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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 .


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.


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 :


''statements'';

''more statements'';

''more statements'';
...

''other statements'';
;

''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.


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).


As a ternary operator


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In , 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';

}



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.


In Haskell


In Haskell 98 , there is only an ''if expression'', no ''if statement'', and the ''else'' part is compulsory.'' Haskell 98 Language and Libraries: The Revised Report ''


Arithmetic IF


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 > 01:

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'.


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 Closure s. Boolean has two subclasses, True and False, which both define the method, True executing the first closure only, False executing the second closure only.2

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


CASE AND SWITCH STATEMENTS


See Also: Switch statement



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


PATTERN MATCHING


See Also: 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'''
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