 Data and processes in computing

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Exercises on Section 4

Exercise 9

For each of the following functions, give the signature and suggest a suitable precondition.

• (a) TOUPPER, which changes a string, containing only lower-case letters, into the corresponding string consisting of upper-case letters. (So, for example, TOUPPER(“word”) = “WORD”.)

• (b) REMOVELAST, which, given a sequence, deletes its last member.

• (c) LAST, which returns the last member of a sequence.

Solution

• (a) This function inputs a string and outputs a string, therefore it has signature TOUPPER : SeqOfCharSeqOfChar. It requires a precondition that the input sequence consists entirely of lower-case letters.

• (b) The members of the sequence may come from any set, which we will denote by X. The input is a sequence and the returned value is also a sequence. So the signature is REMOVELAST : SeqOfXSeqOfX . The input sequence should contain at least one member (otherwise there is no last member to delete), so we need a precondition that the input sequence is not empty.

• (c) This is similar to (a), except that the returned value is an item from X, rather than a sequence. The signature is LAST : SeqOfXX. We again need the precondition that the input sequence is not empty.

Exercise 10

Suppose that the days of the week are represented by numbers, with Mon-day represented by 1, Tuesday represented by 2, and so on, with Sunday represented by 7. Describe a function TOMORROW2, giving the next day with Days represented in this way.

Solution

You may have given this description.

function TOMORROW2(d in Int) return in Int

pre d is in the set {1,2,3,4,5,6,7}.

post The returned value is t, where:

• if d = 1 then t = 2

• if d = 2 then t = 3

• if d = 3 then t = 4

• if d = 4 then t = 5

• if d = 5 then t = 6

• if d = 6 then t = 7

• if d = 7 then t = 1.

Or you may have given a shorter description, such as:

function TOMORROW2(d in Int) return in Int

pre 1 ≤ d ≤ 7.

post The returned value is t, where:

t = d + 1 if 1 = d = 6;

t = 1 if d = 7.

Either of these answers is fine.

Exercise 11

• (a) With s = “abcd”, evaluate each of:

• (i) AT(1, s);

• (ii) PUT(2, s, AT(1, s)).

• (b) For a general sequence s, describe how the sequence PUT(2, s, AT(1, s)) is obtained from s.

The functions AT and PUT were described in Activity 16.

Solution

• (a)(i) With s = “abcd”, AT(1,s) = ‘a’.

• (ii) Then PUT(2,s,AT(1,s)) = PUT(2,“abcd”,‘a’) = “aacd”.

• (b) In general, PUT(2,s,AT(1,s)) is formed by replacing the second member of the sequence s with a copy of the first member of s.

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