Editing Control flow (section)

=== 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.