Introduction to C++

Introducing C++

C++ StructurePrimitive Data TypesI/OCastingStringsControl Flow

Objective

At the end of this lesson, students shouldbe able to write simple C++ programsusing the standard I/O library, the Stringclass, and primitive data types.

Introduction

In CS1410 we will study the C++ programming language.

We do this for a number of reasons (1) We want our students to know at least 2 languages (2) C++ exposes some important ideas, like pointers, that are hidden from the programmer in other languages (3) Seeing the basic principles of programming used in a 2nd

language will significantly increase you programming skills

Starting Point

We will start out assuming that you know C#because that is what we teach in CS 1400.

If you learned another language in yourCS 1400 class, don’t worry. You’ll catch on.

Review: A Simple C# Program

using System;

class Program{ // a constant const int SIZE = 5;

static void Main( ) {

// a local variable double average =32.5; // arithmetic double newValue = average * SIZE;

Console.WriteLine("The average value is {0}", average); Console.ReadLine(); }}

Let’s Convert it to C++

using System;






using System;





C++ is not a pure object orientedlanguage, so all code does not needto be enclosed inside of a class.


using System;

// a constant const int SIZE = 5;



Console.WriteLine("The average value is {0}", average); Console.ReadLine(); }

Main does not have to be static.in C++ it normally returns an int,and it spelled main (no capital M).

Main( ) always returns an integer.int main( )

return 0;


using System;


int main( ) {


Console.WriteLine("The average value is {0}", average); Console.ReadLine(); return 0; }

The syntax of basic declarations,arithmetic, and control statementsare the same as in C#


using System;


int main( ) {


Console.WriteLine("The average value is {0}", average); Console.ReadLine(); return 0; }

C++ I/O is much different from C#(we’ll discuss the details later)

cout << “The average value is “ << average << endl;


using System;


int main( ) {


Console.ReadLine(); return 0; }

To keep the console window open, we’ll use a system call

cout << “The average value is “ << average << endl;system(“PAUSE”);


using System;


int main( ) {


system(“PAUSE”); return 0; }

Namespaces are declared differentlyin C++. Everything we will use is inthe standard (std) namespace.


using namespace std;




int main( ) {



Finally, we need to add a new pre-processor directive, that includesheader files that are required forour program to compile correctly.

The iostream header file is required when doing console I/O.


#include <iostream>

Here’s the Final C++ Program



int main( ) {




#include <iostream>

Header Files

In a C++ program every .cpp file in the project is compiled on its own to produce an object file. Once all of the object files have been produced, they are linked together to create an executable (.exe) file. This means that one .cpp file has noclue of what is defined in the other .cpp files in the program.

So, how do the various bits and pieces of a program createdfrom multiple .cpp files communicate with each other?

Header Files

Header files provide a way for you to advertise theconstants and function interfaces in your code, sothat others can link to your code without having to know the details of the implementation.

Example

In the program we just developed, we have this statement:


The name cout refers to an object of the ostream class.The compiler find out how the cout object works by lookingat the header file <iostream>. To tell the compiler to lookthere we write

#include <iostream>

In this course, you should provide a header file for every.cpp file that you create. In the example we have beenworking on, there is only the .cpp file that contains main().The companion header file for this file should contain• Any #includes required by main( )• Any constants used by main( )• The function prologues and function prototypes for any stand-alone functions that reside in this file.

A Good C++ Code Skeleton

// file prologue

#include “driver.h”int main( ){ // declare local variable variables here // C++ statements


// file prologue

#include <iostream>#include <string>using namespace std;

// constant declarations

// prologues and prototypes// for stand alone functions

driver.cppdriver.h

Primitive Data Types

C++ has fewer primitive data types than does C#,but the primary ones that we will use are exactlythe same … int, double, bool, and char.

There is a major difference between C++ data typesand C# data types. In C++, the size of a data typeis determined by the underlying hardware, it is notdefined by the language.

Assignment and Arithmetic workjust as they do in C#.

C++ Input and Output

In place of C#s Console class, we need twoC++ Objects to do console input and output

cincin is an object of the istream class. To use cin, you must #include iostream in your program.

