Presentation on C++ Programming Language

• Why use C++?
ANSI standard
Compilers available on every platform
All libraries (to my knowledge) have C
and/or C++ API
• Is C++ better than Fortran?
Structure
Object orientation
Reusable code and library creation
Excellent error checking at compile time
C++

Course layout
 Week 1
 Learn about C, the subset of C++
 (not all C covered, some C++ stuff used)
 Week 2
 C++ proper; classes and objects
 Quick question and answer session after each section
C++

Sections
1) Form of a C++ program
2) Data types
3) Variables and scope
4) Operators
5) Statements
6) Arrays
7) Pointers
8) Functions
9) Structures
10) Console I/O
11) File I/O
12) Pre-processor instructions
13) Comments
14) Compiling examples under Unix
C++

Form of a C++ program
#include <iostream.h>
int main(int argc, char* argv[])
{
cout << “Hello Worldn”;
return 0;
}
braces enclosing function
function pre-processor statement
arguments
All C++ programs must have one, and only one, main function
semi-colons
C++

Some differences with Fortran
o C++ is case sensitive: x != X
o You can write it all on one line - statements separated by ‘;’
o There are NO built-in functions but lots of standard libraries
o There is NO built-in I/O but there are I/O libraries
o There are about 60 keywords - need to know about 20?
o C++ supports dynamic memory allocation
o All C++ compilers are accompanied by the C pre-processor
o You can potentially screw up the operating system with C++
C++

Including header files and library functions
 programmer chooses what to include in the executable
C++
Header files declare the functions to be linked in from libraries…
#include <math.h>
#include “D:andy.h”
read & write functions
sqrt, log, sin, abs, etc
My stuff that I use frequently
and don’t want to recreate
If header is in “” the filename must be full (or relative);
if in <> pre-processor searches in pre-defined folders

Microsoft Visual C++
o Each program is created within a ‘project’
o A project can only contain ONE ‘main’ function
o Basically each individual project is a program
o Each project is stored in a folder
o To open a project, double-click the ‘.dsw’ file in the folder
o Click Help>Index and type a query - massive documentation
o Or select (highlight) a keyword and press F1 on keyboard
C++
VC++ is a big application: a separate course!
build
run

Data types
 There are 6 atomic data types:
1) char - character (1 byte)
2) int - integer (usually 4 bytes)
3) float - floating point (usually 4 bytes)
4) double - double precision floating point (usually 8 bytes)
5) bool - true or false (usually 4 bytes)
6) void - explicitly says function does not return a value
and can represent a pointer to any data type
C++ is a programmer’s language - need to know the
basics…
Size of the data types depends on machine architecture
e.g. 16 bit, 32 bit or 64 bit words
Other data types are derived from atomic types e.g. long
intCan use ‘typedef’ to alias your own data type names;
defining C++ classes creates new types
C++

Variables and scope
int a, b, c;
a = 1;
b = c = 0x3F;
float iAmAFloat = 1.234;
double iAmADouble = 1.2e34;
{
int i, a;
for (i=0; i<10; i++) {
a = i;
int b = i;
}
b = 2;
}
‘b’ declared
inside braces;
‘b’ is in scope
inside braces
‘a’ declared outside
loop braces
‘a’ used inside braces, OK
‘b’ is unknown outside braces:
‘b’ is out of scope, ERROR
C++

C++
Operators
Obvious: +, -, *, /
Shorthand: +=, *=, -=, /=
Modulus: %
Decrement: --
Increment: ++
Relational: ==, !=, <, >, <=, >=
Logical: !, &&, ||, &, |, ^, ~
int a, b;
a++;
b--;
means the same as
a = a + 1;
b = b - 1;
5%3 evaluates to 2 (the remainder of division)

Statements C++
A statement is a part of the program that can be executed
Statement categories:
1) Selection
2) Iteration
3) Jump
4) Expression
5) Try (exception handling; look it up)
Statements specify actions within a program. Generally
they are responsible for control-flow and decision making:
e.g. if (some condition) {do this} else {do that}

C++Selection: ‘if’
General form is:
if (expression) {
statement;
}
else if (expression) {
statement;
}
.
.
.
else {
statement;
}
bool flag;
int a, b;
if (a>0 && b>0) {
a = 0;
b = 0;
}
else if (flag) {
a = -1;
}
else {
b = -1;
}
‘expression’ is any condition that evaluates to ‘true’ or ‘false’
‘statement’ could be another ‘if’ i.e. nesting…

