Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
In computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program written in such a language is formed by a sequence of one or more statements. A statement will have internal components .Many languages In computer programming...
Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal...
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, and so on. It is also included in several other types of languages. Its purpose is to allow the value of a variable
Variable (programming)
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
or expression to control the flow of program execution via a multiway branch
Multiway branch
Multiway branch is a computer science term used to describe the change to a program's control flow based upon a value matching a selected criteria. It is a form of conditional statement...
goto is a statement found in many computer programming languages. It is a combination of the English words go and to. It performs a one-way transfer of control to another line of code; in contrast a function call normally returns control...
A label in a programming language is a sequence of characters that identifies a location within source code. In most languages labels take the form of an identifier, often followed by a punctuation character . In many high level programming languages the purpose of a label is to act as the...
s). The main reasons for using a switch include improving clarity, by reducing otherwise repetitive coding, and (if the heuristic
Heuristic
Heuristic refers to experience-based techniques for problem solving, learning, and discovery. Heuristic methods are used to speed up the process of finding a satisfactory solution, where an exhaustive search is impractical...
s permit) also offering the potential for faster execution through easier compiler optimization
Compiler optimization
Compiler optimization is the process of tuning the output of a compiler to minimize or maximize some attributes of an executable computer program. The most common requirement is to minimize the time taken to execute a program; a less common one is to minimize the amount of memory occupied...
in many cases.
History
In his 1952 text Introduction to Metamathematics, Stephen Kleene formally proves that the CASE function (the IF-THEN-ELSE function being its simplest form) is a primitive recursive function
Primitive recursive function
The primitive recursive functions are defined using primitive recursion and composition as central operations and are a strict subset of the total µ-recursive functions...
, where he defines the notion definition by cases in the following manner:
"#F. The function φ defined thus
φ(x1 , ... , xn ) =
φ1(x1 , ... , xn ) if Q1(x1 , ... , xn ),
. . . . . . . . . . . .
φm(x1 , ... , xn ) if Qm(x1 , ... , xn ),
φm+1(x1 , ... , xn ) otherwise,
where Q1 , ... , Qm are mutually exclusive predicates (or φ(x1 , ... , xn) shall have the value given by the first clause which applies) is primitive recursive in φ1, ..., φm+1, Q1, ..., Qm+1. (Definition by cases)." (Kleene 1952:229)
Kleene provides a proof of this in terms of the Boolean-like recursive functions "sign-of" sg and "not sign of" ~sg (Kleene 1952:222-223); the first returns 1 if its input is positive and −1 if its input is negative.
Boolos-Burgess-Jeffrey make the additional observation that "definition by cases" must be both mutually exclusive and collectively exhaustive. They too offer a proof of the primitive recursiveness of this function (Boolos-Burgess-Jeffrey 2002:74-75).
In computer science and recursion theory the McCarthy Formalism of computer scientist John McCarthy clarifies the notion of recursive functions by use of the IF-THEN-ELSE construction common to computer science, together with the four of the operators of primitive recursive functions: zero,...
– its usage replaces both primitive recursion and the mu-operator.
Typical syntax
In most languages, a switch statement is defined across many individual lines using one or two keywords. A typical syntax is:
The first line contains the basic keyword, usually switch, case or select, followed by an expression which is often referred to as the control expression or control variable of the switch statement.
Subsequent lines define the actual cases (the values) with corresponding sequences of statements that should be executed when a match occur.
Each alternative begins with the particular value, or list of values (see below), that the control variable may match and which will cause the control to go to the corresponding sequence of statements. The value (or list/range of values) is usually separated from the corresponding statement sequence by a colon or an implication arrow. In many languages, every case must also be preceded by a keyword such as case or when. An optional default case is typically also allowed, specified by a default or else keyword; this is executed when none of the other cases matches the control expression.
In languages derived from C, a break keyword is used to go to the end of the switch, thus completing execution of the switch statement. In such languages, program execution "falls through" to the statements associated with the next case in the source text when no break is present, thereby behaving like a GOTO mechanism. Notable variations on this in the C-family include C#, in which all blocks must be terminated with a break unless the block is empty (i.e. fallthrough is used as a way to specify multiple values).
