1 #include <stdio.h> 2 #include <stdlib.h>
3
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#define ALIVE 1
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#define DEAD 0
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int main(int argc, char *argv[])
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{
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int BobStatus = ALIVE;
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printf("Bob is "); 10 BobStatus == ALIVE?printf("alive\n"):printf("dead\n"); 11 system("PAUSE"); 12 return 0; 13 }
Purpose of the program
This very short program is designed
to demonstrate the use of the shorthand if - then operator ?:. This program
merely checks the value of the variable BobStatus if it is equal to the define
DEAD then the program prints "Bob is dead" to the screen.
Break down of the code
Line 1 - This line starts with
a '#' hash sign which indicates that it is a preprocessor directive. The
#include directive instructs the preprocessor to copy the contents of the
following file into this source code file at this point. The filename is
surrounded with '<>' brackets this instructs the preprocessor to look in the
system 'include' directory first for this file. So this line copies the contents
of the file 'stdio.h' into the source code file at this point before compiling
starts. The file 'stdio.h' contains the declaration of many basic input output functions
that are used in C programs.
Line 2 - This
line works in the same manner as line 1. The difference is that the preprocessor
copies the contents of the file 'stdlib.h' in to the source code at this point
before compiling starts. The file 'stdlib.h' contains the declaration of many
basic functions used by C programs.
Line 3 - Is
blank.
Line 4 - This
line also starts with a '#' hash sign so is again a preprocessor directive. This
one tells the preprocessor that everywhere it finds 'ALIVE' in the source code
replace it with the value 1.
Line 5 - Like
line 4 this one tells the preprocessor to replace all occurrences of 'DEAD' with
the value 0.
Line 6 - This
line contains the start of our definition of the main function. As you can see
main returns an int value. This value indicates to the operating system whether
or not the program has terminated normally. The main function also takes two
parameters these contain any command line options that are sent to you program
when it starts. E.g. if you start notepad from the command line you would write
notepad if you want to start notepad with a file to edit you would write notepad
readme.txt. readme.txt would be a command line option. This is not a subject
that will be considered further in the book so you may wish to find out more
yourself.
Line 7 - This
line contains the opening brace of the main function. There must be a closing
brace to match every opening brace and vice versa. The closing brace is on line
13.
Line 8 - This
line creates a variable called BobStatus of the type int. It then sets the value
to be ALIVE. When the preprocessor runs it will replace this value with 1.
Line 9 - This
line uses the function printf() which is found in the stdio.h header file. This
function is then instructed to print "Bob is " on to the screen. Since there is
no '\n' new line marker after "Bob is " any further output will appear on the
same line directly after this.
Line 10 - Next
we test the value of the variable BobStatus using the ?: operator. The first
part is the if statement in this case it corresponds to if(BobStatus == ALIVE).
This returns true if BobStatus is equal to ALIVE or false if not. The first
statement after the ? will be executed if BobStatus is equal to ALIVE. So
"alive\n" will be printed out. If BobStatus is not equal to ALIVE then statement
after the : will be executed. Which means that "dead\n" will be printed out. You
can test this by changing line 8 to read int BobStatus = DEAD;
Line 11 - The
function system("PAUSE") is used to pause the program until a key is pressed.
The reason for this is that under windows the console would open, the text would
be output to it then it would close. You would see little more than a flash on
the screen as this happened. Try commenting out the line to see this happen.
Line 12 - The
return statement is used at the end of functions to return an item of data. This
item of data should match the return type declared at the start of the function.
We are in the main function and as you can see in line 6 this should be an
integer. The return statement marks the end of the function any code after this
will not be executed your compiler will usually warn you of this when you
compile the program. The value 0 is a value that tells the
operating system that the program has terminated normally.
Line 13 - This
line contains the closing brace of the main function.