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As with any Turing-complete language, Scheme has a mechanism for comparing different values and branching two or more possibilities based on the comparison.
The basic conditional special forms are (if), (case) and (cond). (If) is used for a simple two-way branch or skip:
(if (condition) (true-expression))or
(if (condition) (true-expression) (false-expression))
The if form returns the value of the clause that it runs; if the condition is false and there is no else clause, the result is undefined.
the (case) special form performs a multi-path branch based on a single value:
(case value (comparison-1 (block-1))
(comparison-2 (block-2))
; ...
(else (default-block)))
If value matches one of the comparison values in a list, then the following expression block is executed.
The expression sections are blocks, and can take more than one expression.
The (cond) special form is the most general of the conditionals; it accepts a set of condition-expression pairs and selects through them in order. As with (case), the expression clause is a block.
(cond ((cond-1) (block-1))
((cond-2) (block-2))
; ...
(else (default-block))
In addition to the selectors and comparators, there are a set of predicates, special forms which accept an argument and determine the arguments membership in a particular type. Predicates are customarily suffixed with a question mark to indicate what they are, but exceptions exist. Important predicates include (null?), (list?), (atom?), (string?), (char?). These dynamic type-checking forms are used in place of the static type checking used in most languages.