Pascal or PASCAL may refer to:-People:* Pascal , a French given name* Pascal , a French and Italian surname* Adam Pascal , American actor and singer, best known for his role of Roger Davis in the Broadway musical Rent* Blaise Pascal , French mathematician and philosopher* Cleo Paskal, environmental...
family (Object Pascal, Modula, Oberon, Ada, etc.) as well as modern BASIC
BASIC
BASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....
dialects influenced by Pascal, do not allow fallthrough. The same goes for most functional languages and many others. Pascal type languages instead permit any number of values per case, given as a comma separated list, as a range, or a combination. Perl
Perl
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
is a language where cases do not fall through by default, but may explicitly do so using a continue keyword.
Compilation
If the range of input values is identifiably 'small' and has only a few gaps, some compilers that incorporate an optimizer
Optimization (computer science)
In computer science, program optimization or software optimization is the process of modifying a software system to make some aspect of it work more efficiently or use fewer resources...
may actually implement the switch statement as a branch table
Branch table
In computer programming, a branch table is a term used to describe an efficient method of transferring program control to another part of a program using a table of branch instructions. It is a form of multiway branch...
A function pointer is a type of pointer in C, C++, D, and other C-like programming languages, and Fortran 2003. When dereferenced, a function pointer can be used to invoke a function and pass it arguments just like a normal function...
s instead of a lengthy series of conditional instructions. This allows the switch statement to determine instantly what branch to execute without having to go through a list of comparisons.
Optimized switch
To optimize a switch statement, the programmer must use a very compact range of possible values to test. Sometimes it is necessary to convert the switch to a more suitable range using an inexpensive transformation.
See algorithmic efficiency
Algorithmic efficiency
In computer science, efficiency is used to describe properties of an algorithm relating to how much of various types of resources it consumes. Algorithmic efficiency can be thought of as analogous to engineering productivity for a repeating or continuous process, where the goal is to reduce...
for an explanation of how the programmer can "assist" the compiler to make an efficient choice.
See also the section 'Compiler generated branch tables' in branch table
Branch table
In computer programming, a branch table is a term used to describe an efficient method of transferring program control to another part of a program using a table of branch instructions. It is a form of multiway branch...
article for why optimization is not always performed as expected and how to solve this.
Checking for optimization
Normally, the only method of finding out if this optimization has occurred is by actually looking at the resultant assembly
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...
output that has been generated by the compiler (and is therefore seldom, if ever, done by HLL
High-level programming language
A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...
programmers).
The first 'C' example below would be eligible for this kind of optimization if the compiler supported it (the range '0' through '9' with zero gaps without a defined case label).
Advantages
In some languages and programming environments, the use of a case or switch statement is considered superior to an equivalent series of if-else statements because it is:
easier to debug (e.g. setting breakpoints on code vs. a call table, if the debugger has no conditional breakpoint capability)
In computer science, program optimization or software optimization is the process of modifying a software system to make some aspect of it work more efficiently or use fewer resources...
implementation may:
execute much faster than the alternative, because it is often implemented by using an indexed branch table
Branch table
In computer programming, a branch table is a term used to describe an efficient method of transferring program control to another part of a program using a table of branch instructions. It is a form of multiway branch...
For example, deciding program flow based on a single character's value, if correctly implemented, is vastly more efficient than the alternative, reducing instruction path length
Instruction path length
In computer performance, the instruction path length is the number of machine code instructions required to execute a section of a computer program. The total path length for the entire program could be deemed a measure of the algorithm's performance on a particular computer hardware...
s considerably.
Disadvantages
When implemented with fall-through as the default path, switch/case statements are a frequent source of bugs among even experienced programmers, given that, in practice, the "break" is almost always the desired path, but not the default behavior of the switch/case construct (at least in C and Java).
Examples
The following are simple examples, written in the various languages, that use switch (or switch-like) statements to print one of several possible lines, depending on the value of an integer entered by the user.
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
does not allow “fall through”; it uses case, when and others. Ada requires full coverage of all possible values for the type in the case statement. If a when others =>
case is not specified, then the code will not compile if either extra cases are specified, or missing.
If at some point in the future, the definition of Digit is modified, the compiler will ensure that the programmer updates the case statement to reflect the changes to the type definition, which ensures that the program is kept up to date and helps to reduce maintenance costs.
