Assignment

Exercise 1

Define a global variable named *counter*, giving it the initial value 0. This variable will be used in subsequent exercises.

(When a Scheme programmer expects that the value of a global variable will be changed by assignments, she conventionally gives it a name beginning and ending with a star. This is not a requirement of the language, but a convention that makes programs more readable.)

Exercise 2

Write a procedure named bump-counter! that takes no arguments and is invoked for its side effect, which is to increase the value of *counter* by 1.

Exercise 3

Write a procedure named reset-counter! that takes no arguments and is invoked for its side effect, which is to change the value of *counter* to 0.

Exercise 4

Write a procedure named report-counter that takes no arguments and returns the current value of *counter*.

> (bump-counter!)
> (report-counter)
1
> (bump-counter!)
> (bump-counter!)
> (bump-counter!)
> (report-counter)
4
> (reset-counter!)
> (report-counter)
0

Exercise 5

Define a procedure named invocations that takes no arguments and returns a natural number indicating the number of times it has been invoked. (Use a static variable to keep track of this number.)

> (invocations)
1
> (invocations)
2
> (invocations)
3
> (invocations)
4

Exercise 6

Define a procedure named parm that takes either one argument or none and controls one static variable. Given no arguments, parm should return the value of its static variable; given one argument, it should assign the argument to the static variable, returning an unspecified value.

Exercise 7

Define a unary procedure named list-keeper. Given any argument other than the symbol :report, list-keeper should return the symbol ok. When the argument is :report, however, list-keeper should return a list of the arguments from all of the invocations of it since the last one with the :report argument:

> (list-keeper ':report)
()
> (list-keeper 'start)
ok
> (list-keeper 'next)
ok
> (list-keeper 2)
ok
> (list-keeper "three")
ok
> (list-keeper 'finish)
ok
> (list-keeper ':report)
(start next 2 "three" finish)
> (list-keeper 'start-again)
ok
> (list-keeper 'keep-going)
ok
> (list-keeper ':report)
(start-again keep-going)

Hint: Save the arguments of the non-report calls in a list that is the value of a static variable.

Exercise 8

Design and write a procedure direction-counter that takes one argument, which must be one of the five symbols north, east, south, west, or :report. Given any of the first four symbols, it should return the symbol ok; given the symbol :report, it should display an appropriately formatted summary of the number of times it has been invoked with each argument since the last report.

> (direction-counter ':report)
north 0
east 0
south 0
west 0
> (direction-counter 'east)
ok
> (direction-counter 'east)
ok
> (direction-counter 'south)
ok
> (direction-counter 'east)
ok
> (direction-counter 'north)
ok
> (direction-counter 'south)
ok
> (direction-counter 'east)
ok
> (direction-counter ':report)
north 1
east 4
south 2
west 0