4 Processes

Processes that can be applied to data

Having looked at some forms of data, we now turn our attention to processes that can be applied to data. Each process that we consider in this section will input data of a specified form, and will result in a corresponding value. For example, one process, which we will call ASC, takes a character as input, and has as its resulting value the integer giving the ASCII code of the input character (as listed in Table 2). Another process, which we will call SIZE, takes a sequence as input, and has as its resulting value the length of the sequence (which will be an integer).

It is important to distinguish between a description of the outcome required when a process is applied to a form of data, and a description of the exact steps to be taken to achieve the desired outcome. Here, we are concerned only with the first of these; that is, with providing an overview of a computing process. You might like to think of this as a “black box” view of the process. We do not, at this stage, care how one obtains the output value.

Certain processes change the state of a named variable. For now, though, we shall not think about processes in that way. Each process that we are concerned with here simply produces a value that depends on one (or more) input value(s). Another important feature of the processes that we shall consider is that they result in a value that is entirely predictable. One can envisage a process that takes a string as input, and which results in a value that is a character that appears in the string. With input “This”, the value resulting from such a process might be any of the four characters ‘T’, ‘h’, ‘i’ or ‘s’. We will not consider a process such as that. Our attention will be confined to processes that, if given the same input twice, will produce the same resulting value each time.

Below, we give a number of examples of processes that may be applied to numbers, characters or sequences. Focus first on Example 1(a) below This process is applied to an input, comprising data of a specified form (a sequence of integers). Application of the process results in a value (the sum of the integers in the input sequence). This resulting value will be referred to as the value returned by the process. Similar comments apply to each of these examples. Each process takes data of a specified form as an input, and each process returns a value, based on this input data. The returned value may also be called the output of the process.

Example 1

• (a) Given a sequence of integers, return the sum of all the numbers in the sequence. For example, given [6, 2, 5, 6] the value returned is 19.
• (b) Given a string, return the first upper-case letter that appears in the string. For example, given the string “my name is Bob”, the value returned is ‘B’.
• (c) Given a string, return the first character in the string. For example, given the string “This is a sentence.”, the value returned is ‘T’.
• (d) Given some integer, x, say, return the integer x ² + x. For example, given the number 4, the value returned is 4² + 4 = 20.
• (e) Given a sequence of items from a set X, and another item from the set X, return the sequence formed by placing the item at the end of the sequence. For example, given the sequence of integers [6, 2, 5, 6] and the integer 1, the value returned is [6, 2, 5, 6, 1].
• (f) Given a string and a character, return the position in the string where the given character first occurs (counting characters from the front of the string). For example, if the string “This is a sentence.” and the character ‘s’ are given, then the value returned is 4, since the first occurrence of ‘s’ in the string is at the fourth place in the sequence [‘T’,‘h’,‘i’,‘s’,‘’,‘i’,‘s’, . . . ].

Here are some key questions to consider about each process.

• How many inputs are required, and of what type?
• What type of value does the process return?
• What is the relationship between the returned value and the input(s)?
• Are there any restrictions on the input(s)?
• Does the process always produce an output? Given any permitted input, is there an associated returned value?
• Is the output produced predictable? Given the same input, will the process always return the same value?

For example, consider Example 1(b) above. In that case the questions can be answered as follows. The process has just one input and this is a sequence of characters. It outputs a character. The output value is the first upper-case letter that appears in the sequence of characters. For this description of the output to yield any value, the input sequence needs to contain at least one upper-case letter. Given that restriction on the input, there will always be an output. The output is predictable.