Conditionals and Conditional Programs

A cond expression must contain a sequence of condition lines, where each line has exactly two parts:

(cond
  [condition1 answer1] 
  [condition2 answer2] 
  ...) 

(We're using square brackets to surround condition-answer pairs. You can use round parentheses if you prefer.) When a poorly-formed cond expression is typed into DrScheme, DrScheme highlights the incorrect part of the expression. If an individual condition line is highlighted, then only that line is incorrect. If multiple lines are highlighted, then the parentheses are not balanced in an individual line (so DrScheme thinks that multiple lines are a single line).

Just like an expression in a definition, no extra parentheses can be added around the condition or answer expressions in a cond-line. Parentheses cannot be placed around else.

Beyond the Shape Rules

The rules concerning the shape of a cond expression do not specify the kind of condition expressions that are allowed. In fact, a condition expression must have a Boolean value. For example, according to the shape rules for forming a cond expression, the following is a legal expression:

(cond
  [1 2]) 

If you enter this expression in DrScheme, DrScheme tentatively accepts the expression, but then checks whether the condition 1 is #t or #f. It is neither, so DrScheme highlights the 1 and prints an error message.

The following is another legal expression according to the shape rules:

(cond
  [#t 5] 
  [1 2]) 

In this case, DrScheme evaluates the expression and returns 5. Although 1 would be an illegal condition value if the second condition is ever tested, DrScheme never tests it since the first condition is always #t.

The difference between errors reported because a cond expression has the wrong shape (syntax errors) and error reported because the expression returns the wrong kind of result (run-time errors) is explained in 7 Syntax and Semantics in the book.

When a cond expression is evaluated, at least one of the conditions must be #t. If no condition is #t, then DrScheme prints the error ``no matching cond case.'' (This is another kind of run-time error.) For example, the following expression generates the ``no matching cond clause'' error, since there is only one condition and it is #f:

(cond
  [#f 0]) 

A cond expression with an else clause never generates the ``no matching case'' error because the else clause always matches.

Automatic Parenthesis Correction

When you type an close parenthesis ) where an open bracket [ needs to be closed, DrScheme automtically changes ) to ] to close the bracket. For example, typing ) after

(define (positive? n)
 (cond 
   [(> n 0) #t 

produces ], instead of ). Similarly, ] may be automatically converted to ). Automatic parenthesis correction is handy, but if it bothers you, turn it off via the Edit|Preferences menu item.