C++A note on conditional expressions
A condition is one or more expressions that evaluate to true or false
Expressions can be linked together by logical operators
bool flag;
double a;
(!flag)
(a>0.0 && flag)
(a==0.0 || !flag)
(a<0.0 || (flag && a>0.0))
evaluates ‘true’ if flag is ‘false’
evaluates ‘true’ if a is
zero or flag is false

C++Selection: ‘switch’
This statement only works with integers and chars -
it only checks for identical values (not less or more than).
Good for using with menu choices…
switch (ch) {
case ‘a’:
doMenuOptionA();
break;
case ‘b’:
doMenuOptionB();
break;
.
.
.
}
ch of type ‘char’
(could be ‘int’)
This is a label,
and so has
a ‘:’ after it
Note! As in ‘if’, there
is NO ‘;’ after braces
control jumps here
if ch equals ‘b’
control jumps to
end brace

Iteration: ‘for’ loop C++
Huge amount of variation allowed, very flexible: generally
for (initialisation; condition; increment) { statements }
int i, a[10];
int sum = 0;
for (i=0; i<10; i++) {
a[i] = i;
sum = sum + i;
}
Initially i is
set to zero
Keeps looping while
i is less than 10
(i.e. condition is ‘true’)
i increments by one
on each iteration

C++
Iteration: ‘while’ loop
General form is
while (condition) { statements }
char c = ‘0’;
while (c != ‘q’) {
cout << “Type a key (q to
finish): “;
cin >> c;
}
cout << “Finishedn”;
use console I/O
initialise the char variable
The loop body will execute
forever - until “q” is pressed

C++
Jump: ‘goto’, ‘return’, ‘break’ and ‘continue’
Personal bigotry: using ‘goto’ just shows bad design; do not use
‘return’ is used to exit a function, usually with a value
int a = 0;
while (true) {
a++;
if (a >= 10) {
break;
}
}
int count = 0;
int i;
for (i=0; i<10; i++) {
if (i<3 || i>6) {
continue;
}
count++;
}
infinite loop
jump out of loop
(if loops nested, only
jump out of inner)
Jump to beginning
of loop and begin
next iteration

C++
Expressions
An expression statement is anything that ends with a ‘;’
func();
a = b + c;
;
float root = (-b + sqrt(b*b
- 4.0*a*c)) / (2.0*a);
int flagset = 0x0001 |
0x0100;
bool check = (flag &
0x0001);
function call
empty statement - perfectly valid
bitwise OR of two integers
bitwise AND of two ints; result is true or false

C++Arrays (I)
o In C++ arrays can be either fixed or variable length;
there is a very close link between arrays and pointers
o An array can be of any data type, including your own
o C++ arrays are indexed from 0; the last element at N-1
int a[5];
for (i=0; i<10; i++) {
a[i] = i;
}
first element at 0, last at 4
may crash when i>4; no
bounds checking by default

C++
Arrays (II)
Multi-dimensional arrays specified by:
double elements[100][100][100]; // about 8Mb!
for (i=0; i<100; i++) {
for (j=0; j<100; j++) {
for (k=0; k<100; k++) {
elements[i][j][k] = initElement(i, j, k);
}
}
} function that returns a ‘double’
Arrays can be initialised with contents:
float coeff[] = { 1.2, -5.456e-03, 200.0, 0.000001 };
compiler sizes array automatically

C++Arrays (III)
Character strings are arrays of type char
Many functions dedicated to manipulating strings <string.h>
C++ offers the standard ‘string’ class
Many library functions require you to use char* variables
char myName[128];
int len;
strcpy(myName, “Andy
Heath”);
char* fullname = myName;
len = strlen(fullname);
char* surname = &myName[5];
len = strlen(surname);
char* argv[];
terminator
length is 10
length is 5
array of strings passed from command line
A N D Y H a

C++Pointers (I)
oA pointer is a variable; just like any other type of variable
o The variable contains the memory address of a certain data type
o The address is in either stack memory or heap memory
o Any atomic or user-defined data type can be pointed to
o The value of the pointer’s data type can be obtained
o Obtaining the value pointed to in memory is called dereferencing
o Two unary operators are used frequently with pointers: * and &
o Using pointers incorrectly will almost certainly crash things
o Pointers are the most useful feature of C++ (in my opinion)

Pointers (II) C++
float a = 3.142;
float* pa = &a;
pa = 1000
double b = 1.2e12;
double* pb = &b;
double bb = *pb;
bb = 1.2e12
int c = 7;
int* pc = &c;
int* pc1 = pc + 1;
*pc1 = 22;
potentially disastrous: something
important may be at address 1028!
3.142
1.2e12
7
?22?
stack
memory
address
1000
1004
1008
1012
1016
1020
1024
1028

C++Pointers (III)
The name of an array is also a pointer:
float e[10], *pe;
pe = e = &e[0];
Can use a pointer as index into array:
for (int i=0; i<10; i++) {
*pe = float(i);
pe++;
}
Stack memory is allocated at compile time, when the size of
an object (e.g. an array) is already known. Unless declared ‘static’,
stack memory is freed when object is no longer in scope. E.g. when a
function returns, the memory it’s variables occupied will be reused.