A list of values for a particular case can be combined using '|' as shown below, or a range of values
may be specified using ".." to indicate the extents of the range. e.g., when 0 .. 4 => Put_Line ("Small Digits);
In the example below, there is no need to check for values outside the range of 0 to 9 because the type is guaranteed to have a value within the range for the type.
type Digit is new Integer range 0 .. 9;
n : Digit;
...
case n is
when 0 =>
Put_Line ("You typed zero");
when 1 | 9 =>
Put_Line ("n is a perfect square");
when 2 =>
Put_Line ("n is a prime number");
Put_Line ("n is an even number");
when 3 | 5 | 7 =>
Put_Line ("n is a prime number");
when 4 =>
Put_Line ("n is a perfect square");
Put_Line ("n is an even number");
when others =>
Put_Line ("n is an even number");
end case;
ALGOL 60 is a member of the ALGOL family of computer programming languages. It gave rise to many other programming languages, including BCPL, B, Pascal, Simula, C, and many others. ALGOL 58 introduced code blocks and the begin and end pairs for delimiting them...
a switch is effectively an array of labels (branch table
Branch table
In computer programming, a branch table is a term used to describe an efficient method of transferring program control to another part of a program using a table of branch instructions. It is a form of multiway branch...
). A switch declaration defines its values (i.e., for each index value the name of a label occurring somewhere in the program). A goto statement can specify as destination, instead of a fixed label, an "array element" of this switch, i.e., the switch identifier with, in brackets, the index.
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
and similarly-constructed languages, the lack of break keywords to cause fall through of program execution from one block to the next is used extensively. For example, if n=2, the fifth case statement will produce a match to the control variable. The next line outputs "n is an even number.". Execution then continues through the next 3 case statements and to the next line, which outputs "n is a prime number." this is a classic example of omitting the break line to allow for fall through. The break line after a case block causes the switch statement to conclude. If the user types in more than one digit, the default block is executed, producing an error message by executing the default code.
switch (n) {
case 0:
printf("You typed zero.");
break;
case 4:
printf("n is an even number.");
case 1:
case 9:
printf("n is a perfect square.");
break;
case 2:
printf("n is an even number.");
case 3:
case 5:
case 7:
printf("n is a prime number.");
break;
case 6:
case 8:
printf("n is an even number.");
break;
default:
printf("Only single-digit numbers are allowed.");
break;
}
Clojure
Instead of switch/case syntax, clojure uses cond macro
(cond (= n 5) 5
(= n 3) 3
:else "n is not equal to 5 or 3")
C#
In C#, every case block that contains any statements must have a reachable end point, or triggers a compilation error. Usually, this is a break statement, but any jump statement can be used – such as return, goto or throw – or the switch can simply end with an infinite loop. Case fall-through is only permitted when there are no statements between one case statement and the next. If fall-through is otherwise desired, it must be made explicit with the goto case construct.
C# also allows the use of non-integer case values, such as Strings.
switch (n)
{
case 0:
Console.WriteLine("You typed zero.");
break;
case 1:
case 4:
case 9:
case 16:
Console.WriteLine("n is a perfect square.");
break;
case 2:
Console.WriteLine("n is an even number.");
goto case 3;
case 3:
case 5:
case 7:
Console.WriteLine("n is a prime number.");
break;
case 6:
case 8:
case 10:
Console.WriteLine("n is an even number.");
break;
default:
Console.WriteLine("Only single-digit numbers are allowed.");
break;
}
Eiffel is an ISO-standardized, object-oriented programming language designed by Bertrand Meyer and Eiffel Software. The design of the language is closely connected with the Eiffel programming method...
's multi-branch instruction uses inspect, when, and else. It does not have the “fall through” behavior. Also, the else part is optional. However, an omitted else differs from an included, but empty else. If the else is empty and a case is processed that is not specified in one of the when parts, control passes through the else. But if the else is omitted, it is assumed that all cases should be identified in a when part. In this case an exception will occur as a result of processing a case not handled in a when part.
inspect
n
when 0 then
print ("You typed zero%N")
when 1, 9 then
print ("n is a perfect square%N")
when 2 then
print ("n is a prime number%N")
print ("n is an even number%N")
when 3, 5, 7 then
print ("n is a prime number%N")
when 4 then
print ("n is a perfect square%N")
print ("n is an even number%N")
when 6, 8 then
print ("n is an even number%N")
else
print ("Only single digit numbers are allowed%N")
end
Go is a compiled, garbage-collected, concurrent programming language developed by Google Inc.The initial design of Go was started in September 2007 by Robert Griesemer, Rob Pike, and Ken Thompson. Go was officially announced in November 2009. In May 2010, Rob Pike publicly stated that Go was being...
has an explicit fallthrough statement which can be used at the end of a case statement to indicate that control falls through next case clause in a expression "switch" statement.
