Another looping construct that most languages provide in one way or another is the familiar For Statement. In Pascal the simplest of these loops have a syntax that can be expressed
as exemplified by
the equivalent of which would probably be coded in a C-like language as
The variable exemplified by i is usually known as the control variable. In fact, even though every reader is doubtless familiar with For Statements and has probably used them correctly on many occasions, they have semantic properties that are deceptively complex and many languages impose quite severe constraints on them, affording computer language lawyers ample opportunity to point out subtleties that the beginner might not have considered. We shall explore a few of these in Chapter 13. For the moment let us adopt a syntax suggested by Pascal and pose the following problems.
(a) Allowing the associated Statement to alter the value of the control variable would allow for fairly chaotic behaviour. Consider, for example, the following meaningless code.
How could you attribute the production for a For Statement to prevent such statements?
(b) One school of thought maintains that in a For Statement the control variable should be implicitly declared at the start of the loop, so that it is truly local to the loop – in other words, that a new scope should apply to this variable. Examine the implications of this carefully and come up with a way of implementing it.
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