HamCall (April 1991)/HAMCALL CD-ROM (Buckmaster)(April 1991).BIN/prgming/ctutor/chap06.txt

Text File | 1990-10-14 | 9KB | 236 lines

                                                     Chapter 6

                                            DEFINES AND MACROS



AIDS TO CLEAR PROGRAMMING
______________________________________________________________

Load and display the file named DEFINE.C for    ==============
your first look at some defines and macros.        DEFINE.C
Notice lines 2 through 5 of the program, each   ==============
starting with the word "#define".  This is
the way all defines and macros are defined.  Before the actual
compilation starts, the compiler goes through a preprocessor
pass to resolve all of the defines.  In the present case, it
will find every place in the program where the combination
"START" is found and it will simply replace it with the 0
since that is the definition.  The compiler itself will never
see the word "START", so as far as the compiler is concerned,
the zeros were always there.  Note that if the string is found
in a string constant or in a comment, it will not be changed.

It should be clear to you by now that putting the word "START"
in your program instead of the numeral 0 is only a convenience
to you and actually acts like a comment since the word "START"
helps you to understand what the zero is used for.

In the case of a very small program, such as that before you,
it doesn't really matter what you use.  If, however, you had
a 2000 line program before you with 27 references to "START",
it would be a completely different matter.  If you wanted to
change all of the "START"s in the program to a new number, it
would be simple to change the one #define, but difficult to
find and change all of the references to it manually, and
possibly disastrous if you missed one or two of the
references.

In the same manner, the preprocessor will find all occurrences
of the word "ENDING" and change them to 9, then the compiler
will operate on the changed file with no knowledge that
"ENDING" ever existed.

It is a fairly common practice in C programming to use all
capital letters for a symbolic constant such as "START" and
"ENDING" and use all lower case letters for variable names.
You can use any method you choose since it is mostly a matter
of personal taste.


IS THIS REALLY USEFUL?
______________________________________________________________

When we get to the chapters discussing input and output, we
will need an indicator to tell us when we reach the
end-of-file of an input file.  Since different compilers use

                                                           6-1

                                Chapter 6 - Defines and Macros

different numerical values for this, although most use either
a zero or a minus 1, we will write the program with a "define"
to define the EOF used by our particular compiler.  If at some
later date, we change to a new compiler, it is a simple matter
to change this one "define" to fix the entire program.  In
most C compilers, the EOF is defined in the STDIO.H file.  You
can observe this for yourself by listing the STDIO.H file that
was supplied with your compiler.


WHAT IS A MACRO?
______________________________________________________________

A macro is nothing more than another define, but since it is
capable of at least appearing to perform some logical
decisions or some math functions, it has a unique name.
Consider line 4 of the program on your screen for an example
of a macro.  In this case, anytime the preprocessor finds the
word "MAX" followed by a group in parentheses, it expects to
find two terms in the parentheses and will do a replacement
of the terms into the second definition.  Thus the first term
will replace every "A" in the second definition and the second
term will replace every "B" in the second definition.  When
line 13 of the program is reached, "index" will be substituted
for every "A", and "count" will be substituted for every "B".
Once again, it must be stated that string constants and
comments will not be affected.  Remembering the cryptic
construct we studied a couple of chapters ago will reveal that
"mx" will receive the maximum value of "index" or "count".
In like manner, the "MIN" macro will result in "mn" receiving
the minimum value of "index" or "count".

The results are then printed out.  There are a lot of
seemingly extra parentheses in the macro definition but they
are not extra, they are essential.  We will discuss the extra
parentheses in our next example program.  Be sure to compile
and execute DEFINE.C before going on to the next program.


LETS LOOK AT A WRONG MACRO
______________________________________________________________

Load the file named MACRO.C and display it on    =============
your screen for a better look at a macro and        MACRO.C
its use.  The second line defines a macro        =============
named "WRONG" that appears to get the cube of
"A", and indeed it does in some cases, but it fails miserably
in others.  The second macro named "CUBE" actually does get
the cube in all cases.

Consider the program itself where the CUBE of i+offset is
calculated.  If i is 1, which it is the first time through,
then we will be looking for the cube of 1+5 = 6, which will
result in 216. When using "CUBE", we group the values like

                                                           6-2

                                Chapter 6 - Defines and Macros

this, (1+5)*(1+5)*(1+5) = 6*6*6 = 216.  However, when we use
WRONG, we group them as 1+5*1+5*1+5 = 1+5+5+5 = 16 which is
a wrong answer.  The parentheses are therefore required to
properly group the variables together.  It should be clear to
you that either "CUBE" or "WRONG" would arrive at a correct
answer for a single term replacement such as we did in the
last program.  The correct values of the cube and the square
of the numbers are printed out as well as the wrong values for
your inspection.

In line 5 we define "ADD_WRONG" according to the above rules
but we still have a problem when we try to use the macro in
line 23 and 24.  In line 24 when we say we want the program
to calculate 5*ADD_WRONG(i) with i = 1, we get the result 5*1
+ 1 which evaluates to 5 + 1 or 6, and this is most assuredly
not what we had in mind.  We really wanted the result to be
5*(1 + 1) = 5*2 = 10 which is the answer we get when we use
the macro named "ADD_RIGHT(i)", because of the extra
parentheses in the definition given in line 6.  A little time
spent studying the program and the result will be worth your
effort in understanding how to use macros.

In order to prevent the above problems, most experienced C
programmers include parentheses around each variable in a
macro and additional parentheses around the entire expression.

The remainder of the program is simple and will be left to
your inspection and understanding.



WHAT IS AN ENUMERATION VARIABLE?
______________________________________________________________

Load and display the program named ENUM.C for   ==============
an example of how to use the "enum" type            ENUM.C
variable.  Line 4 contains the first "enum"     ==============
type variable named "result" which is a
variable which can take on any of the values contained within
the parentheses.  Actually the variable "result" is an "int"
type variable but can be assigned any of the values defined
for it.  The names within the parentheses are "int" type
constants and can be used anywhere it is legal to use an "int"
type constant.  The constant "win" is assigned the value of
0, "tie" the value 1, "bye" the value 2, etc.

In use, the variable "result" is used just like any "int"
variable would be used as can be seen by its use in the
program.  The "enum" type of variable is intended to be used
by you, the programmer, as a coding aid since you can use a
constant named "mon" for control structures rather than the
meaningless (at least to you) value of 1.  Notice that "days"
is assigned the values of days of the week in the remainder
of the program.  If you were to use a "switch" statement, it

                                                           6-3

                                Chapter 6 - Defines and Macros

would be much more meaningful to use the labels "sun", "mon",
etc, rather than the more awkward 0, 1, 2, etc.


PROGRAMMING EXERCISE
______________________________________________________________

1.   Write a program to count from 7 to -5 by counting down.
     Use #define statements to define the limits. (Hint, you
     will need to use a decrementing variable in the third
     part of the "for" loop control.










































                                                           6-4