Haskell
Haskell's case construct, unlike C-influenced languages, has no fall-through behaviour. It is an expression
Expression (programming)
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
which returns a value, and it can deconstruct values using pattern matching
Pattern matching
In computer science, pattern matching is the act of checking some sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually has to be exact. The patterns generally have the form of either sequences or tree structures...
.
case list of
(f:r) -> "Not empty, first item is " ++ show f
[] -> "List is empty!"
OCaml, F#
OCaml and F#'s match construct is like Haskell's case above.
(* OCaml *)
match list with
f::r -> "Not empty, first item is " ^ string_of_int f
| [] -> "List is empty!"
// F#
match list with
| f::r -> "Not empty, first item is " + string f
| [] -> "List is empty!"
Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal...
does not allow “fall through”, but has ranges and comma separated literal lists.
case age of
0,1: writeln('baby');
2,3,4: writeln('toddler');
5..12: writeln('kid');
13..19: writeln('teenager');
20..25: writeln('young');
else writeln('old');
end;
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
Perl 6 is a major revision to the Perl programming language. It is still in development, as a specification from which several interpreter and compiler implementations are being written. It is introducing elements of many modern and historical languages. Perl 6 is intended to have many...
) has a powerful built in switch statement called given, where the cases are called when:
use feature 'switch';
given ($foo) {
when (undef) {
say '$foo is undefined';
}
when ("foo") {
say '$foo is the string "foo"';
}
when ([1,3,5,7,9]) {
say '$foo is an odd digit';
continue; # Fall through
}
when ($_ < 100) {
say '$foo is numerically less than 100';
}
when (\&complicated_check) {
say 'a complicated check for $foo is true';
}
default {
die "I don't know what to do with $foo";
}
}
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
does not have direct language support for the switch statement; 'if'/'elif' is often used for that. However, it is possible to emulate this behaviour, through a dictionary
Associative array
In computer science, an associative array is an abstract data type composed of a collection of pairs, such that each possible key appears at most once in the collection....
of functions.
Here is an example that does not use a “fall through” mechanism. The default case is mimicked by using dict.get's fallback parameter:
In QBasic, the switch statement is called "Select Case", and fall-through to later blocks is not supported. The "Select Case" statement is more expressive because it allows conditionals within cases.
SELECT CASE age
CASE IS < 12: PRINT "Have some juice!"
CASE 13 TO 20: PRINT "Have a soda!"
CASE IS >= 21: PRINT "Have a beer!"
END SELECT
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
doesn’t allow “fall through”; it uses case, when and else:
case n
when 0
puts 'You typed zero'
when 1, 9
puts 'n is a perfect square'
when 2
puts 'n is a prime number'
puts 'n is an even number'
when 3, 5, 7
puts 'n is a prime number'
when 4, 6, 8
puts 'n is an even number'
else
puts 'Only single-digit numbers are allowed'
end
Also can be used to assign a value, in a more compact way:
result = case n
when 0 then 'none'
when 1..9 then 'valid'
else 'too much'
end
puts 'n is ' + result
Shell script
Bash and similar shell scripting languages offer a case construct using the OR operator, |, to separate the selections, and the ) symbol to separate the list of selections from the action to be taken. Fall through is done using ;& whereas ;; acts as a case break.
case $n in
0) echo 'You typed 0.';;
1|9) echo "$n is a perfect square.";;
3|5|7) echo "$n is a prime number.";;
4) echo "$n is a perfect square.";& # fall through
2|6|8) echo "$n is an even number.";;
*) echo 'Only single-digit numbers are allowed.';;
esac
SQL
SQL has a case/when/then/else/end expression:
CASE N WHEN 1 then "one" WHEN 2 THEN "two" ELSE "i can not count that high" END
This can be shortened to an equivalent decode construct.
SELECT decode(N, 1, "one", 2, "two");
FROM "table_name"
Visual Basic .NET , is an object-oriented computer programming language that can be viewed as an evolution of the classic Visual Basic , which is implemented on the .NET Framework...
, the switch statement is called "Select Case", and fall-through to later blocks is not supported.
However, ranges and various constructs from If statements are both supported
Select Case n
Case Is < -5
MsgBox("n is less than -5")
Case -4 To -1
MsgBox("n is between -4 and -1")
Case 0
MsgBox("n is 0")
Case 2, 4, 6, 8
MsgBox("n is even")
Case 1, 3, 5, 7, 9
MsgBox("n is odd")
Case Else
MsgBox("only single-digit numbers are allowed.", vbCritical)
End Select
Visual FoxPro is a data-centric object-oriented and procedural programming language produced by Microsoft. It is derived from FoxPro which was developed by Fox Software beginning in 1984. Fox Technologies merged with Microsoft in 1992, after which the software acquired further features and the...
