1

C++• C++ Overview• Input/Output• New Data Types• Expressions• Declarations, Operators• Control Structures

2

C++

Bjarne Stroustrup

159.234

Designed and implemented a new language in the early 1980’s.

Initially called “C with ClassesC with Classes” it became known as C++, an incremental step up from C.

It was designed to deliver:the flexibility and efficiency of C withthe facilities for program organisation of the Simula language (usually referred to as object-oriented programming).

of AT&T Bell Laboratories

3

C++

A word from the inventor

159.234

"C makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do, it blows away your whole leg." - Bjarne Stroustrup

http://www.research.att.com/~bs/C++.html

4

Object Oriented Programming (OOP) language?

A programming language in which programmers define an abstract data type consisting of both:

• data and • the operations (functions) that can be applied to data.

5

C++

AIMS

159.234

Make programming more enjoyable.

Be a general purpose language that:is a better Csupports data abstractionsupports object-oriented programmingsupports generic programming

6

C++

DESIGN PRINCIPLES

159.234

Don’t get involved in a quest for perfection.

Be useful immediately.

Don’t try to force people.

It is more important to allow a useful feature than to prevent every misuse.

Leave no room for a lower-level language below C++ (except assembler).

7

C++

Language rules:

159.234

Provide as good support for user-defined types as for built-in types.

Locality is good.

Preprocessor usage should be eliminated.

No gratuitous incompatibilities with C.

If you don’t use a feature, you don’t pay for it.

8

Comments

COMMENTS

159.234

C++ allows the use of //// as a commentall text to the end of the line is ignored.

Use /* ... */ /* ... */ for large multi-line blocks of comments// for small comments.// allows nesting of comments.Commenting out a large block of code does not work in C, if the block already contains a small comment./*/* for (i=0;i<10;i++) { k = i*i; /*/*k is the square of i*/*/ }*/*/

/*/* for (i=0;i<10;i++) { k = i*i; // this is ok }*/*/

9

Comments

COMMENTS

159.234

Sometimes its useful to use a pre-processor directive to comment a large chunk of code:

#if 0#if 0 commented out code with /*/* comments */*/#endif#endif

10

Input and Output

Input and Output:

159.234

C++ uses a new way to perform console I/O.Its major advantage over printf and getchar, is that it is both consistent and safer.

printf("%printf("%ss",i); /* but i is an ",i); /* but i is an integerinteger */ */ is not detected as an error by most compilersis not detected as an error by most compilers.

C++ instead uses the notion ofstreamsmanipulatorsoverloading

it overloads the two operators >> operators >> and <<<<

11

C++ streamsC++ streams

A stream is a sequence of bytes moving to/from a device or file.

12

cin

Input and Output

159.234

#include <iostream>#include <iostream>...

cout << "Hello World“ << endl;cout << "Hello World“ << endl;

writes "Hello World" on the console and then goes to the next line.

cout << "i = " << i; cout << "i = " << i; //outputs the value of i

For input: cin >> i;cin >> i; //The >> point the way the data is going.

There is no need to use pointers!

To get a complete line of text (the newline is read and discarded) cin.getline(s, max_no_of_chars);cin.getline(s, max_no_of_chars);

13

Text FormattingTo alter the appearance of output, we insert manipulatorsmanipulators into the stream, or call functions.

159.234

#include <iostream>

endl outputs a new linedec uses decimal valueshex uses hexadecimal values

width(int w) field width of next output

fill(char c) fill charactersetf(flag) set special flags ios::left left align ios::right right align

cout.fill('0'); cout.setf(ios::right); cout << hours << ":"; cout.width(2); cout << minutes << "."; cout.width(2); cout << seconds << endl;

sprintfsprintf is still used to format a string before displaying it.

14

C++

NEW SIMPLE TYPES

159.234

C++ defines two new simple types: boolBoolean variables can represent either true or false. bool test; test = true; test = (x<5);

bool, true and falseare new keywords in C++ and cannot be used as identifiers. On output the values are shown as 0 (false) and 1 (true). wchar_t is a 'wide' character type is designed to allow alphabets with many characters (e.g. Japanese characters).

15

C++Enumerated types:

159.234

#define#define is used to link names to constants.

Use of the pre-processor is frowned on in C++.

Enumerated typesEnumerated types are one way of replacing it.

enum day enum day {{sun,mon,tue,wed,thu,fri,sat};};

defines a new type day.

Enumerated types allow the compiler to check the values associated with them.

d1 = 1; // will produce a warning// will produce a warning

enum is implemented using integers.

