sahar mosleh california state university san marcospage 1 introduction to c++

Sahar Mosleh California State University San Marcos

Introduction to

C++


Our first C++ program:

#include <iostream>

using namespace std;

// This is the “Hello World!” program int main ()

int main(){ cout<<“Hello World!”;}


Like other languages, C++ program

• Consists of a series of statements as defined by the language syntax

• Running a C++ program is a complex process as computers are not able to do anything directly with C++ code

• Must go through the compilation process


Source Program (Hello World)

Preprocessor

Compilation of Source Program

Object Program (Hello World!)

• This compilation process effectively translates a C++ program into a form your computer can understand.

• Machine language• Differs depending on what kind of machine you use.

Compilation Process


• The compilation process requires a program called a compiler

• We will use C++ compiler in Unix for this course


Comments

#include <iostream> using namespace std;

// This line is a comment int main () {

cout<< “Hello World!”; }


Comments


// This line is a comment // This line is also a comment

int main () {

cout<< “Hello World!”;

}


Comments


/* Comments can look like this too!*/

int main () {

cout<< “Hello World!”; }


Comments

#include <iostream>using namespace std;

/* Comments of this sort can span many

lines */ int main () // another comment

{cout<< “Hello World!”;

}


Comments


/* Comments of this sort can span many lines*/

int /* a funny place for a comment */ main (){

cout<< “Hello World!”;}


Comments

• Are ignored by the compiler.

• Are for the exclusive benefit of the programmer.

• Used to describe code that otherwise might be misinterpreted.

• Also used for other relevant information such as your name, student number, course and assignment number, and so on.


Terminologies


What Are Valid Instructions?• In your algorithm, what are the things you can tell a computer

to do?

• That depends on the primitive commands that the computer can understand.

• "Make coffee" is not one of them but "Add 4 and 3" is.

• A program will usually have the following form:

• Section for taking INPUT data• Section for PROCESSING it• Section for OUTPUTTing results

• What you can say in these sections depends heavily on the concept of VARIABLES which we will cover next.


Variables

• Every data item used by your program must be stored in a box in the memory. These boxes in the memory are variables.

• A box can only contain one item at a time, but the box content can be changed again and again.

• e.g. INPUT : two numbers • Need 2 boxes to store the inputs.

• Num1• Num2

• Usually, the result of computation is also stored in a box until it is further modified or given out as the OUTPUT.

• e.g. OUTPUT: sum of the 2 numbers• Need 1 box to store the sum.

• Sum


• Almost every instruction in your program will have to deal with these boxes, for example:

• Take data from the user and put it in a box

• Copy data from one box to another

• Perform calculation on a box.

• Use box contents and place the result in a box.

• Display the box content on the screen.

• Box manipulation commands are the primitive commands you have to work with in developing any algorithm.


• e.g. Instructions to the computer:• 1) Take data from the user and put it in Num1.• 2) Take data from the user and put it in Num2.• 3) Sum gets the result of Num1 + Num2.• 4) Display what's in Sum.

• Although you don't want to develop your high-level algorithm with these primitive commands in mind, you should start thinking about the "boxes", and how they will be used, as you get closer to the programming language level.

• Thus, another way to think of "adding details" is "start thinking in terms of boxes.


Variable Declarations• Before a variable (box) can be used in your instructions, you

have to list it for the Instruction Follower (Computer).

• This is similar to the way Ingredients are listed at the top of a recipe.

• Why?? The computer has to be able to:• 1. reserve the boxes in its memory, labeled with appropriate

names.• 2. make sure no bad data will go into the boxes by knowing

what types of data can go into them.• So, a variable declaration has two components:

• 1. the name of the box• 2. the type of the contents allowed

• e.g. Variable Num1 will contain an integer. Variable Gender will contain a character.


Variable Names

• In most programming languages, the variable name cannot contain blank spaces.

• e.g. myName is Ok in C++• my_Name is Ok in C++• my Name is not Ok in C++

• The name should describe the purpose of the variable.


Variable Types

• Content types found in most programming languages are• integer (for e.g. -1, 0, 1, 2, 3 ...)• real number (for e.g. 23.45, 0.345)• character (for e.g. 'A', 'a', 'B')• string (for e.g. "Happy Birthday")

• Thus, a type defines the set of values which can go into a variable.


First Set of Primitive Commands

• We will write our algorithms in restricted English for now.

• But you must obey the following grammar rules carefully.

• Note that < > are placeholders to be replaced by actual items.

• First, always state at the top of your algorithm,


• Instructions to the computer:

• For Variable Declarations:• Variable <name> will contain a <type>.• e.g. Variable Num1 will contain an integer.

• For INPUTs:• Take data from the user and put it in <name>.• e.g. Take data from the user and put it in Num1.


