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These are called "Boolean" variables in reference to the notable 19th century mathematician George Boole. Boole's greatest work was entitled An Investigation of the Laws of Thought on Which Are Founded the Mathematical Theories of Logic and Probabilities, published in 1854. Boole reduced logic (the analysis of the truth or falsity of complex propositions) to a simple algebra, thereby incorporating logic into mathematics. Boole's two-valued, or binary, algebra is the simplest form of his more general boolean algebra.

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Technically, the result of applying a C logical operator is of the particular C numeric data type called int.

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Alcock rather misleadingly talks of certain characters being stored in a certain "order" and/or "contiguously"; what he means is that characters are encoded in a certain order or contiguously.

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Its actually even more complicated than this, but I will spare you any further details!

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This possibility is not pursued in detail by Alcock at this stage, but is raised implicitly in Exercise 4 of the chapter.

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Note that the names of separate variables must be textually different, even if only "slightly" so.

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The prototype for the exit() function is in the header file stdlib.h, so you should #include this header at the top of your file if you intend to use the exit() function.

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In particular, you might have a look at the standard library facilities for "process control" described in pagese 171-176 of Illustrating C - though this is not for the faint hearted!

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In general when I refer to the name of a function I include a pair of parentheses after the name, as in main(). I adopt this convention so that, whenever I use a name, you can easily tell whether it is intended as the name of a function or of a variable. In fact, the compiler works much the same way: when it sees a name in an expression, it looks to see whether the name is followed by a left parenthesis, and, if so, it knows that the name is supposed to refer to some function.

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I use the term "sub-function" loosely in this kind of context to distinguish between one "calling" function and a second function which is "called". However, "sub-function" is not a technical term of the C language: to C all functions are "equal" - there is no kind of precise distinction between "functions" and "sub-functions".

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Note that the function for printing out the result will not have to vary or be modified...

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A technical exception here is in the case of array arguments. C has some special rules for what happens when an array name is used in an expression (without any index) - such as when an array is specified as an argument. The nett effect is that arrays "sort of" get passed by reference: but the details of this are beyond the scope of the current discussion.

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Both printf() and scanf() are exceptions to this rule - they can deal with varying numbers of arguments. But this is highly exceptional: most of the library functions we deal with will only accept fixed numbers of arguments; and all of the user defined functions covered in this course will only accept fixed numbers of arguments.

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Again, printf() and scanf() are exceptions to this general rule; but also again, it is beyond the scope of this course to consider this very exceptional behaviour in any further detail!

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In some cases, the prototype for a particular function may be included in several different header files; in such a case you just have to insure that at least one of them is #include'd in your file.

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When I say that the name of the function is "replaced" by the returned value, I do not, of course, mean literally that the text of your program is altered. Rather I mean that the way to understand what happens next at execution time is to imagine the return value appearing in the place occupied by the function name.

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Other functions may be "active" - main() is active all the time, for example - but be temporarily suspended while a sub-function is executing. Their variables still exist, and will become visible again in the Watch window as execution returns to them...

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OK: this is a little white lie. It can be done, but only using the mechanisms of struct data types. This is beyond the scope of the current discussion, but have a browse in Chapter 8 of Illustrating C if you want to explore this further...