Conditionals (if-else) in Programming

Generalized if[-else]: if-elseif-...[-else]

A common generalization of if-[else] is a sequence of one or more conditions and associated branches.

The final conditon usually allows a special else, equivalent to using literal true.

The semantics of this generalization can be achieved by repeated nesting inside the else branches of if-else. The common syntactic convention is then not to increase the indentation with each nesting. Java, C and C++ take this approach.

 if (c1) {
   s1();
 } else {
   if (c2) {
     s2();
   }
 }

 if (c1) {
   s1();
 } else if (c2) {
   s2();
 }

 if (c1) {
   s1();
 } else {
   if (c2) {
     s2();
   } else {
     if (c3) {
       s3();
     } else {
       s4();
     }
   }
 }

 if (c1) {
   s1();
 } else if (c2) {
   s2();
 } else if (c3) {
   s3();
 } else {
   s4();
 }

Some languages also have a dedicated syntax for the generalization.

In Python, indentation has semantics interfering with unindenting of a nested if, so an extension to the if-else syntax is provided.

 if c1:
   s1()
 else:
   if c2:
     s2()
   else:
     if c3:
       s3()
     else:
       s4()

 if c1:
   s1()
 elif c2:
   s2()
 elif c3:
   s3()
 else:
   s4()

In Scheme, unindenting a nested if doesn't change the semantics, but breaks the syntactic convention for indentation of parentheses. A separate syntax that doesn't break this convention is provided that also removes the redundant "if"s, but now requires "else" be explicit.

 (if c1
   (s1)
   (if c2
     (s2)
     (if c3
       (s3)
       (s4))))

 (cond
   (c1 (s1))
   (c2 (s2))
   (c3 (s3))
   (else (s4)))