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Lesson 1: The basics of
C++
I am writing this for those people who want to learn how
to program in
C++, especially those who had trouble. It is for those
of you who want a sense of accomplishment every time
your program works perfectly. If you want the sense of
accomplishment, read on.
C++ is a programming language. It is a programming
language of many
different dialects, just like each language that is
spoken has many dialects. In C though, they are not
because the "speakers" live in the North, South, or grew
up in some other place, it is because there are so many
compilers. There are about four major ones: Borland C++,
Microsoft Visual C++, Watcom C/386, and DJGPP. You can
download DJGPP http://www.delorie.com/djgpp/ or you may
already have another compiler.
Each of these compilers is a little different. The
library functions of one will have all of the standard
C++ functions, but they will also have other functions
or, continuing the analogy, words. At times, this can
lead to confusion, as certain programs will only run
under certain compilers, though I do not believe this to
be the case with the programs in these tutorials.
++++++++++++++++++++++++++++++++++++++++++++++++++++++
If you don't have a compiler, I strongly suggest you get
one. A simple one is good enough for
my tutorials, but get one.
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C++ is a different breed of programming language. It has
only a few keywords for DOS, and it has no keywords to
use for output. This means that almost everything is
stored in a header file. This gives the use of many
functions. But lets see a real program...
#include <iostream.h>
int main()
{
cout<<"HEY, you, I'm alive! Oh, and Hello World!";
return 0;
}
That does not look too hard, right? Lets break down the
program and then look at it. The #include is a
preprocessor directive which tells the compiler to put
code in the header file iostream.h into our program! By
including header files, you ~can gain access to many
different functions. For example, the cout function
requires iostream.h.
The next thing is int main() what this is saying is that
there is a function called main, and that it returns an
integer, hence int. Then those little braces ( { and } )
are used to signal the beginning and ending of
functions, as well as other code blocks. If you have
programmed in Pascal, you will know them as BEGIN and
END.
The next line of the program may seem strange. If you
have programmed in other languages you might think that
print would be used to display text. However, in C++ the
cout function is used to display text. It uses the <<
symbols, known as insertion operators. The quotes tell
the compiler that you want to output the literal string
as-is. The ; is added to the end of all function calls
in C++.
The penultimate line of code is ordering main to return
0. When one returns a value to main, it is passed on to
the operating system. As a note, declaring int main() or
void main() both will generally work. It is accepted
practice to some to declare main as a void, but to
others it is
extremely upsetting. Previously, these tutorials had
used void main, however, this is NO LONGER recommended,
as it does not conform to the ANSI standard.
After, the brace closes off the function. You can try
out this program if you want, just cut and paste it into
the IDE of a compiler such as DJGPP, or save it to a
file ending with a .cpp extension, and use a
command-line compiler to compile and link it.
Comments are extremely important to understand. When you
declare that an area is a comment, the compiler will
IGNORE it. To comment it is possible to use either // ,
which declares that the entire line past that point is a
comment, or it is possible to use /* and then */ to
block off everything between the two as a comment.
Certain compilers will change the color of a commented
area, but some will not. Be certain not accidently
declare part of your code a comment. Note that this is
what is known as "commenting-out" a section of code, and
it is useful when you are debugging.
So far you should be able to write a simple program to
display information typed in by you, the programmer.
However, it is also possible for your program to accept
input. the function you use is known as cin>>.
Wait! Before you can receive input you must have a place
to store input! In programming, these locations where
input and other forms of data are stored, are called
variables. There are a few different types of variables,
which must be stated. The basic types are char, int, and
float.
Char is used to create variables that store characters,
int is used to create variables that store integers
(numbers such as 1, 2, 0, -3, 44, -44), and float is
used to delare numbers with decimal places. In fact,
they are all keywords that are used in front of variable
names to tell the compiler that you have created a
variable. That is known as "declaring a variable". When
you declare a variable, or variables, you must end the
line with a semi-colon, the same as if you were to call
a function. If you do not declare the variable you are
attempting to use, you will receive numerous error
messages and the program will not run.
Here are some examples of declaring variables:
int x;
int a, b, c, d;
char letter;
float the_float;
It is not possible, however, to declare two variables of
different types with the same name.
#include <iostream.h>
int main()
{
int thisisanumber;
cout<<"Please enter a number:";
cin>>thisisanumber;
cout<<"You entered: "<<thisisanumber;
return 0;
}
Let's break apart this program and examine it line by
line. Int is the keyword that is used when delcaring a
variable which is an integer. The cin>> sets the value
of thisisanumber to be whatever the user types into the
program when prompted. Keep in mind that the variable
was declared an integer, which means the output will be
in the form of an integer. Try typing in a sequence of
charaters, or a decimal when you run the example program
to see what you get as a response. Notice that when
printing out a variable, there are not any quotation
marks. If there were quotation marks, the output would
be "You Entered: thisisanumber." Do not be confused by
the inclusion of two separate insertion operators on a
line. It is allowable, as long as you make certain to
have each separate output of variable or string with its
own insertion operator. Do not try to put two variables
together with only one << because it will give you an
error message. Do not forget to end functions and
declarations with the semi-colon(;). Otherwise you will
get an error message when you try to compile the
program.
Now that you know a little bit about variables, here are
some ways to manipulate them. *, -, +, /, =, ==, >, <
are all operators used on numbers, these are the simple
ones. The * multiplies, the - subtracts, and the + adds.
Of course, the most important for changing variables
is the equal sign. In some languages, = checks if one
side is equal to the other side, but in C++ == is used
for that task. However, the equal sign is still
extremely useful. It sets the left side of the equal
sign, which must be one AND ONLY one variable, equal to
the right side. The right side of the equal sign is
where the other operators can be used.
Here are a few examples:
a=4*6; //(Note use of
comments and of semi-colon) a is 24
a=a+5; // a equals the original value of a with five
additional units
a==5 //Does NOT assign five to a. Rather, it checks to
see if a equals 5.
The other form of equal, ==, is not a way to assign a
value to a variable. Rather, it checks to see if the
variables are equal. It is useful in other areas of C++
such as if statements and loops.
You can probably guess what the < and > are for. They
are greater than and less than checks.
For example:
a<5 //Checks to see if a is
less than five
a>5 //Checks to see if a is greater than five
a==5 //Checks to see if a equals five, for good measure |
