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Thursday, March 1, 2012

C LANGUAGE TUTORIAL 6.Defines and Macros

6.Defines and Macros


6.1 Defines And Macros Are Aids To Clear Programming

Load and display the file named define.c for your first look at some defines and macros.

#define START  0  /* Starting point of loop */
#define ENDING 9  /* Ending point of loop  */
#define MAX(A,B)  ((A)>(B)?(A):(B))  /* Max macro definition */
#define MIN(A,B)  ((A)>(B)?(B):(A))  /* Min macro definition */
main( )
{
int index,mn,mx;
int count = 5;
for (index = START;index <= ENDING;index++) {
mx = MAX(index,count);
mn = MIN(index,count);
printf("Max is %d and min is %d\n",mx,mn);
}
}

Notice the first four lines 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. 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 the START, it
would be a completely different matter. If you wanted to change all of the STARTS in the
program to a new number, it would be simple to change the one #define, but difficult, and
possible disastrous if you missed one or two of the references.

In the same manner, the preprocessor will find all occurrence 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.

6.2 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 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 (end-of-file) 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. End-of-line is another indicator that is not universal. This will make more sense when
we get to the chapters on input and output.

6.3 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 the
third line 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 12 of the program is reached, "index" will be
substituted for every "A", and "count" will be substituted for every "B". 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, they are essential. We will discuss the extra parentheses in our next
program.
Compiler and run define.c.

6.4 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
its use.

#define WRONG(A) A*A*A /* Wrong macro for cube */
#define CUBE(A) (A)*(A)*(A) /* Right macro for cube */
#define SQUR(A) (A)*(A) /* Right macro for square */
#define START 1
#define STOP 9
main( )
{
int i,offset;
offset = 5;
for (i = START;i <= STOP;i++) {
printf("The square of %3d is %4d, and its cube is %6d\n",
i+offset,SQUR(i+offset),CUBE(i+offset));
printf("The wrong of  %3d is %6d\n",i+offset,WRONG(i+offset));
}
}

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

6.5 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.

next lesson


C LANGUAGE TUTORIAL 7.STRINGS AND ARRAYS

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