In order for the program to be non-linear (that is, depending on the situation, different instructions are executed), programming languages use logical expressions, the result of which can be either true (true) or false (false). The result of logical expressions is usually used to determine the path of program execution.
Simple Boolean expressions are the result of relation operations between two operands (values). In the examples below, the operands are the values of the variables x and y. Operands can be numbers, symbols, and other data types. Everything that can be compared with each other. However, it is not recommended to compare real numbers because of the peculiarities of their storage in computer memory.
Pascal provides the following relational operators:
- less: x< y
- greater: x> y
- equals: x = y
- not equal: x<>y
- less or equal: x<= y
- greater than or equal: x> = y
Boolean types
The result of a Boolean expression is always a Boolean (Boolean) value. Boolean data type (boolean) can take only two values (true or false). These values are ordered as follows: false< true. Это значит, что данные булевого типа являются не только результатом операций отношения, но и могут выступать в роли операндов операции отношения. Также к ним можно применять функции ord, succ, pred, процедуры inc и dec.
A boolean value occupies 1 byte in memory.
In the example, six Boolean variables are assigned the values of simple Boolean expressions. The values stored in such variables are then displayed on the screen.
In addition to the boolean type, Pascal introduces three more boolean types - bytebool (takes 1 byte), wordbool (takes 2 bytes) and longbool (takes 4 bytes).
For all boolean types, false is 0, and true is any nonzero value. Boolean variables of different boolean types behave differently when you perform operations on them. Below is an example implemented in the FreePascal language (the result is displayed in the comments).
var b: boolean; wb: wordbool; begin b: = false; b: = pred (b); writeln (b, "", ord (b)); // TRUE 255 writeln (b = true); // TRUE wb: = false; wb: = pred (wb); writeln (wb, "", ord (wb)); // TRUE -1 b: = true; b: = succ (b); writeln (b, "", ord (b)); // TRUE 2 wb: = true; wb: = succ (wb); writeln (wb, "", ord (wb)); // FALSE 0 end.
Logical operations
You can use logical operators to form complex logical expressions. Boolean operators are often used in relation to simple Boolean expressions.
The Pascal programming language provides the following logical operations:
true xor true = false
true xor false = true
false xor true = true
false xor false = false
- Conjunction (logical multiplication, intersection) - and. The expression a and b gives true only if a and b are true. In all other cases, the expression values a and b are false.
true and true = true true and false = false false and true = false false and false = false
- Disjunction (logical addition, union) - or. The expression a or b evaluates to false only if a and b are both false. In all other cases, the result is true.
true or true = true true or false = true false or true = true false or false = false
- Negation (inversion) - not. The expression not a has the opposite meaning of a.
not true = false not false = true
- Exclusive OR - xor. The expression a xor b evaluates to true only if only one of the operands evaluates to true.
The sequence of execution of logical operators: not, and, or.
In Pascal, the logical operators (and, or, xor, not) are executed first, and only then the relational operators (>,> =,<, <=, <>, =), so don't forget to place parentheses in complex logical expressions.
Complex Boolean expressions may not be processed until the end if continuing the calculation does not change the result. If a boolean expression must be processed to the end, then this is ensured by including the compilation directive (B +).
Standard boolean functions
- odd (x) = true if x is odd (x is an integer type);
- eoln (x) = true if the end of the line of the text file x was encountered;
- eof (x) = true if end of file x is encountered.
Otherwise, these functions are false.
False, a. 1. Uttering falsehood; unveracious; given to deceit; dishnest; as, a false witness. ... ... The Collaborative International Dictionary of English
false- adj 1: not genuine, authentic, or legitimate compare counterfeit 2 a: not true or correct; esp: intentionally or knowingly untrue or incorrect injured by false accusations b: intended to mislead or deceive: decept ... Law dictionary
false- W3S3 adj ▬▬▬▬▬▬▬ 1¦ (untrue) ¦ 2¦ (wrong) ¦ 3¦ (not real) ¦ 4¦ (not sincere) ¦ 5 false economy 6 under false pretences 7 false move / step 8 false imprisonment / arrest ▬▬▬▬▬▬▬ adj. falser, falsest 1.not true; in error; incorrect; mistaken 2. untruthful; lying; dishonest 3. disloyal; unfaithful adjective ** 1.) not true: The report was dismissed as totally false. a false statement / claim / accusation ─ opposite TRUE 2.) made to look like something real: ARTIFICIAL: false eyelashes a) not real and intended to trick people: a… ... Usage of the words and phrases in modern English
false- adjective 1.not true or real, but intended to look real in order to deceive people: false and misleading advertisements Firms issuing false certificates might be subject to lawsuits. 2.a false economy something that you ... ... Financial and business terms
This article is about a Unix utility. An article about an esoteric programming language is called FALSE. false (translated from English "false") is a console command for UNIX compatible operating systems, the only action of which is to return the value 1, ... ... Wikipedia
False- is the antonym of the adjective true.False is the 2nd album of Gorefest, False (album) .False may also refer to: * FALSE, an esoteric stack oriented programming language * false, a Unix utilityee also * Contradiction * Falsity * Lie * Falsework, a ... ... Wikipedia
false- fȯls adj, fals · er; fals est 1) not corresponding to truth or reality a test for HIV which gave false results 2) artificially made false teeth 3) of a kind related to or resembling another kind that is usu. designated by the…… Medical dictionary
Books
- False Delicacy, a Comedy, Kelly Hugh. The book is a reprinted edition. Despite the fact that serious work has been done to restore the original quality of the publication, on some pages you may find ...
