Class 3: Types & Expressions I


Sections 1.2-1.3 of Absolute C++




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.


User Input

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.]



double x, y;
cin >> x >> y;

User Types



x gets the value 12.0, y gets the value 25.0.

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:


double x, y, z;
cin >> x >> y >> z;

User Types

12.0 25.0 


x gets the value 12.0, y gets the value 25.0, and z gets the value 7.1

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.


What can we do with doubles?

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 doubles, 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

         Computing Slopes

         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.

Assoc Prof Christopher Brown

Last modified by LT M. Johnson 08/15/2007 09:11 AM