Conditional structures

From Rosetta Code
Revision as of 21:17, 26 January 2007 by Ce (talk | contribs) (→‎Compile-Time Control Structures: New subsection: template metaprogramming)

Template:Language Feature

Here, we document the conditional structures offered by different programming languages. Common conditional structures are if-then-else and switch.

AppleScript

if-then-else

if myVar is "ok" then return true

set i to 0
if i is 0 then
	return "zero"
else if i mod 2 is 0 then
	return "even"
else
	return "odd"
end if

C

if-then-else

Compiler: GCC 4.1.2 

int main (int argc, char ** argv) {
  int input = 2;

  if ( 3 == input ) {
    // Do something
  }



  if ( 3 == input ) {
    // Do something
  } else {
    // Do something else
  }
}

switch

Compiler: GCC 4.1.2 

int main (int argc, char ** argv) {
  int input = 42;

  switch (input) {
    case 0:
      // Do something, because input = 0
      break;
    case 1:
      // Do something, because input = 1
      break;
    case 2:
      // Do something, because input = 2
    default:
      // Do something else.
      break; // Optional
  }
}

C++

Run-Time Control Structures

if-then-else

Compiler: GCC 4.1.2 

int main (void) {
  int input = 2;

  if ( 3 == input ) {
    // Do something
  }



  if ( 3 == input ) {
    // Do something
  } else {
    // Do something else
  }
}

?:

Compiler: GCC 4.0.2

Conditionals in C++ can also be done with the ?: operator. The arguments are expressions, and a?b:c is an expression as well. However, since many things in C++ are expressions (this especially includes assignments and function calls), ?: can be used for those, too. However, the if/else construct is usually more readable and therefore preferred. 

int main()
{
  int input = 2;
  int output = (input == 2? 42 : 4711);  // sets output to 42
  int output2 = (input == 3? 42 : 4711); // sets output2 to 4711
  void do_something();
  void do_something_else();
  input == 1? do_something() : do_something_else(); // only calls do_something_else()
}

switch

Compiler: GCC 4.1.2 

int main (void) {
  int input = 42;

  switch (input) {
    case 0:
      // Do something, because input = 0
      break;
    case 1:
      // Do something, because input = 1
      break;
    case 2:
      // Do something, because input = 2
      // Because there is no 'break', we also fall through
      // into the default case, executing it right after case 2:
    default:
      // Do something else.
      break; // Optional
  }
}

Compile-Time Control Structures

Preprocessor Techniques

Conditional compile based on if a certain macro exists, 

#ifdef FOO
// compile this only if macro FOO exist
#endif

Conditional compile based on if a macro doesn't exist 

#ifndef FOO
// only compiled if macro FOO does not exist
#endif

Conditional compile based on if a certain macro exists, with else clause 

#ifdef FOO
// compile this only if macro FOO exist
#else
// compile this only if macro FOO does not exist
#endif

Conditional compile based on expression 

#if defined(FOO) && FOO == 1
// only compiled if macro FOO is compiled and expands to a constant expression evaluating to 1
#endif

Chain of conditionals 

#if defined(FOO)
// only compiled if macro FOO is defined
#elif defined(BAR)
// only compiled if macro FOO is not defined, but macro BAR is
#else
// only compiled if neither FOO nor BAR is defined
#endif

Typical usage: Include guards 

#ifndef FOO_H_ALREADY_INCLUDED
#define FOO_H_ALREADY_INCLUDED
// header content
#endif

If the header is included the first time, the macro FOO_H_ALREADY_INCLUDED will not be defined, thus the code between #ifndef and #endif will be executed. The first thig this code does is to define that macro, so that the next time the header is included, the code will be ignored. This effectively avoids multiple inclusion.

Template metaprogramming

Selecting a type depending on a compile time condition 

template<bool Condition, typename ThenType, typename Elsetype> struct ifthenelse;

template<typename ThenType, typename ElseType> struct ifthenelse<true, ThenType, ElseType>
{
  typedef ThenType type;
};

template<typename ThenType, typename ElseType> struct ifthenelse<false, ThenType, ElseType>
{
  typedef ElseType type;
};

// example usage: select type based on size
ifthenelse<INT_MAX == 32767, // 16 bit int?
           long int,         // in that case, we'll need a long int
           int>              // otherwise an int will do
  ::type myvar;

ColdFusion

if-elseif-else

Compiler: ColdFusion any version 

<cfif x eq 3>
 do something
<cfelseif x eq 4>
 do something else
<cfelse>
 do something else
</cfif>

switch

Compiler: ColdFusion any version 

<cfswitch expression="#x#">
 <cfcase value="1">
  do something
 </cfcase>
 <cfcase value="2">
  do something
 </cfcase>
 <cfdefaultcase>
  do something
 </cfdefaultcase>
</cfswitch>

Java

if-then-else

   if(s.equals("Hello World"))
   {
       foo();
   }
   else if(s.equals("Bye World"))
   {
       bar();
   }
   else
   {
       deusEx();
   }

ternary

   s.equals("Hello World") ? foo : bar

switch

   switch(c) {
   case 'a':
      foo();
      break;
   case 'b':
      bar();
   default:
      foobar();
   }

JavaScript

if-then-else

   if(s=="Hello World")
   {
       foo();
   }
   else if(s=="Bye World")
   {
       bar();
   }
   else
   {
       deusEx();
   }

