hot air. There are some specifics that can affect argument granularity:
Since Clojure data structures are immutable, data hiding and access
functions/methods have little relevance. So the repetition caused by
accessing parts of a complex structure is trifling.
What appear in object-design as associations are rendered by small
collections of typed object pointers in each object. In Clojure,
these tend to become single(ton) global maps. There are a standard
Clojure functions for accessing and manipulating hierarchical
structures of this kind (get-in and assoc-in, for examples).
Where we are looking for dynamically-bound compliance with an
interface, Clojure protocols and datatypes are cleaner and more
adaptable than most object systems. In this case, the whole object is
passed.
Do both. To begin with, provide functions that transform the simplest structures, then add convenience functions for handling more complex structures which compose the functions that handle the simple ones.
I am learning C from 'Programming in C' by Stephen Kochan.
Though the author is careful from the beginning only not to confuse the students with jargon, but occasionally he has used few terms without explaining their meaning. I have figured out the meaning of many such terms with the help of internet.
However, I could not understand the exactly meaning of the phrase 'language construct', and unfortunately the web doesn't provide a good explanation.
Considering I am a beginner, what does 'language construct' mean?
First, you need to understand what a constructed language Formal Language is. All programming languages are constructed formal languages (read the reference). You may then read a little bit about compiler construction, including this reference as well.
Going back to your question, consider this: The English language (a natural language) has tokens 'A-Z/0-9/,;"...' which we use to build "words" and we use languages rules to build sentences out of words. So, in the English language, a construct is what we build out of tokens.
Consider this brick-and-mortar example: Imagine if you set out to build a house, the basic materials you might use are: sand, iron, wood, cement, water (just five for simplicity). Anything you build out of these 4 or 5+ items would be a "construct", which in turn helps you build your house.
I have intentionally omitted details to further simplify the answer; hope this is helpful.
A language construct is a piece of language syntax. For example, the following is a language construct in C that lets you control the flow of a program:
if ( condition ) {
/* when condition is true */
} else {
/* when condition is false */
}
They usually use the term language construct because these are parts of most programming languages, but may be written differently, depending on the language. For example, a similar language construct in bourne shell would be:
if COMMAND; then
# when command returns 0
else
# when command returns anything else
fi
The function of this construct is the same, however, the way it's written is a bit different.
Hope this helps. If you need more detail, you may want to do a bit more research. As one of the comments suggests, Wikipedia may be helpful.
They are the base units from which the language is built up. They can't be used as a function rollback. They are directly called by the parser.
It includes all the syntax, semantics and coding styles of a language.
For more clarification you may refer to this question.
Wikipedia definition:
A language construct is a syntactically allowable part of a program that may be formed from one or more lexical tokens in accordance with the rules of a programming language.
The term Language Constructs is often used as a synonym for control structure, and should not be confused with a function.
Without seeing the context that the phrase is used in, I cannot be sure, but generally the phrase 'language construct' just means the combination of keywords, grammar and structure of a coding language. Basically, how to format/write/construct a piece of code.
Let say you want to create a class containing methods and properties, so:
Construct is an architecture of a class you are about to create. The architecture of the class consists of methods and properties created by you by using predefined utilities (such as: 'if', 'else', 'switch', 'break', etc)
That's my take on construct.
In reference to a programming language
Language Constructs mean the basic constructs of a programming languge e.g
1. Conditions (if, else, switch)
2. Loops (For, While, Do-while) etc
C is a structural language so while compiling your code everything thing goes statement by statement. Thus it becomes necessary to place your statement properly. This placing i.e. putting your statement correctly is your language construct else there may be syntax error or logical error.
Language constructs according to the GCSE book are basic building block of a programming language. that are
1. Sequential,
2. Selection, if, if/else
3. Iteration, while, for
Language construct is a piece of language syntax.
Example:
A declaration of a variable is a language construct:
{
int a; // declaration of a variable "a"
}
A language construct is a piece of syntax that the compiler has intimate knowledge about, usually because it needs to handle it specially. Typical examples of language constructs are the short-circuiting operators found in many imperative languages. Because these operators require lazy evaluation in an otherwise eager language, they must be handled specially by the compiler.
So, a stricter definition of a language construct may be: a syntactical form that is handled specially by the compiler, having functionality that cannot be implemented by a user.
I'm starting to learn Lisp with a Java background. In SICP's exercise there are many tasks where students should create abstract functions with many parameters, like
(define (filtered-accumulate combiner null-value term a next b filter)...)
in exercise 1.33. In Java (language with safe, static typing discipline) - a method with more than 4 arguments usually smells, but in Lisp/Scheme it doesn't, does it? I'm wondering how many arguments do you use in your functions? If you use it in production, do you make as many layers?
SICP uses a subset of Scheme
SICP is a book used in introductory computer science course. While it explains some advanced concepts, it uses a very tiny language, a subset of the Scheme language and a sub-subset of any real world Scheme or Lisp a typical implementation provides. Students using SICP are supposed to start with a simple and easy to learn language. From there they learn to implement more complex language additions.
Only positional parameters are being used in plain educational Scheme
There are for example no macros developed in SICP. Add that standard Scheme does have only positional parameters for functions.
Lisp and Scheme offer also more expressive argument lists
In 'real' Lisp or Scheme one can use one or more of the following:
objects or records/structures (poor man's closures) which group things. An object passed can contain several data items, which otherwise would need to be passed 'spread'.
defaults for optional variables. Thus we need only to pass those that we want to have a certain non-default value
optional and named arguments. This allows flexible argument lists which are much more descriptive.
computed arguments. The value or the default value of arguments can be computed based on other arguments
Above leads to more complicated to write function interfaces, but which are often easier to use.