:
Do Case
Case field_1 = "X"
Replace field_b With 1
Case field_1 = "Y"
Replace field_b With 2
Case field_1 = "Z"
Replace field_b With 3
Endcase
Visual Basic is the third-generation event-driven programming language and integrated development environment from Microsoft for its COM programming model...
, the switch statement is called "Select Case", and fall-through to later blocks is not supported. Short-circuit evaluation is used. But also, may doing exactly like C, using GOSUB
GOSUB
GOSUB is a command in many versions of the BASIC computer programming language. A GOSUB statement jumps to a line elsewhere in the program. That line and the following lines up to a RETURN are used as a simple kind of a subroutine without parameters or local variables.The GOSUB command may be used...
behind Select Case. the block Select Case then call to GOSUB label, where each BREAK in C it's a RETURN (to gosub).
Select Case n
Case "s"
MsgBox "Case values of any type are supported"
Case "s"
MsgBox "This block is not an error but will never be selected"
Case ArbitraryFunction
MsgBox "Any Expression which can be evaluated at runtime may be used as a case value or selector"
Case 2+3: MsgBox "The colon is a general language feature, not a part of the switch statement"
MsgBox "Each block is terminated by the following Case or End statement"
Case Else
MsgBox "Case values which do not match the selector type will cause an exception", vbCritical
End Select
WebDNA is a server-side scripting, interpreted language containing an optional embedded, ram-resident, proprietary, searchable data structure. It has the ability to connect directly to other SQL servers if desired. Its primary use is in creating dynamic web page applications competing against the...
example is easy to understand:
[text]x=5[/text]
[switch value=[x]]
[case value=1]
The value of x was 1
[/case]
[case value=2]
The value of x was 2
[/case]
[default]
The value of x was neither 1 nor 2; it was [x]
[/default]
[/switch]
Windows PowerShell is Microsoft's task automation framework, consisting of a command-line shell and associated scripting language built on top of, and integrated with the .NET Framework...
employs a construct whereby the action to be taken is enclosed in a scriptblock (i.e. curly braces), with the selector placed directly before it. The selector can consist of regular expressions if the "-regex" parameter is inserted after the "switch" command; similarly, wildcards are supported using the "-wildcard" parameter. In either case, the wildcard or regex must be enclosed in quote marks.
Akin to C-based implementations, if a "break" statement is not included at the end of a scriptblock, the switch statement will continue to test each case and execute further scriptblocks.
switch (n)
{
0 { Write-Host 'You typed 0' }
{ ($_ -eq 1) -or ($_ -eq 4) -or ($_ -eq 9) }
{ Write-Host 'n is a perfect square' }
{ (($_ % 2) -eq 0) -and ($_ -ne 0) }
{ Write-Host 'n is an even number' }
{ ($_ -eq 2) -or ($_ -eq 3) -or ($_ -eq 5) -or ($_ -eq 7) }
{ Write-Host 'n is an prime number' }
default { Write-Host 'Only single-digit numbers are allowed' }
}
Symbolic constants
In many (but not all) circumstances, using symbolic names rather than explicit literal integers makes the source code easier to read and maintain. This is often achieved via enumeration
Enumeration
In mathematics and theoretical computer science, the broadest and most abstract definition of an enumeration of a set is an exact listing of all of its elements . The restrictions imposed on the type of list used depend on the branch of mathematics and the context in which one is working...
s and has no influence on the performance or behavior of the program. This style of switch statement is commonly used for finite state machine
Finite state machine
A finite-state machine or finite-state automaton , or simply a state machine, is a mathematical model used to design computer programs and digital logic circuits. It is conceived as an abstract machine that can be in one of a finite number of states...
implementation. Here are some examples, given in Pascal (one of the first languages implementing enumerations, along with Algol68) as well as in C (which has a tradition of constants in all capitals, although not enforced by the compiler.)