This object represents the standard input stream. The cin object is created automatically for you.

keyboard buffercin program

keyboard buffer

cout is an object of the ostream class. This objectrepresents the standard output stream. It

is also created automatically for you.

output buffercout program

cout

display buffer

The Stream Insertion Operator, <<

a binary operator (it takes two operands)the left hand operand must be an output streamthe right hand operand

– is converted into text– the text data is then copied into the stream

display buffer

int a = 5; a 0000 0000 0000 0101

the integer 5

0000 0000 0011 0101

the character 5

cout

cout << a;

5

multiple pieces of data are output bycascading the << operator …

cout << “The answer is “ << a;

If you want data to appear on a new line, you mustexplicitly add the newline character to the outputstream. There is no WriteLine operation.

Special characters are added to the streamusing the escape character \

\t\netc …

cout << “The answer is “ << a << ‘\n’;

the endl stream manipulator can be added to the outputstream. It does two things:

It adds a new line to the stream.

It forces the buffer to be output

cout << “Hello” << endl;

Formatting Numbers

Output formatting in C++ is quite different fromC#’s output formatting.

To display a double or a float in standard decimal notation

cout.setf(ios::fixed);

setf( ) is a function in the cout object. It is responsible forsetting formatting flags. We will study these in much moredetail in a later section. In this case, we are setting theios::fixed flag. This makes the output appear as a normaldecimal number instead of in scientific notation.

cout.setf(ios::showpoint);

the ios::showpoint flag guarantees that a decimal pointwill be displayed in the output.

cout.precision(2);

the cout.precision( ) function determines how many digitswill be displayed after the decimal point.

The default formatting, is the“general” format. In this caseprecision defines the number ofdigits in total to be displayed

Example

double price = 78.5;cout.setf(ios::fixed);cout.setf(ios::showpoint);cout.precison(2);cout << “The price is $” << price << endl;

The stream extraction operator, >>

Also a binary operatorThe left operand must be an input streamAny initial white space is skipped, then the stream is read up

to the next white space character ( tab, space, new-line )If necessary, the text just read is converted to match the

type of the right operand. An error occurs if the conversion cannot be done.

The data is stored in the right operand

keyboard buffercin

int a;cin >> a;

a

0000 0000 0011 0101

the character 5

0000 0000 0000 0101

5 72 hello

reading stops when white space is encountered

In C#, we always read a string from theConsole, and then used a Parse method toconvert the string to the desired data type.

In C++, this conversion happens automatically.

However, no exception occurs if the conversioncannot be done.

Failed Input

Consider the following:

A program contains the statements int number = 0; cin >> number

What happens if the user types the letter “t”?

The stream extraction operator is not able toconvert the letter “t” into a proper integer value.Three important things happen, without warning:

1. No input occurs, so the variable number contains whatever value it had previously.

2. The letter “t” remains in the input buffer, so a subsequent read operation will try to read it again.

3. The object cin sets itself to a “failed” state, and subsequent input operations will all fail.

Handling Failed Input

As noted, if input fails, no data is input.

We can detect when this happens by testing the state ofthe input stream object.

if (cin.fail( ) ) { cout << “Invalid input occurred”; . . . }

There is a well known idiom in C++ that makeshandling failed input much easier. The expression

cin >> number;

has a value, which is the value of the cin object itself.if the value of cin is “good”, we can go ahead and process the data. The statement to do this looks like

if (cin >> number) { // process the input

}

Recall that once the stream object fails, all subsequentread operations will fail. How do we fix that?

The stream objects have a member function namedclear( ), that resets the failed state back to good.

cin.clear( );

Using the Stream State to Control a Loop

cout << “\nEnter an integer value (or ‘q’ to quit): “;while (cin >> number){ // process the data

}cin.clear( ); // clear the fail statestring dummyValue; // get the ‘q’ out of the buffercin >> dummyValue;. . .