Use this to create variables that can store days: dayday d1,d2; d1 = tue; d2 = d1; if (d2 == wed) { ... }

16

C++

Enumerated types:

159.234

In the above example sunsun is represented by 00, mon by 1 etc.There is no easy way to print out enumerated types. switch (d1) {switch (d1) {

case case sunsun::cout << "Sunday";cout << "Sunday";break;break;

case case monmon::......}}

Another way of avoiding the pre-processor is by using const: const int MAX = 4000;const int MAX = 4000;

MAX is a non-changeable integer variable.You can initialise constants but notnot assign to them.

17

C++

Values of expressions:

159.234

We are familiar with the normal arithmetic operators: + - * /a + b is an expression and its value is the sum of the contents of the two variables.

However, in C++, the following statements below: b = 2; c = 3; a = b + c;

can also been written as: a = (b=2) + (c=3);

DON’T do thisDON’T do this..

18

C++

Values of expressions:

159.234

However consider:

a = b = c = 1;a = b = c = 1;

This sets several variables to one - and is used.

The brackets have been omitted - if we insert them, we get:

a = (b = (c = 1));a = (b = (c = 1));

19

C++

Comma operator:Comma operator:

159.234

for (i=0;i<10;i++) { }In most uses of a for loop, the initialisation phase and the increment phase only have one expression.

To initialise two variables we can use the comma operator for (i=0,j=0;i<10;i++,j++) { }

The comma operator can be used anywhere: i = 0, j = 0; //this is OK.

Its value is the value of the left-hand expression. k = (i=4),(j=5);

k gets the value 4. But DON’T do this.But DON’T do this.

20

C++

The conditional operator:

159.234

A 'ternary'ternary' operator - it takes three arguments: expr1 ? expr2 : expr3expr1 ? expr2 : expr3

If expr1 is true then expr2 is evaluatedotherwise expr3 is evaluated.It allows a shorthand form of an if statement: if (y < z) { x = yy; } else { x = zz; }can also be written: x = (y < z) ? y : z;

21

C++

break

159.234

break is used in a switch statement to stop execution falling through from one case to the next:

switch (c) { case 'A': … do something break;break; case 'B': … etc break;break; }

break can also be used in loops as well.

while (true) { if (a == b) {

break;break;}

... }

break ends the inner-most loop that encloses it.

22

C++

continue

159.234

continuecontinue goes to the beginning of the loop.

Inside a for loop the 'increment' expression is evaluatedis evaluated after a continue :

for (i=0; i<total; i++) { cin >> c; if (c > 'A') { continue;continue; // i is incremented } }

This is different from a while loop

i = 0; while (i<total) { cin >> c; if (c > 'A') { continue;continue; //i is not incrementedi is not incremented } i++; }

23

C++

goto

159.234

The use of goto is frowned upon! Try not to use it.Try not to use it.

It can be helpful in getting out of deeply nested loops, when a break isn't much use.

Explicit jumps around pieces of code are used a lot in assembler programming.

A label has the same syntax as an identifier, with a colon added at the end

for (i=0; i<10; i++) { for (j=0; j<20; j++) { ... if (a[i][j] == 10) goto found;goto found; } } ...found:found: cout << "I found it! " << endl;

Better: create a function and use return.

24

C++

Declarations:

159.234

C allows declarations at the start of a { } block.

Variables only exists while the block is being executed.

We normally declare variables at the start of a function, but this is also ok:

int main () { int i; for (i=0;i<10;i++) { int temp;int temp; temp = i; } }

temp only exists within the for loop.temp only exists within the for loop.

25

C++DECLARATIONS

159.234

C++ allows declarations anywhere.

The variable exists from that point to the end of the enclosing block.

for (int j=0int j=0;;j<10;j++) { cout << j; int int kk = j; = j; cout << kk; }

j exists while the loop is executing and maybe longer depending on compiler settings.

kk exists from its declarationfrom its declaration to the end of the loop.

for (int j=0;j<10;j++) { cout << j << kk; // is an error!is an error! int k = j;int k = j; cout << k; }

26

C++ streamsC++ streams

A stream is a sequence of bytes moving to/from a device or file.

27

Input from KeyboardInput from Keyboard

Store in i the value entered from the keyboard:

cin >> i;cin >> i;

By default cin skips all white spaces (blank, tab, newline). Get a complete line of text (the newline character is read & discarded): cin.getline(s, maxNoOfChars);cin.getline(s, maxNoOfChars);  

s must be a string and maxNoOfChars an integer(getline is a method of the cin object, set up for us by the system )

28

Reading From FileReading From File

#include <fstream.h>ifstream inFile;inFile.open(“data.txt”);inFile >> n1 >> n2;inFile.close(();

#include <stdio.h>

FILE *f;

f = fopen(“data.txt”, “r”);

fscanf(f,"%d %d",&n1,&n2);

fclose(f);

