LISP - Do Construct

The do construct is also used for performing iteration using LISP. It provides a structured form of iteration.

The syntax for do statement −

(do ((variable1    value1   updated-value1)
      (variable2   value2   updated-value2)
      (variable3   value3   updated-value3)
   ...)
   (test return-value)
   (s-expressions)
)

The initial values of each variable is evaluated and bound to the respective variable. The updated value in each clause corresponds to an optional update statement that specifies how the values of the variables will be updated with each iteration.

After each iteration, the test is evaluated, and if it returns a non-nil or true, the return-value is evaluated and returned.

The last s-expression(s) is optional. If present, they are executed after every iteration, until the test value returns true.

Example

Create a new source code file named main.lisp and type the following code in it −

(do ((x 0 (+ 2 x))
   (y 20 ( - y 2)))
   ((= x y)(- x y))
   (format t "~% x = ~d  y = ~d" x y)
)

When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result returned is −

x = 0  y = 20
x = 2  y = 18
x = 4  y = 16
x = 6  y = 14
x = 8  y = 12