Like the arithmetic operations, the test operations can take more than
two arguments. For example, (< 1 2 3) is legal Scheme
notation for ``1 < 2 and 2 < 3.'' The and and
or operators also take any number of arguments. The
not operation always takes one argument. The and,
or, and not operators work on Boolean values only.
Exercise Notes
Exercise 3.3.1 The = operator works on numbers,
only, so trying to test a Boolean function using the pattern
(= (in-interval-1? 0) false)
false, you
can create a test claim using not, instead:
(not (in-interval-1? 0))
true, it needs no
wrapper.
A cond expression must contain a sequence of
condition lines, where each line has exactly two parts:
(cond [condition1 answer1] [condition2 answer2] ...)
cond expression is typed into DrScheme, DrScheme highlights
the incorrect part of the expression as a syntax error. 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 Syntax
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 syntax for forming a cond
expression, the following is a legal expression:
(cond [1 2])
1 is
true or false. It is neither, so DrScheme highlights the
1 and prints an error message. This is a run-time error.
The following is another legal expression according to the syntax
rules:
(cond [true 5] [1 2])
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 true.
When a cond expression is evaluated, at least one of the
conditions must be true. If no condition is true,
then DrScheme prints the run-time error ``no matching cond case.''
For example, the following expression generates the ``no matching
cond clause'' error, since there is only one condition and it is
false:
(cond [false 0])
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) true
], 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.
No DrScheme notes for this section.