An Introduction to the Imperative Part of C++

[Ibwe Salimu]

1. Introducing C++


1.1 Some Remarks about Programming


Programming is a core activity in the process of performing tasks or
solving problems with the aid of a computer. An idealised picture is:

[problem or task specification] - COMPUTER - [solution or
completed task]

Unfortunately things are not (yet) that simple. In particular, the
"specification" cannot be given to the computer using natural
language. Moreover, it cannot (yet) just be a description of the
problem or task, but has to contain information about how the problem
is to be solved or the task is to be executed. Hence we need
programming languages. Click here for a more detailed view of the
problem solving pipeline.

There are many different programming languages, and many ways to
classify them. For example, "high-level" programming languages are
languages whose syntax is relatively close to natural language,
whereas the syntax of "low-level" languages includes many technical
references to the nuts and bolts (0's and 1's, etc.) of the computer.
"Declarative" languages (as opposed to "imperative" or "procedural"
languages) enable the programmer to minimise his or her account of how
the computer is to solve a problem or produce a particular output.
"Object-oriented languages" reflect a particular way of thinking about
problems and tasks in terms of identifying and describing the
behaviour of the relevant "objects". Smalltalk is an example of a pure
object-oriented language. C++ includes facilities for object-oriented
programming, as well as for more conventional procedural programming.

Proponents of different languages and styles of languages sometimes
make extravagant claims. For example, it is sometimes claimed that
(well written) object-oriented programs reflect the way in which
humans think about solving problems. Judge for yourselves!

(BACK TO COURSE CONTENTS)


1.2 The Origins of C++


C++ was developed by Bjarne Stroustrup of AT&T Bell Laboratories in
the early 1980's, and is based on the C language. The name is a pun -
"++" is a syntactic construct used in C (to increment a variable), and
C++ is intended as an incremental improvement of C. Most of C is a
subset of C++, so that most C programs can be compiled (i.e. converted
into a series of low-level instructions that the computer can execute
directly) using a C++ compiler.

C is in many ways hard to categorise. Compared to assembly language it
is high-level, but it nevertheless includes many low-level facilities
to directly manipulate the computer's memory. It is therefore an
excellent language for writing efficient "systems" programs. But for
other types of programs, C code can be hard to understand, and C
programs can therefore be particularly prone to certain types of
error. The extra object-oriented facilities in C++ are partly included
to overcome these shortcomings.

(BACK TO COURSE CONTENTS)


1.3 ANSI C++


The American National Standards Institution (ANSI) provides "official"
and generally accepted standard definitions of many programming
languages, including C and C++. Such standards are important. A
program written only in ANSI C++ is guaranteed to run on any computer
whose supporting software conforms to the ANSI standard. In other
words, the standard guarantees that ANSI C++ programs are portable. In
practice most versions of C++ include ANSI C++ as a core language, but
also include extra machine-dependent features to allow smooth
interaction with different computers' operating systems. These machine
dependent features should be used sparingly. Moreover, when parts of a
C++ program use non-ANSI components of the language, these should be
clearly marked, and as far a possible separated from the rest of the
program, so as to make modification of the program for different
machines and operating systems as easy as possible.

(BACK TO COURSE CONTENTS)


1.4 The C++ Programming Environment in UNIX

The best way to learn a programming language is to try writing
programs and test them on a computer! To do this, we need several
pieces of software:

An editor with which to write and modify the C++ program components or
source code,
A compiler with which to convert the source code into machine
instructions which can be executed by the computer directly,
A linking program with which to link the compiled program components
with each other and with a selection of routines from existing
libraries of computer code, in order to form the complete machine-
executable object program,
A debugger to help diagnose problems, either in compiling programs in
the first place, or if the object program runs but gives unintended
results.
There are several editors available for UNIX-based systems. Two of the
most popular editors are emacs and vi. For the compiler and linker, we
will be using the GNU g++ compiler/linker, and for the debugger we
will be using the GNU debugger gdb. For those that prefer an
integrated development environment (IDE) that combines an editor, a
compiler, a linking program and a debugger in a single programming
environment (in a similar way to Microsoft Developer Studio under
Windows NT), there are also IDEs available for UNIX (e.g. Eclipse,
xcode, kdevelop etc.)

Appendix A.1 of these notes is a brief operational guide to emacs and g
++ so that you can get going without delay. If you are interested in
learning more about UNIX you can click here.

(BACK TO COURSE CONTENTS)


1.5 An Example C++ Program

Here is an example of a complete C++ program:

// The C++ compiler ignores comments which start with
// double slashes like this, up to the end of the line.

