Editing Control flow (section)

== Choice ==
=== If-then-(else) statements ===
{{main article|Conditional (computer programming)}}
Conditional expressions and conditional constructs are features of a [[programming language]] that perform different computations or actions depending on whether a programmer-specified [[Boolean data type|Boolean]] ''condition'' evaluates to true or false. 
* <code>IF..GOTO</code>. A form found in unstructured languages, mimicking a typical machine code instruction, would jump to (GOTO) a label or line number when the condition was met.
* <code>IF..THEN..(ENDIF)</code>. Rather than being restricted to a jump, any simple statement, or nested block, could follow the THEN key keyword. This a structured form.
* <code>IF..THEN..ELSE..(ENDIF)</code>. As above, but with a second action to be performed if the condition is false. This is one of the most common forms, with many variations. Some require a terminal <code>ENDIF</code>, others do not. [[C (programming language)|C]] and related languages do not require a terminal keyword, or a 'then', but do require parentheses around the condition.
* Conditional statements can be and often are nested inside other conditional statements. Some languages allow <code>ELSE</code> and <code>IF</code> to be combined into <code>ELSEIF</code>, avoiding the need to have a series of <code>ENDIF</code> or other final statements at the end of a compound statement.

{| class="wikitable"
|-
! [[Pascal (programming language)|Pascal]]:
! [[Ada (programming language)|Ada]]:
|-
|<syntaxhighlight lang="pascal">
if a > 0 then
  writeln("yes")
else
  writeln("no");
</syntaxhighlight>
|<syntaxhighlight lang="ada">
if a > 0 then
      Put_Line("yes");
else
      Put_Line("no");
end if;
</syntaxhighlight>
|-
! [[C (programming language)|C]]:
! [[Shell script]]:
|-
|<syntaxhighlight lang="c">
if (a > 0) { 
    puts("yes");
}
else {
    puts("no");
}
</syntaxhighlight>
|<syntaxhighlight lang="bash">
if [ $a -gt 0 ]; then
      echo "yes"
else
      echo "no"
fi
</syntaxhighlight>
|-
! [[Python (programming language)|Python]]:
! [[Lisp (programming language)|Lisp]]:
|-
|<syntaxhighlight lang="python">
if a > 0: 
    print("yes")
else:
    print("no")
</syntaxhighlight>
|<syntaxhighlight lang="lisp">
(princ
  (if (plusp a)
      "yes"
      "no"))
</syntaxhighlight>
|}
Less common variations include:
* Some languages, such as early [[Fortran]],{{efn|In Fortran, this statement was deemed obsolescent in Fortran-90, and deleted as of Fortran 2018.}} have a ''three-way'' or ''[[arithmetic if]]'', testing whether a numeric value is negative, zero, or positive.
*Some languages have a [[Functional programming|functional]] form of an <code>if</code> statement, for instance [[Lisp (programming language)|Lisp's]] <code>cond</code>.
*Some languages have an [[Operator (programming)|operator]] form of an <code>if</code> statement, such as C's [[ternary operator]].
* [[Perl]] supplements a C-style <code>if</code> with <code>when</code> and <code>unless</code>.
* [[Smalltalk]] uses <code>ifTrue</code> and <code>ifFalse</code> messages to implement conditionals, rather than any fundamental language construct.

=== Case and switch statements ===
{{Main article|Switch statement}}
[[Switch statement]]s (or ''case statements'', or ''multiway branches'') compare a given value with specified constants and take action according to the first constant to match. There is usually a provision for a default action ("else", "otherwise") to be taken if no match succeeds. Switch statements can allow compiler optimizations, such as [[lookup table]]s. In [[dynamic language]]s, the cases may not be limited to constant expressions, and might extend to [[pattern matching]], as in the [[shell script]] example on the right, where the <code>*)</code> implements the default case as a [[glob (programming)|glob]] matching any string. Case logic can also be implemented in functional form, as in [[SQL]]'s <code>decode</code> statement.<!--Perl's implementation of case as a lookup table-->

{| class="wikitable"
|-
! [[Pascal (programming language)|Pascal]]:
! [[Ada (programming language)|Ada]]:
|-
|<syntaxhighlight lang="pascal">
case someChar of
  'a': actionOnA;
  'x': actionOnX;
  'y','z':actionOnYandZ;
  else actionOnNoMatch;
end;
</syntaxhighlight>
|<syntaxhighlight lang="ada">
case someChar is
  when 'a' => actionOnA;
  when 'x' => actionOnX;
  when 'y' | 'z' => actionOnYandZ;
  when others => actionOnNoMatch;
end;
</syntaxhighlight>
|-
! [[C (programming language)|C]]:
! [[Shell script]]:
|-
|<syntaxhighlight lang="c">
switch (someChar) {
  case 'a': actionOnA; break;
  case 'x': actionOnX; break;
  case 'y':
  case 'z': actionOnYandZ; break;
  default: actionOnNoMatch;
}
</syntaxhighlight>
|<syntaxhighlight lang="bash">
case $someChar in 
   a)    actionOnA ;;
   x)    actionOnX ;;
   [yz]) actionOnYandZ ;;
   *)    actionOnNoMatch  ;;
esac
</syntaxhighlight>
|-
! [[Lisp (programming language)|Lisp]]:
! [[Fortran]]:
|-
| <syntaxhighlight lang="lisp">
(case some-char
  ((#\a)     action-on-a)
  ((#\x)     action-on-x)
  ((#\y #\z) action-on-y-and-z)
  (else      action-on-no-match))
</syntaxhighlight>
| <syntaxhighlight lang="fortran">
select case (someChar)
  case ('a')
    actionOnA
  case ('x')
    actionOnX
  case ('y','z')
    actionOnYandZ
  case default
    actionOnNoMatch
end select
</syntaxhighlight>
|}