Does cond need else Scheme?

Does cond need else Scheme?

The else clause of a cond is optional; if present, that branch will be taken “by default”—if none of the other conditions evaluates to a true value, the else branch will be taken.

How do you write a conditional statement in a Scheme?

Conditions in Scheme Conditional expressions are written with the relational operator (which tells us whether they’re equal, not equal, less than, etc.) before the two values being compared. Compound conditions are written by placing the logical operator (and, or, & not) before the simple logical expression(s).

What is a Scheme expression?

A Scheme expression is a construct that returns a value, such as a variable reference, literal, procedure call, or conditional. Expression types are categorized as primitive or derived. Primitive expression types include variables and procedure calls.

How does let work in Scheme?

In Scheme, you can use local variables pretty much the way you do in most languages. When you enter a let expression, the let variables will be bound and initialized with values. When you exit the let expression, those bindings will disappear.

How does cond work in scheme?

Just as in the above example, cond checks the first argument, finds that ‘birthday does not equal ‘party and goes on to the next argument. Here, ‘cake does equal ‘cake, so Scheme immediately returns ‘have-a-piece. (It does not then go on to return ‘boooooring, since it already found a case that was true).

What is a conditional expression in scheme?

Just as in BASIC, C, Pascal, and most other programming languages, Scheme has a conditional expression. It is the keyword cond. Here is an example (try it out): cond works by searching through its arguments in order.

Why does the Little Schemer always use else instead of T?

The Little Schemer always uses else instead of #t in this context. Either of these is perfectly acceptable to Scheme, but the latter can make your code easier to read. So, in the above example, cond asks if (equal? ‘birthday ‘party) returns #t when evaluated.

What happens if there are no cond-clause s in a form?

If no cond-clause s are present, the result is # . If only a [ else then-body …+] is present, then the then-body s are evaluated. The results from all but the last then-body are ignored. The results of the last then-body, which is in tail position with respect to the cond form, are the results for the whole cond form.