# Preconditions and Postconditions

## Exercises

Start DrScheme.

### Exercise 1: Are they all real?

a. Write the `all-real?` procedure described in the accompanying reading.

b. What preconditions should `all-real?` have?

c. Is it necessary to test those preconditions? Why or why not?

### Exercise 2: Differentiating Between Errors

Revise the definition of `greatest-of-list` given in the corresponding reading so that it prints a different (and appropriate) error message for each error condition.

I'd recommend that you use `cond` rather than `if` in writing this revised version.

### Exercise 3: When Can You Count Between?

Revise the definition of the `count-from` procedure presented in the reading on recursion with natural numbers so that it enforces the precondition that its first argument be less than or equal to its second argument.

### Exercise 4: An Odd Factorial

Here is a procedure that computes the product of all of the odd natural numbers up to and including `number`:

```(define odd-factorial
(lambda (number)
(if (= number 1)
1
(* number (odd-factorial (- number 2))))))
```

a. What precondition or preconditions does `odd-factorial` impose on its argument?

b. What will happen if these preconditions are not met?

c. Revise the definition of `odd-factorial` as a husk-and-kernel program in which the husk enforces the precondition.

d. How can we be certain, in this case, that none of the recursive calls we make to the kernel procedure violates the precondition?

### Exercise 5: Finding Values

a. Describe (using the six-P style), define, and test a procedure named `index` that takes a symbol `sym` and a list `ls` of symbols as its arguments and returns the index of `sym` in `ls`. You should use 0-based indices (so that the initial value in a list is at index 0).

```> (index 'gamma (list 'alpha 'beta 'gamma 'delta))
2
> (index 'easy (list 'easy 'medium 'difficult 'impossible))
0
> (index 'the (list 'and 'the 'cat 'sat 'on 'the 'mat))
1
```

b. Arrange for `index` to signal an error (by invoking the `error` procedure) if `sym` does not occur at all as an element of `ls`.

### Exercise 6: Substitution

Describe (using the six P style), define, and test a procedure named `substitute` that takes three arguments -- a symbol `new`, another symbol `old`, and a list `ls` of symbols -- and returns a list just like `ls` except that every occurrence of `old` has been replaced with an occurrence of `new`. Use the husk-and-kernel structure to make sure that `new` and `old` are symbols and that `ls` is a list of symbols before starting into the recursion.

```> (substitute 'alpha 'omega (list 'phi 'chi 'psi 'omega 'omega)
(phi chi psi alpha alpha)
> (substitute 'starboard 'port (list 'port 'starboard 'port 'port))
(starboard starboard starboard starboard)
> (substitute 'in 'out null)
()
> (substitute "in" 'out null)
substitute: expected a symbol as first parameter
> (substitute 'in 'out (list 'alpha "beta" 23))
substitute: expected a list of symbols as third parameter
```

## History

Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

This document was generated by Siteweaver on Tue May 6 09:29:08 2003.
The source to the document was last modified on Mon Feb 24 13:14:29 2003.
This document may be found at `http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2003S/Labs/prepost.html`.

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Samuel A. Rebelsky, rebelsky@grinnell.edu