Functions (I) C++
A function is a distinct body of code that returns a value -
even if the value is nothing at all
// this is some function
int func(double a, double b)
{
if (a < b) {
return -1;
}
else {
return 1;
}
}
// this is the main function
{
return func(1.0, 2.0);
}

C++Functions (II)
This is the same function - the code is written in a different order
int func(double, double);
// this is the main function
{
return func(1.0, 2.0);
}
// this is some function
int func(double a, double b)
{
if (a < b) {
return -1;
}
else {
return 1;
}
}
Prototype
(this is what is
in header files)

C++Functions (III)
o A function returns a value (except if it is of type ‘void’)
o The value is specified with the ‘return’ keyword
o The value returned must be the same type as the function
double logit(double z)
{
return log(z);
}
int addInts(int a, int b,
int c)
{
return a + b + c;
}
A function can call itself - recursion
int factorial(int n)
{
if (n > 0) {
return n*factorial(n-1);
}
else {
return 1;
}
}

Functions (IV) C++
Arguments passed to functions in one of three ways:
1) By value (default)
2) By reference
3) Thru a pointer to the argument
{
float x = 2.0f;
func(x);
cout << x;
}
void func(float x)
{
x = pow(10, x);
}
{
float x = 2.0f;
func(x);
cout << x;
}
void func(float& x)
{
x = pow(10, x);
}
{
float x = 2.0f;
func(&x);
cout << x;
}
void func(float* px)
{
*px = pow(10, *px);
}
C++ default Like Fortran Thru a pointer

C++Structures
oStructures are a C concept
o Replaced in C++ with the ‘class’ concept
o Objects of a class can be completely defined as a new data type
class Andy {
// definition of stuff
};
int main(int argc, char*
argv[])
{
Andy a, b, c;
a = b + c;
cout << a;
}
a, b and c are
of type Andy
If we choose to, we can give ‘Andy’
objects the ability to do addition,
and to write themselves to the console
but that’s for NEXT WEEK…

C++Console I/O
oThe console is a command window (DOS or terminal)
o C++ uses the ‘<<’ and ‘>>’ operators to write and read
o C++ recognises what sort of data you want to read or
write and acts accordingly
o Use “manipulators” to format I/O - look it up!
{
char* name = “Andy Heath”;
float number = 1.2345;
int menuChoice;
cin >> menuChoice;
cout << name << “ “ << number <<
“n”;
return 0;
}
read an ‘int’ token from
the keyboard - tokens
separated by white-space
(space, newline, tab)
write variables and constants to screen;
use newline (‘n’) to finish line

File I/O C++
oSimilar to console I/O
o Different types of stream for input and output
o File streams defined in <fstream.h>
ifstream inFile;
ofstream outFile;
float number;
inFile.open(“readme.txt”);
outFile.open(“writeme.txt”);
while (!inFile.eof()) {
inFile >> number;
outFile << number;
}
inFile.close();
outFile.close();
Keep going until the
end of file is reached
These are methods of the base
classes ‘ios’ and ‘fstream’…
‘inheritance’ covered next week!

C++
Pre-processor instructions
Pre-processor instructions allow the programmer to choose
what source code the compiler sees - very useful for writing
programs that are portable between Unix and Windows
ifdef WIN32
#include <someWindowsHeader.h>
void myFunc(/* Windows data
types */);
#else
#include <someUnixHeader.h>
void myFunc(/* Unix data types
*/);
#endif
Can use verbose output in a function to help with debugging
#ifdef _DEBUG
cout << “Debug: at end of ‘func1’ x = “ << x << “n”;
#endif
Look at Project>Settings and the “C/C++” tab for Pre-processer defs

C++Comments
Two types of comments:
/* blah blah blah */
// blah blah blah
/*
* this function computes the square root of
infinity
*/
double sqrtInf()
{
double infinity = NaN; // is infinity a
number?
return 1.0; // near enough since I haven’t
got a clue
}
VC++ colours comments green by default

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