This code will process user input until a non-integer value is typed:

you can also cascade the stream extraction operator:

cin >> a >> b;

You can control the size of an input field withthe setw( n ) stream manipulator.

cin >> setw(5) >> title;

But … keep in mind that this can leave data in the buffer.

cin.get

The get function of the istream class works similarto the stream extraction operator. With no parameter,it gets one character from the input stream.

cin.get( );

cin.ignore( )

This function reads in a character and ignoresit. The character read in is discarded.

cin.ignore( );

This version of the function reads in n charactersand ignores them.

cin.ignore(n);

This version of the function reads in n charactersor until it encounters the delimiter character,and ignores the characters read.

cin.ignore(n, ‘\n’);

Why is ignore useful? Try the following code…

int numItems;string description;

cout << “\nenter the number of items and description: “;cin >> numItems;getline(cin, description);

When prompted, enter the data on two lines.

Suppose that the user enter 5 for thenumber of items. The contents of thestream object cin look like this:

5 nl

The steam object contains a pointer thatalways points to the next character to read.At this stage it is pointing to the 5. The newlinecharacter was put into the stream when the userpressed the Enter key.

When this statement is executed

cin >> numItems;

5 nl

The character 5 is read from the stream, convertedinto the integer 5, and stored in the variable numItems.The pointer into the stream now points to the new line character.

Now the user enters A p p l e s andpresses the Enter key. The stream looks like this:

5 nl A p p l e s nl

Now the program executes this statement:

getline(cin, description);

5 nl

The getline command reads data until it encounters a new linecharacter. The characters it reads are stored in the stringvariable description. The new line character is discarded.

A p p l e s nl

5 nl

The getline command found the newline character and quit. Nothing gets stored in the variable description. Thecharacters A p p l e remain in the stream.

A p p l e s nl

int numItems;string description;

cout << “\nenter the number of items and description: “;cin >> numItems;cin.ignore( ); // gets rid of the newline charactergetline(cin, description);

When prompted, enter the data on two lines.

The Solution

5 nl

The cin.ignore( ) command moves the stream pointerover 1 character, so the newline character is ignored.It now points to the A.

A p p l e s nl

Now the program executes this statement:

getline(cin, description);

5 nl

The getline command reads data until it encounters a new linecharacter. The characters A p p l e s are reads from the Stream and stored in the string variable description.

The new line character is discarded.

A p p l e s nl

In C++, you can use the same style of castas you did in C#, but the preferred way tocast in C++ is to write, for example

Casting

int number = static_cast<int> (myDoubleValue);

Strings

C++ has a string class what is similar to C#’s string class.

Declaring a String(you must #include <string>)

string myName;

string myName = “Prof. deBry”;

Reading a line into a string

This function is similar to cin.get( )except thatit reads an entire line of data, including spaces,and the data is stored in a string object. This isthe preferred way of reading in a line of data.It is equivalent to C#’s ReadLine method.

getline(cin, stringName);

Additional Notes on the String Class

Use + to concatenate two strings

Use the member function size( ) to get the length of a string

The [ ] operator can be used to access or modify individualcharacters of a string. No bounds checking is performed.

The c_str( ) member function returns a char* based string.

The C11 Numeric Conversion Functions

The C++11 standard contains new functions to convertthe contents of a string into a numeric value. These aresimilar to C#’s Parse methods.

stoi – converts a string to an int

stod– converts a string to a double

+ 6 more for the other C++ numeric data types

The C11 Numeric Conversion Functions

These functions attempt to convert the first part ofa string into a numeric value. If a conversioncannot be performed, an invalid_argumentexception is thrown.

Use getline and the numeric conversion functionsin the string class to avoid the problems with cin.

int numItems;string description;string inputValue;

cout << “\nenter the number of items and description: “;getline(cin inputValue); // get the user input in the stringnumItems = stoi(inputValue). // convert it to an integergetline(cin, description);

Alternate Solution

This is similar to the way you read an integer in C#.

C++ Control Flow

With a few minor exceptions, the statements that controlflow through a C++ program look and work the same as they do in C#

Switch

In C#, each case must contain a break statement. Itis illegal to drop through from one case to the next.

switch (num){case 1: a = a + 5; b = b + 3;case 2: a = a + 6; b = b + 4;case 3: a = a – 7; b = b – 1;}

if num = 3, only this code executes

if num = 2, then this code executes

if num =1, thenthis code executes

This is valid code in C++

There is no foreach statement in C++

Instead, C++ uses a range based for. Itlooks like this:

int a[ ] = {1, 2, 3};for(int n : x){ cout << n << endl;}

Introduction to C++

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