Pascal (using an enumeration)
var state : (StateReady, StateSet, StateGo, StateFail);
case state of
StateReady: begin
state:=succ(state);
if x < 0 then state:=StateFail
end;
StateSet: begin
state:=succ(state);
if y > 0 then state:=StateFail
end;
switch( state )
{
case STATE_READY:
state = STATE_SET;
if( x < 0 ) state = STATE_FAIL;
break;
case STATE_SET:
state = STATE_GO;
if( y > 0 ) state = STATE_FAIL;
break;
case STATE_GO:
printf( "go!\n" );
break;
case STATE_FAIL:
exit( -1 );
}
Alternative uses
Many languages evaluate expressions inside switch blocks at runtime, allowing a number of less obvious uses for the construction. This prohibits certain compiler optimizations, so is more common in dynamic and scripting languages where the enhanced flexibility is more important than the performance overhead.
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
, a constant can be used as the "variable" to check against, and the first case statement which evaluates to that constant will be executed:
switch(true) {
case ($x
'hello'):
foo;
break;
case ($z
'howdy'): break;
}
switch(5) {
case $x: break;
case $y: break;
}
This feature is also useful for checking multiple variables against one value rather than one variable against many values. COBOL also supports this form (and others forms) in the EVALUATE statement.
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
, due to its handling of equality, the statement can be used to test for variable’s class:
case input
when Array: puts 'input is an Array!'
when Hash: puts 'input is a Hash!'
end
Ruby also returns a value that can be assigned to a variable, and doesn’t actually require the case to have any parameters (acting a bit like an else if statement):
catfood = case
when cat.age <= 1: junior
when cat.age > 10: senior
else normal
end
Alternatives
A series of nested if-else conditionals that examine the target one value at a time.
In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...
, which contains, as keys, the case values and, as values, the part under the case statement.
(In some languages, only actual data types are allowed as values in the lookup table. In other languages, it is also possible to assign functions as lookup table values, gaining the same flexibility as a real switch statement. See Control table
Control table
Control tables are tables that control the program flow or play a major part in program control. There are no rigid rules concerning the structure or content of a control table - its only qualifying attribute is its ability to direct program flow in some way through its 'execution' by an associated...
article for more detail on this).
Lua does now support Case/Switch statements: http://lua-users.org/wiki/SwitchStatement . This lookup technique is one way to implement switch statements in the Lua language - which has no built-in switch.
In some cases, lookup tables are more efficient than non-optimized switch statements since many languages can optimize table lookups - whereas switch statements are not optimized unless the range of values is small with few gaps. A non-optimized, non-binary search lookup, however, will almost certainly be slower than either a non-optimized switch or the equivalent multiple if-else statements.
Control tables are tables that control the program flow or play a major part in program control. There are no rigid rules concerning the structure or content of a control table - its only qualifying attribute is its ability to direct program flow in some way through its 'execution' by an associated...
(that may be implented as a simple lookup table) can also be customized to accommodate multiple conditions on multiple inputs if required and usually exhibits greater 'visual compactness' than an equivalent switch (that can occupy many statements).
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
In computer science, polymorphism is a programming language feature that allows values of different data types to be handled using a uniform interface. The concept of parametric polymorphism applies to both data types and functions...
In computer science, efficiency is used to describe properties of an algorithm relating to how much of various types of resources it consumes. Algorithmic efficiency can be thought of as analogous to engineering productivity for a repeating or continuous process, where the goal is to reduce...
- general discussion on improving speed of algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
In computer programming, a branch table is a term used to describe an efficient method of transferring program control to another part of a program using a table of branch instructions. It is a form of multiway branch...
- an extremely fast, optimized form of a switch statement used mostly in Assembler languages and compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
Control tables are tables that control the program flow or play a major part in program control. There are no rigid rules concerning the structure or content of a control table - its only qualifying attribute is its ability to direct program flow in some way through its 'execution' by an associated...
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
In computer science, Duff's device is an optimized implementation of a serial copy that uses a technique widely applied in assembly language for loop unwinding. Its discovery is credited to Tom Duff in November of 1983, who at the time was working for Lucasfilm. It is perhaps the most dramatic...
Loop unwinding, also known as loop unrolling, is a loop transformation technique that attempts to optimize a program's execution speed at the expense of its binary size...
technique that makes use of a switch statement.
Conditional statement - A switch statement is a type of conditional statement
In computer science and recursion theory the McCarthy Formalism of computer scientist John McCarthy clarifies the notion of recursive functions by use of the IF-THEN-ELSE construction common to computer science, together with the four of the operators of primitive recursive functions: zero,...
Multiway branch is a computer science term used to describe the change to a program's control flow based upon a value matching a selected criteria. It is a form of conditional statement...
- a switch statement is a type of multiway branch
The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.