OCaml

if-then-else

Compiler: OCaml 3.09 

let condition = true

if condition then
  ()//do something
else
  ()//do something else

match-with

match expression with
 | 0 -> ()//do something
 | 1 -> ()//do something
 | n when n mod 2 = 0 -> ()//do something 
 | _ -> ()//do something

Pascal

if-then-else

Compiler: Turbo Pascal 7.0 

IF condition1 THEN
  procedure1
ELSE
  procedure3;

IF condition1 THEN
  BEGIN
    procedure1;
    procedure2;
  END
ELSE
  procedure3;

IF condition 1 THEN
  BEGIN
    procedure1;
    procedure2;
  END
ELSE
  BEGIN
    procedure3;
    procedure4;
  END;

Perl

if-then-else

Interpreter: Perl 5.8.8 

#!/usr/bin/perl -w

use strict;

my $condition1 = 0;
my $condition2 = 1;

if ( $condition1 ) {
 # Do something
}

# post-conditional if
do_something() if $condition1;



#!/usr/bin/perl -w

use strict;

my $condition1 = 0;
my $condition2 = 1;

if ( $condition1 ) {
 # Do something
} elsif ( $condition2 ) {
 # Do somethine else
}



#!/usr/bin/perl -w

use strict;

my $condition1 = 0;
my $condition2 = 1;

if ( $condition1 ) {
 # Do something
} else {
 # Do something else
}



#!/usr/bin/perl -w

use strict;

my $condition1 = 0;
my $condition2 = 1;

if ( $condition1 ) {
 # Do something
} elsif ( $condition2 ) {
 # Do something else
} else {
 # Do that other thing
}

($condition) ? print "Then\n" : print "Else\n";

# or 

my $var = ($condition) ? "Then" : "Else";

unless

Interpreter: Perl 5.8.8 

#!/usr/bin/perl -w

use strict;

my $condition1 = 0;

unless ( $condition1 ) {
  # Do something
}

# post-conditional unless
do_something() unless $condition1;

unless ( $condition1 ) {
  # Do something
} else {
  # Do something else
}

switch

Interpreter: Perl 5.8.8 

#!/usr/bin/perl -w

use strict;
use Switch;

$input = 42;

switch ($input) {
   case 0 {
     # Do something, because input = 0
   }
   case 1 {
     # Do something, because input = 1
   }
   case "coffee" {
     # Do something, because input = coffee
   }
   else {
     # Do something else.
   }
}

goto

Interpreter: Perl 5.8.8

Typically dispised by most Perl programmers 

goto LABELB;

LABELA: # note labels end with a colon not semi-colon
goto END;

LABELB:
goto LABELA;

END:
exit(0);

PHP

if

Interpreter: PHP 3.x & 4.x & 5.x 

<?php

$foo = 3;

if ($foo == 2)
  //do something

if ($foo == 3)
  //do something
else
  //do something else

if ($foo != 0)
{

  //do something

}

else
{

  //do another thing

}

?>

switch

Interpreter: PHP 3.x & 4.x & 5.x 

<?php

switch ($i)
{

  case "apple":
      echo "i is apple";
      break;

  case "bar":
      echo "i is bar";
      break;

  case "cake":
      echo "i is cake";
      break;

}

?>

Python

if-then-else

if x == 0:
    foo()
elif x == 1:
    bar()
elif x == 2:
    baz()
else:
    boz()

Ruby

if-then-else

if s == 'Hello World'
  foo
elsif s == 'Bye World'
  bar
else
  deus_ex
end


case-when-else

case cartoon_character
when 'Tom'
  chase
when 'Jerry'
  flee
else
  nil
end

ternary

s == 'Hello World' ? foo : bar


SmallTalk

ifTrue/ifFalse

 "Conditionals in Smalltalk are really messages sent to Boolean objects"
  (( balance) > 0)
      ifTrue: [Transcript cr; show: 'still sitting pretty!'.]
      ifFalse: [Transcript cr; show: 'No money till payday!'.].

Visual Basic .NET

if-then-else

Basic 

Dim result As String, a As String = "pants", b As String = "glasses"

If a = b Then
  result = "passed"
Else
  result = "failed"
End If

Condensed 

Dim result As String, a As String = "pants", b As String = "glasses"

If a = b Then result = "passed" Else result = "failed"

If a = b Then
  result = "passed"
Else : result = "failed"
End If

If a = b Then : result = "passed"
Else
  result = "failed"
End If

if-then-elseif

Dim result As String, a As String = "pants", b As String = "glasses"

If a = b Then
  result = "passed"
ElseIf a <> b Then
  result = "failed"
Else
  result = "impossible"
End If

select-case-else

Dim result As String, a As String = "pants", b As String = "glasses"

Select Case a
  Case b
    result = "match"
  Case a : result = "duh"
  Case Else
    result = "impossible"
End Select

inline-conditional

Imports Microsoft.VisualBasic

...

Dim result As String = CType(IIf("pants" = "glasses", "passed", "failed"), String)

generic-inline-conditional

Compiler: Microsoft (R) Visual Basic Compiler version 8.0 

Imports Microsoft.VisualBasic

...

Function IIf2(Of T)(ByVal condition As Boolean, ByVal truepart As T, ByVal falsepart As T) As T
  If condition Then Return truepart Else Return falsepart
End Function

...

Dim result As String = IIf2("pants" = "glasses", "passed", "failed") ' type is inferred


See Also

Retrieved from "https://rosettacode.org/wiki/Conditional_structures?oldid=15641"