C-way

C++ way

29

Sending Data To FileSending Data To File

#include <fstream.h>

•••ofstream outFile;outFile.open(“result.txt”);outFile << n1 << n2;outFile.close();

#include <stdio.h>

FILE *f;

f = fopen(“result.txt”, “w”);

•••

fprintf(f,"%d %d",n1,n2);

fclose(f);

C-wayC-way

C++ C++ wayway

30

File Name Selected By UserFile Name Selected By User

char* fileName; .. . cout << "Enter file name: "; cin >> fileName; ifstream inFile; inFile.open(fileName); if( !inFile){

cerr <<"No file-error here!"; exit(EXIT_FAILURE);//include <stdlib.h>

} inFile.close(); cout <<"Done";.. .

31

Reading data ( assume char ch; char str[60]; )

cin

Need #include<iostream>

myFile Need #include<fstream>

Doing:

cin >>ch myFile >> ch Skip white spaces

cin.get(ch) myFile.get(ch) Reads every char ch

cin.get(str,40,‘#’)

cin.get(str,40)Read C-strings (leaves termin. ch. in the stream), appends ‘\0’.

cin.getline(str,60,’#’)

cin.getline(str,60)

myFile.getline(str, 60,’#’)

myFile.getline(str,60)

Read C-strings (removes termin. ch. from stream), appends ‘\0’.

32

Output data

cout

#include <iostream>

myFile #include<fstream>

Doing:

cout <<value myFile<<value Write out value (char, int, C/C++ strings …)

cout.put(ch) myFile.put(ch) Write out char ch

33

Attention!1. Check for the existence of the file.

if(!myFile!myFile){ //no file to work with cerr <<“Failed to find/create file<<endl; return 1; //notify the system something was wrong}

This (common) trick works because myFile will have a NULL pointerNULL pointer if the file open operation failed. NULL is generally the same as zero so !NULL evaluates true.

When the file is in a different directory, e.g “A:\mystuff\input.txt”We must use: ifstream myFile(“A:\\\\mystuff\\\\input.txt”);

34

Attention!2. Mixing numbers and strings when working with files needs careful consideration.

while(inFile >> num1 >> num2 >> ch >> word)while(inFile >> num1 >> num2 >> ch >> word)

Is OK for this inFile:23.4 56 M Demlow

12 4 A Smith

But it does not work for 23.4 56 M Ann Demlow

12 4 A Smith

35

#include <iostream>#include <fstream>

using namespace std;

int main(){

cout << “Hi there!" << endl; int i; while( cin >> i )while( cin >> i ){ cout << i << endl; } return 0;}

• Output is:Hi there!1122

Terminated with ^D or ^Z

36

#include <iostream>#include <fstream>

using namespace std;

int main(){

ifstream myInputStream( "testinput.txt" ); if( !myInputStream ){ cerr << "error opening input file" << endl; exit(1); }

ofstream myOutputStream( "testoutput.txt" ); if( !myOutputStream ){ cerr << "error opening output file" << endl; exit(1); }

while( myInputStream >> i ){ myOutputStream << i * 10 << endl; }

myInputStream.close(); myOutputStream.close();

return 0;}

• Input of:1 2 3 4 5 6 7 8 9 0• Gives output file:1020304050607080900

37

SummarySummary memory

Keyboard

Monitor

cin

cout

cerr

>>…

<<…

----

-----

----

Disk file ifstream var

Disk file

----

-----

----

ofstream var

38

ExerciseExercise

Write a complete program that will read the following data from a text file, then generate an output file.

31 2.2 3 4.45.5 6.6 7 8.81.1 2.1 3.1 4.1

Store these items into an array of struct

Use dynamic memory allocation for the array

Example: Number of items

x1, y1, x2 ,y2

39

ExerciseExercise

wBound.x1 = 1, wBound.y1 = 2.2, wBound.x2 = 3, wBound.y2 = 4.4wBound.x1 = 5.5, wBound.y1 = 6.6, wBound.x2 = 7, wBound.y2 = 8.8wBound.x1 = 1.1, wBound.y1 = 2.1, wBound.x2 = 3.1, wBound.y2 = 4.1

Use the following format for the output fileoutput file.

wBound.x1 = v1, wBound.y1 = v2, wBound.x2 = v3, wBound.y2 = v4

Where vivi is a value read from the file

Sample output file: out.txt

40

SummarySummary

Next: Formatting output, program structure, types in C++, enum, const. Textbook p.30-34,37(const), 38, 383, browse p. 414-417

File streams work like streams coming from/going to the keyboard (cin)/ screen (cout), except that the other end is a file instead of an input/output device.

To use cin/cout, we must include <<ioiostream>stream>To use ifstream/ofstream, we must include <fstream><fstream>