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Booleans and Conditionals

In Scheme, falsity is represented by the value false, written #f. Conceptually, #f is a pointer to a special object, the false object.

Predicates are procedures that return either #t or #f, and don't have side effects. Calling a predicate is like asking a true/false question--all you care about is a yes or no answer.

Try out the "greater-than" predicate >.

Scheme>(> 1 2)

Here we told Scheme to apply the predicate procedure to 1 and 2; it returned #f and Scheme printed that.

The important thing about #f is its use in conditionals. If the first subexpression (the condition) of an if expression returns the value #f, the second subexpression is not evaluated, and the third one is; that value is returned as the value of the if expression.

Try just using the literal value #f as the first subexpression of an if, i.e., the "condition" that controls which branch is taken.

Scheme>(if #f 1 2)

Here the second subexpression was just the literal 2, so 2 was returned.

Now try it using the predicate >

Scheme>(if (> 1 2) 1 2)

This is clearer if we indent it like this, lining up the "then" part (the consequent) and the "else" part (the alternative) under the condition.

Scheme>(if (> 1 2)

This is the right way to indent code when writing a Scheme program in an editor, and most Scheme systems will let you indent code this way when using the system interactively--the you can hit <RETURN>, and type in extra spaces. Scheme won't try to evaluate the expression until you write the last closing parenthesis and hit <RETURN>. This helps you format your code readably even when typing interactively, so that you can see what you're doing.

The false value makes a conditional expression (like an if) go one way, and a true value will make it go another. In Scheme, any value except #f counts as true in conditionals. Try this:

Scheme> (if 0 1 0)

What result value does Scheme print?

One special value is provided, called the true object, written #t. There's nothing very special about it, though--it's just a handy value to use when you want to return a true value, making it clear that all you're doing is returning a true value.

Scheme>(if #t 1 2)
Scheme>(if (> 2 1) 1 2)

Now let's interactively define the procedure min, and then call it:

Scheme> (define (min a b)
           (if (< a b)
Scheme> (min 30 40)

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