- False Dmitri, a Russian romance and tragedy described by British eye-witnesses, 1604-1612, Howe Sonia E. The book is a reprinted edition. Despite the fact that serious work has been done to restore the original quality of the publication, on some pages you may find ...
True-false [͵tru: ʹfɔ: ls] a log.
truthful; having the truth values "true" and "false"
true-false questions - "yes-no" questions
New Comprehensive English-Russian Dictionary. 2001 .
See what "true-false" is in other dictionaries:
true-false- adjective offering a series of statements each of which is to be judged as true or false a true false test Ant: multiple choice * * * true false "TROO FLS", adjective. having to do with or containing statements which must be marked as either…… Useful english dictionary
true-false- adjective Allowing only the answers true or false. Somehow he got one true false question wrong out of twenty… Wiktionary
true-false test- true ′ false ′ test n. edu a test requiring one to mark statements as either true or false Etymology: 1920–25 ... From formal English to slang
true-false test- ☆ true false test n. a test, as one typically given in school, consisting of a series of statements to be identified as either “true” or “false”… English World dictionary
True / False Film Festival- Infobox Film Festival name = True / False Film Festival caption = opening = closing = awardsreceived = awardsgiven = films = directors = producers = writers = starring = host = Ragtag Programming for Film and Media Art date = number = 40 50…… Wikipedia
true-false test- (ˈ) ̷ ̷ˈ ̷ ̷ noun: an objective test consisting of a series of statements each of which is to be marked as either true or false * * * / trooh fawls /, Educ. a test requiring one to mark statements as true or false. * * * n. a test…… Useful english dictionary
true-false test- noun Date: 1924 a test consisting of a series of statements to be marked as true or false ... New Collegiate Dictionary
true-false test- / trooh fawls /, Educ. a test requiring one to mark statements as true or false. * * *… Universalium
The True False Identity- Infobox Album | Name = The True False Identity Type = Album Artist = T Bone Burnett Released = 2006 Recorded = Genre = Rock Length = 59:30 Label = DMZ Producer = T Bone Burnett Reviews = * Allmusic Rating | 3.5 | 5…… Wikipedia
False- is the antonym of the adjective true.False is the 2nd album of Gorefest, False (album) .False may also refer to: * FALSE, an esoteric stack oriented programming language * false, a Unix utilityee also * Contradiction * Falsity * Lie * Falsework, a ... ... Wikipedia
true-falsetest- true false test (tro͞oʹfôlsʹ) n. A test in which statements are to be marked either true or false. * * *… Universalium
Books
- The true-blue laws of Connecticut and New Haven and the false blue-laws invented by the Rev. Samuel Peters to which are added specimens of the laws and ... blue-laws of England in the reign of James I, Trumbull J Hammond. The book is a reprinted edition. Despite the fact that serious work has been done to restore the original quality of the publication, on some pages you may find ...
Perl does not have a special boolean type, but documentation often mentions that a function returns a "boolean" value. Sometimes it is simply written that the function returns true or false.
So where is the truth?
Perl does not have a special boolean type, but any scalar value when checked with if will return true or false. So you can write
If ($ x eq "foo") ()
If ($ x) ()
the first expression checks if the contents of the variable are the same $ x with the string "foo", and the second will check if the value of $ x itself is true.
What values are true or false in Perl?
It's pretty straightforward. Let's refer to the documentation:
Number 0, strings "0" and "", empty list "()" and "undef" are considered false in boolean context. All other values are considered true. Negation of the true value with "!" or "not" returns a special false value. It counts as "" when used as a string, and 0 when used as a number.
From perlsyn, "Truth and Falsehood" section.
Thus, the following scalar values are considered false:
- undef - undefined value
- 0 is number 0, even if you write it as 000 or 0.0
- "" empty line.
- "0" is a string containing the single number 0.
All other scalar values are true, including these:
- 1 any number other than zero
- "" a string with one or more spaces
- "00" is two or more 0s per line
- "0 \ n" 0 and line feed
- "true"
- "false" yes, even the string "false" is considered true.
I think this is because Larry Wall, the creator of Perl, is generally positive about the world. Apparently, he thinks that very few things in the world are really bad and false, and most of them are true.