In Lisp it is good style to have descriptive names for arguments and also provide online documentation for the interface. The development environment will display information about the interface of a function, so this information is typically only a keystroke away or is even display automatically.
It's also good style for any non-trivial interface which is supposed to be used interactively by the user/developer to check its arguments at runtime.
Example for a complex, but readable argument list
When there are more arguments, then Common Lisp provides named arguments, which can appear in any order after the normal argument. Named arguments provide also defaults and can be omitted:
(defun order-product (product
&key
buyer
seller
(vat (local-vat seller))
(price (best-price product))
amount
free-delivery-p)
"The function ORDER-PRODUCT ..." ; documentation string
(declare (type ratio vat price) ; type declarations
(type (integer 0) amount)
(type boolean free-delivery-p))
...)
We would use it then:
(order-product 'sicp
:seller 'mit-press
:buyer 'stan-kurilin
:amount 1)
Above uses the seller argument before the buyerargument. It also omits various arguments, some of which have their values computed.
Now we can ask whether such extensive arguments are good or bad. The arguments for them:
the function call gets more descriptive
functions have standard mechanisms to attach documentation
functions can be asked for their parameter lists
type declarations are possible -> thus types don't need to be written as comments
many parameters can have sensible default values and don't need to be mentioned
Several Scheme implementations have adopted similar argument lists.
Like Smalltalk or Lisp?
EDIT
Where control structures are like:
Java Python
if( condition ) { if cond:
doSomething doSomething
}
Or
Java Python
while( true ) { while True:
print("Hello"); print "Hello"
}
And operators
Java, Python
1 + 2 // + operator
2 * 5 // * op
In Smalltalk ( if I'm correct ) that would be:
condition ifTrue:[
doSomething
]
True whileTrue:[
"Hello" print
]
1 + 2 // + is a method of 1 and the parameter is 2 like 1.add(2)
2 * 5 // same thing
how come you've never heard of lisp before?
You mean without special syntax for achieving the same?
Lots of languages have control structures and operators that are "really" some form of message passing or functional call system that can be redefined. Most "pure" object languages and pure functional languages fit the bill. But they are all still going to have your "+" and some form of code block--including SmallTalk!--so your question is a little misleading.
Assembly
Befunge
Prolog*
*I cannot be held accountable for any frustration and/or headaches caused by trying to get your head around this technology, nor am I liable for any damages caused by you due to aforementioned conditions including, but not limited to, broken keyboard, punched-in screen and/or head-shaped dents in your desk.
Pure lambda calculus? Here's the grammar for the entire language:
e ::= x | e1 e2 | \x . e
All you have are variables, function application, and function creation. It's equivalent in power to a Turing machine. There are well-known codings (typically "Church encodings") for such constructs as
If-then-else
while-do
recursion
and such datatypes as
Booleans
integers
records
lists, trees, and other recursive types
Coding in lambda calculus can be a lot of fun—our students will do it in the undergraduate languages course next spring.
Forth may qualify, depending on exactly what you mean by "no control structures or operators". Forth may appear to have them, but really they are all just symbols, and the "control structures" and "operators" can be defined (or redefined) by the programmer.
What about Logo or more specifically, Turtle Graphics? I'm sure we all remember that, PEN UP, PEN DOWN, FORWARD 10, etc.
The SMITH programming language:
http://esolangs.org/wiki/SMITH
http://catseye.tc/projects/smith/
It has no jumps and is Turing complete. I've also made a Haskell interpreter for this bad boy a few years back.
I'll be first to mention brain**** then.
In Tcl, there's no control structures; there's just commands and they can all be redefined. Every last one. There's also no operators. Well, except for in expressions, but that's really just an imported foreign syntax that isn't part of the language itself. (We can also import full C or Fortran or just about anything else.)
How about FRACTRAN?
FRACTRAN is a Turing-complete esoteric programming language invented by the mathematician John Conway. A FRACTRAN program is an ordered list of positive fractions together with an initial positive integer input n. The program is run by updating the integer (n) as follows:
for the first fraction f in the list for which nf is an integer, replace n by nf
repeat this rule until no fraction in the list produces an integer when multiplied by n, then halt.
Of course there is an implicit control structure in rule 2.
D (used in DTrace)?
APT - (Automatic Programmed Tool) used extensively for programming NC machine tools.
The language also has no IO capabilities.
XSLT (or XSL, some say) has control structures like if and for, but you should generally avoid them and deal with everything by writing rules with the correct level of specificity. So the control structures are there, but are implied by the default thing the translation engine does: apply potentially-recursive rules.
For and if (and some others) do exist, but in many many situations you can and should work around them.
How about Whenever?
Programs consist of "to-do list" - a series of statements which are executed in random order. Each statement can contain a prerequisite, which if not fulfilled causes the statement to be deferred until some (random) later time.
I'm not entirely clear on the concept, but I think PostScript meets the criteria, although it calls all of its functions operators (the way LISP calls all of its operators functions).
Makefile syntax doesn't seem to have any operators or control structures. I'd say it's a programming language but it isn't Turing Complete (without extensions to the POSIX standard anyway)
So... you're looking for a super-simple language? How about Batch programming? If you have any version of Windows, then you have access to a Batch compiler. It's also more useful than you'd think, since you can carry out basic file functions (copy, rename, make directory, delete file, etc.)
http://www.csulb.edu/~murdock/dosindex.html
Example
Open notepad and make a .Bat file on your Windows box.
Open the .Bat file with notepad
In the first line, type "echo off"
In the second line, type "echo hello world"
In the third line, type "pause"
Save and run the file.
If you're looking for a way to learn some very basic programming, this is a good way to start. (Just be careful with the Delete and Format commands. Don't experiment with those.)