Conditional (programming)

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In computer science, conditional statements, conditional expressions and conditional constructs are features of a programming language which perform different computations or actions depending on whether a programmer-specified condition evaluates to true or false (see boolean datatype). Apart from the case of branch predication, this is always achieved by selectively altering the control flow based on some condition.

In imperative programming languages, the term "conditional statement" is usually used, whereas in functional programming, the terms "conditional expression" or "conditional construct" are preferred, because these terms all have distinct meanings.

Although dynamic dispatch is not usually classified as a conditional construct, it is another way to select between alternatives at runtime.

[edit] If-Then(-Else)

The if-then construct (sometimes called if-then-else) is common across many programming languages. Although the syntax varies quite a bit from language to language, the basic structure (in pseudocode form) looks like this:

If (condition) Then
   (statements)
Else
   (statements)
End If

When an interpreter finds an If, it expects a boolean condition - for example, x > 0, which means "the variable x contains a number that is greater than zero" - and evaluates that condition. If the condition is true, the statement block following the Then is executed. Otherwise, the execution continues in the following block - either in the Else block (which is usually optional), or if there is no Else block, then after the End If.

After either the block after the Then or the block after the Else has been executed, control returns to the point after the End If.

In early programming languages - and in particular, in some dialects of BASIC in the 1980s - an if-then statement could only contain GOTO statements. This led to a hard-to-read style of programming known as spaghetti programming. As a result, structured programming, which allowed (virtually) arbitrary statements to be put in statement blocks inside an if statement, gained in popularity, until it became the norm.

[edit] Else If parts

By using Else If, it is possible to combine several conditions. Only the statements following the first condition that is found to be true will be executed. All other statements will be skipped. The statements of the final Else will be executed if none of the conditions are true. This example is written in the Ada programming language:

if condition then
    -- statements;
elseif condition then
    -- more statements;
elseif condition then
    -- more statements;
...
else condition then
    -- other statements;
end if;

elseif, in Ada, is simply syntactic sugar for else followed by if. In Ada, the difference is that only one end if is needed, if one uses elseif instead of else followed by if.

In some other languages, such as C, else if literally just means else followed by if - so no syntactic sugar is needed or provided.

[edit] If expressions

Many languages support if expressions, which are similar to if statements, but return a value as a result. Thus, they are true expressions (which evaluate to a value), not statements (which just perform an action).

[edit] As a ternary operator

Main article: ?: [sic]

In C and C-like languages conditional expressions take the form of a ternary operator called the conditional expression operator, ?:, which follows this template:

(condition)?(evaluate if condition was true):(evaluate if condition was false)

This means that conditions can be inlined into expressions, unlike with if statements, as shown here using C syntax:

//Invalid
my_variable = if(x > 10) { "foo" } else { "bar" };
//Valid
my_variable = (x > 10)?"foo":"bar";

To accomplish the same as the second (correct) line above, using a standard if/else statement, this would take more than one line of code (under standard layout conventions):

if (x > 10) {
  my_variable = 'foo';
}
else {
  my_variable = 'bar';
}

[edit] As a function

In Visual Basic and some other languages, a function called IIf is provided, which can be used as a conditional expression. However, it does not behave like a true conditional expression, because both the true and false branches are always evaluated; it is just that the result of one of them is thrown away, while the result of the other is returned by the IIf function.

[edit] In Haskell

In Haskell 98, there is only an if expression, no if statement, and the else part is compulsory.^[1]

[edit] Arithmetic IF

This section is missing citations or needs footnotes. Using inline citations helps guard against copyright violations and factual inaccuracies. (May 2007)

Fortran 77 has an "arithmetic if" statement which is halfway between a computed IF and a case statement, based on the trichotomy x < 0, x = 0, x > 0^[2]:

IF (e) label1, label2, label3

Where e is any numeric expression (not necessarily an integer); this is equivalent to

IF (e < 0) GOTO label1
IF (e = 0) GOTO label2
IF (e > 0) GOTO label3

Because this arithmetic IF is equivalent to multiple GOTO statements, it is considered to be an unstructured control statement, and should not be used if more structured statements can be used. In practice it has been observed that most arithmetic IF statements referenced the following statement with one or two of the labels.

This was the only conditional control statement in the original implementation of Fortran on the IBM 704 computer. On that computer it could be implemented quite efficiently using instructions such as 'Branch if accumulator negative'.

[edit] Alternative implementation

In contrast to other languages, in Smalltalk the conditional statement is not a language construct but defined in the class Boolean as an abstract method that takes two parameters, both closures. Boolean has two subclasses, True and False, which both define the method, True executing the first closure only, False executing the second closure only.^[3]

var := condition 
    ifTrue: [ 'foo' ]
    ifFalse: [ 'bar' ]

[edit] Case and switch statements

Main article: Switch statement

Switch statements (in some languages, case statements) compare a given value with specified constants and take action according to the first constant to match. The example on the left is written in Pascal, and the example on the right is written in C.

case someChar of
  'a': actionOnA;
  'x': actionOnX;
  'y','z':actionOnYandZ;
end;

switch (someChar) {
  case 'a': actionOnA; break;
  case 'x': actionOnX; break;
  case 'y':
  case 'z': actionOnYandZ; break;
  default: actionOnNoMatch;
}

[edit] Pattern matching

Main article: Pattern matching

Pattern matching is a more sophisticated alternative to both if-then-elseif, and the simple case or switch statements mentioned above. It is available in some programming languages such as the ML language family. Here is a simple example written in the O'Caml language:

match fruit with
 | "apple" -> cook pie
 | "coconut" -> cook dango_mochi
 | "banana" -> mix;;

The true power of pattern matching, however, comes from the ability to concisely (a) match on data structure patterns, and (b) bind variables at the same time. Here is an example written in Haskell which illustrates both of these features:

map _ []      = []
map f (h : t) = f h : map f t

This code defines a function map, which applies the first argument (a function) to each of the elements of the second argument (a list), and returns the resulting list. The two lines are not two separate definitions of the function, but rather definitions of what to do in two cases - one case where the list is empty (just return an empty list) and the other case where the list is not empty.

Pattern matching is not strictly speaking always a choice construct, because it is possible in Haskell to write only one alternative, which is guaranteed to always be matched - in this situation, it is not being used a choice construct, but simply as a way to bind variables. However, it is frequently used as a choice construct in the languages in which it is available.

[edit] Branch predication

Main article: Branch predication

In assembly language, branch predication is a feature of certain CPU instruction sets which permits conditional execution of instructions, without having to perform costly conditional jumps.

[edit] Choice system cross reference

This table refers to the most recent language specification of each language. For languages that do not have a specification, the latest officially released implementation is referred to.

1. ^a  This refers to pattern matching as a distinct conditional construct in the programming language - as opposed to mere string pattern matching support, such as regular expression support.
2. ^{a  b}  The often-encountered else if in the C family of languages, and in Haskell, is not a language feature but a set of nested and independent if then else statements combined with a particular source code layout. However, this also means that a distinct else-if construct is not really needed in these languages.
3. ^a  In Haskell, a separate constant choice construct is not needed, because the same task can be done with pattern matching.

[edit] See also

Look up then, else in Wiktionary, the free dictionary.

• Branch
• Dynamic dispatch for another way to make execution choices

[edit] References

1. ^ Haskell 98 Language and Libraries: The Revised Report
2. ^ American National Standard Programming Language FORTRAN (1978-04-03). Retrieved on 2007-09-09.
3. ^ VisualWorks: Conditional Processing (2006-12-16). Retrieved on 2007-09-09.