/* Comments can also be written starting with a slash
followed by a star, and ending with a star followed by
a slash. As you can see, comments written in this way
can span more than one line. */

/* Programs should ALWAYS include plenty of comments! */

/* Author: Rob Miller and William Knottenbelt
Program last changed: 30th September 2001 */

/* This program prompts the user for the current year, the user's
current age, and another year. It then calculates the age
that the user was or will be in the second year entered. */

#include <iostream>

using namespace std;

int main()
{
int year_now, age_now, another_year, another_age;

cout << "Enter the current year then press RETURN.\n";
cin >> year_now;

cout << "Enter your current age in years.\n";
cin >> age_now;

cout << "Enter the year for which you wish to know your age.\n";
cin >> another_year;

another_age = another_year - (year_now - age_now);

if (another_age >= 0) {
cout << "Your age in " << another_year << ": ";
cout << another_age << "\n";
} else {
cout << "You weren't even born in ";
cout << another_year << "!\n";
}

return 0;
}
Program 1.5.1

This program illustrates several general features of all C++ programs.
It begins (after the comment lines) with the statement

#include <iostream>


This statement is called an include directive. It tells the compiler
and the linker that the program will need to be linked to a library of
routines that handle input from the keyboard and output to the screen
(specifically the cin and cout statements that appear later). The
header file "iostream" contains basic information about this library.
You will learn much more about libraries of code later in this course.

After the include directive is the line:

using namespace std;


This statement is called a using directive. The latest versions of the
C++ standard divide names (e.g. cin and cout) into subcollections of
names called namespaces. This particular using directive says the
program will be using names that have a meaning defined for them in
the std namespace (in this case the iostream header defines meanings
for cout and cin in the std namespace).

Some C++ compilers do not yet support namespaces. In this case you can
use the older form of the include directive (that does not require a
using directive, and places all names in a single global namespace):

#include <iostream.h>

Much of the code you encounter in industry will probably be written
using this older style for headers.

Because the program is short, it is easily packaged up into a single
list of program statements and commands. After the include and using
directives, the basic structure of the program is:

int main()
{
First statement;
...
...
Last statement;

return 0;
}
All C++ programs have this basic "top-level" structure. Notice that
each statement in the body of the program ends with a semicolon. In a
well-designed large program, many of these statements will include
references or calls to sub-programs, listed after the main program or
in a separate file. These sub-programs have roughly the same outline
structure as the program here, but there is always exactly one such
structure called main. Again, you will learn more about sub-programs
later in the course.

When at the end of the main program, the line

return 0;

means "return the value 0 to the computer's operating system to signal
that the program has completed successfully". More generally, return
statements signal that the particular sub-program has finished, and
return a value, along with the flow of control, to the program level
above. More about this later.

Our example program uses four variables:

year_now, age_now, another_year and another_age

Program variables are not like variables in mathematics. They are more
like symbolic names for "pockets of computer memory" which can be used
to store different values at different times during the program
execution. These variables are first introduced in our program in the
variable declaration

int year_now, age_now, another_year, another_age;
which signals to the compiler that it should set aside enough memory
to store four variables of type "int" (integer) during the rest of the
program execution. Hence variables should always be declared before
being used in a program. Indeed, it is considered good style and
practice to declare all the variables to be used in a program or sub-
program at the beginning. Variables can be one of several different
types in C++, and we will discuss variables and types at some length
later.

(BACK TO COURSE CONTENTS)



1.6 Very Simple Input, Output and Assignment


After we have compiled the program above, we can run it. The result
will be something like

Enter current year then press RETURN.
1996
Enter your current age in years.
36
Enter the year for which you wish to know your age.
2001
Your age in 2001: 41
The first, third, fifth and seventh lines above are produced on the
screen by the program. In general, the program statement

cout << Expression1 << Expression2 << ... << ExpressionN;
will produce the screen output

Expression1Expression2...ExpressionN
The series of statements

cout << Expression1;
cout << Expression2;
...
...
cout << ExpressionN;
will produce an identical output. If spaces or new lines are needed
between the output expressions, these have to be included explicitly,
with a " " or a "\n" respectively. The expression endl can also be
used to output a new line, and in many cases is preferable to using
"\n" since it has the side-effect of flushing the output buffer
(output is often stored internally and printed in chunks when
sufficient output has been accumulated; using endl forces all output
to appear on the screen immediately).

The numbers in bold in the example screen output above have been typed
in by the user. In this particular program run, the program statement

cin >> year_now;
has resulted in the variable year_now being assigned the value 2001 at
the point when the user pressed RETURN after typing in "2001".
Programs can also include assignment statements, a simple example of
which is the statement

another_age = another_year - (year_now - age_now);
Hence the symbol = means "is assigned the value of". ("Equals" is
represented in C++ as ==.)

(BACK TO COURSE CONTENTS)



1.7 Simple Flow of Control


The last few lines of our example program (other than "return 0") are:

if (another_age >= 0) {
cout << "Your age in " << another_year << ": ";
cout << another_age << "\n";
}
else {
cout << "You weren't even born in ";
cout << another_year << "!\n";
}
The "if ... else ..." branching mechanism is a familiar construct in
many procedural programming languages. In C++, it is simply called an
if statement, and the general syntax is

if (condition) {
Statement1;
...
...
StatementN;
} else {
StatementN+1;
...
...
StatementN+M;
}
Th