Sections 1.21.3 of Absolute C++
Lecture
Suppose I want to compute something like (3.55  17.017)^3. I can do this with C++, but "^" doesn't mean exponentiation in C++, so instead I need to write:
cout << (3.55  17.017)*(3.55  17.017)*(3.55  17.017) << endl;
This is a bit of a hassle. Normally we would think of first computing (3.55  17.017), then taking the resulting value and cubing it. Hopefully we think something like: "let x be (3.55  17.017) and compute x^3." We need a variable in which to store the value (3.55  17.017)!
So we try creating the following program: 
#include <iostream> int main() { x = 3.55  17.017; std::cout << x*x*x << std::endl; return 0; } 
When we compile this, we get an error message like
error line 4: 'x' is an undeclared identifier
What the compiler is saying is this: "x??? you never told me there was going to be an x!" If you want to use x to store the value (3.55  17.017), you need to first tell the compiler that the name x is going to stand for a number  this is called declaring the variable x.
The statement double
x;
declares that x is a variable of type double,
which means that it's a variable that can stand for numbers with decimal
points. The type of a variable tells you what kind of objects can be
stored in the variable. There are many different types in C++, and in
fact understanding types is one of the most important skills you'll learn in
this course.
The =
operator assigns a value to the variable x.
To make the program work, we write: 
#include <iostream> int main() { double x; //declare x as an object x = 3.55  17.017; //assign x the value 3.55  17.017 std::cout << x*x*x << std::endl; //print out x^3 return 0; } 
Once x has been declared as a variable of
type double, it can be used in the program just like a normal number, either to
print or to use in arithmetic expressions, like
cout
<< x*x*x <<
endl
,
which does both.
What really goes on here is that space in your
computer's memory is allocated for one object of type double
, and given the name x.
When you ask for the value of x (by involving it in an arithmetic
expression or by trying to print it) the computer fetches a copy of the value
from memory. When you use the = operator, the computer takes the value on the
right hand side, and copies it into the space reserved for x.
Thus, strange sequences like:
double x;
x = 3.5;
x = 2.4;
... make perfect sense.
After the statement double x;
,
space is reserved for x, though we have no idea what actual value is in there 
at this point x is uninitialized. The
statement x = 3.5;
copies
the value 3.5 into the space reserved for x. Finally, the statement
x = 2.4;
copies the value 2.4 into the
space reserved for x, thus overwriting the 3.5 that had been there
previously.
Legal Variable Names
Can you use any word or letter to represent a variable name? The answer is NO. A variable name must begin with a letter (lowercase or uppercase) or an underbar ( _ ). It may be any length you want (Anything after the first 32 characters will be ignored.) but it can only contain letters, digits, and the underbar. Except in special situations, the use of the underbar to begin a variable name should be avoided. There are a special class of keywords reserved for use by C++ and you may not use one of them to name a variable. Examples of keywords are double, cout, and endl. As we proceed, it will become obvious what keywords cannot be variable names but a complete list can be found in Appendix A of your textbook.
C++ distinguished between uppercase and lowercase. As a result, Answer and answer will be considered different variable names. A very common mistake that beginning programmers make is to be sloppy in writing variable names, sometimes using capitals and sometimes not. It is not good programming practice to use two variables names that are spelled the same except for capitalization because it leads to errors. Your source code will be more easy to understand by mere mortals (interpret this to mean the instructor grading your programs) if you use meaningful variables names.
We've already seen how to output information from a
program using cout
.
In C++ (and in many other places) we refer to an output stream, the
idea being that each thing we write goes out sequentially in the order we write
it. In exactly the same way, we read from an input stream. Not
surprisingly (given that our output stream is
cout
) our input stream object is
called cin
.
[Note: Really it's called std::cin
,
but if we use using namespace std
we may drop the std::
part. It's defined in the iostream
library, just like cout
.]
Code 
double x, y; cin >> x >> y; 
User Types 
12.0 25.0 
Effect 

You may only read into variables, so cin
>> 1.5
will cause a compiler
error. (And what would it mean????) When reading into double x
, cin
skips any spaces, tab's, or newlines until it comes
across a number. So, for example:
Code 
double x, y, z; cin >> x >> y >> z; 
User Types 
12.0 25.0 7.1 
Effect 

Putting this together, we can construct a very
simple program Addition
Calculator, which reads in two numbers from the user and adds them together.
Notice that the variable that contains the sum of the two numbers input by the
user is actually called sum
.
This is just to enhance the readability of my code. I could've called the
variable "George" and it would've worked just the same.
double
s?Very few of the mathematical operations we
typically use with real numbers are part of the core C++ language. In fact, the
only operations you'd recognize from math class are +, , *, and /. Operations
like exponentiation, square roots, logs, and so on are not part of the core
language, but they are available to you in libraries. Libraries extend
the functionality of C++. There is a set of standard libraries that
are an official part of the language (Here
is some online documentation on the standard libraries.), and the iostream
library we've been using is one example. There is a library called cmath
that allows you to perform other operations on double
s, like square roots and sines
and logarithms.
Here is a list of problems and solutions. With even the very basic construction of input, output, variables, and expressions, we can write some useful programs.
· Fahrenheit  Celsius Conversion
·
Roots
of Quadratic Polynomials This requires the
sqrt
function, so it includes a
brief description of the cmath
library. "Libraries" extend the base C++ language.
· Compound Interest Calculation. There are two solutions given for this problem, Version 1, and Version 2. Version 2 uses variable reassignment for the first time.
Last modified by LT M. Johnson 08/15/2007 09:11 AM