• For Processing:

• <name> gets the result of <equation using box names and/or numbers>.

• e.g. Num2 gets the result of Num1 * 5 + 4.

• Add <number> to <name>.• e.g. Add 5 to Num1.

• Subtract <number> from <name>.• e.g. Subtract 1 from Num2.


• For Outputs:

• Display what's in <name>.

• e.g. Display what's in Num2.

• Also, make sure every instruction has a step number in front of it.

• e.g. •1) Take data from the user and put it in Num1.•2) .....

• Now, let's practice.


• Ex1: Ask the user to enter the base and the height of a triangle. Find the area of the triangle and print it on the screen for the user.

• INPUTS: the base and height of a triangle • OUTPUT: area of the triangle• Give the high level algorithm:• Give the low level algorithm using the language defined

above:


• Abstract Form of the algorithm:

• Input:• Get base and height

• Process:• Find the area

• Output:• Print the area on the screen


• More detail format:

• Input:• Create two boxes called base and height• Get the values of base and height• Store them in base and height boxes

• Process:• Create another box called area• Multiply base by height and divide it by 2 • Store the result in area box

• Output:• Print the content of the box called area on the

screen


• Another example:

• Find the average of the marks for the 5 courses taken by a student last year


Constants vs. Variables

Sets of Values

Functions


Review

• You may use characters, digits and underscore in a variable name.

• Make sure you do not use blanks in variable names.

• No value is erased from a box until another value is placed in there.

• * WE ARE STILL WORKING WITH ALGORITHMS • * WE ARE NOT USING C++ YET.


Constant vs. Variables

• Constants are values directly written in your algorithm, not referred to by a variable name:

• Given Add 10 to Num.• 10 is a constant.• Num is a variable.

• Thus, an equation can consist of constants and/or variables.• e.g. Num gets the result of 3 * A / B.• *Inter:* What is wrong with 30 gets the result of A / B


Sets of Values

• The concept of SETS will come up again and again in computer science.

• For this course, we can think of the variable type as further restricted by the “acceptable set of values.”

• This does not mean that the computer will automatically check acceptable values for you.)


• For example:

• Variable Age will contain an integer.• The set of acceptable ages = {1, 2, .......... 120}


• Variable HwScore will contain an integer.• The set of acceptable scores = {0, 1, ........ 100}



• Variable Grade will contain a character.

• The set of acceptable grades = {'A', 'B', 'C', 'D', 'E', 'F'}

• The above examples showed sets which have finite number of elements and the elements can be counted as "1st one", "2nd one" etc.

• How about this example?

• Variable Area will contain a real number.

• The set of acceptable areas = { ????? }

• In this case, you cannot really list the elements and count them. In fact, there are infinite number of elements.

• You could write this, for example, as { x | 0 < x <= 200} • This says any area x must be between 0 and 200(inclusive).

*Inter:* How would you describe a set of acceptable values for GPA?

*


• Rule:* In this course, for every variable you use in your algorithm, determine the acceptable set of values and write it down.

• e.g.• Variable Age will contain an integer. // {0, 1, ...., 120}• Variable Sum will contain an integer. // {0, 1, ...., 240}• Variable Num will contain an integer. // {x | x is any integer}

• Why should we do this?• Allow you to think in more depth about the data and results.• Can later help make your algorithm force the user to enter only

acceptable values from the keyboard.• Can be referenced during debugging.


Functions• Given a value, a function returns you a value.

• e.g. abs(-2) returns 2.• e.g. ceil(3.4) returns 4.• e.g. floor(3.9) returns 3.

• Notice that parentheses surround the given value.

• There are many pre-defined functions in a programming language.

• For example, abs, ceil, floor are available in C++.• The set of values you can give to a function is called the

Domain.• The set of values you can get back from a function is called

the Range.• e.g.

•The Domain of abs is integers.•The Range of abs is positive integers.

• You should be aware of the Domain and the Range of each function you use.


Terminology

• When you use a function, you "call" it. e.g. call abs. When you give it a value, you "pass" it.

• e.g. call abs passing it -3. When it gives you a computed value, it is “returned.”

• e.g. abs(-3) returns 3.


Calling a function

• How do you call a function?

• Write its name, and then the value to be passed in parentheses. e.g. abs (-3)

• You may pass a variable to a function.

• This means the function will take the value contained in the variable and use it to compute a new value.

• e.g. abs (Num)

• Where can I call it in my algorithm?• Any place where a constant can be used.

• e.g. We can store it ( ex: Result = abs (Num) * 3).• e.g. We can use it (ex: xyz = xyz + abs (Num) ).• e.g. We can display it (ex: print abs (Num).

• The expression abs (Num) stands for the value returned.• Thus, if Num contained -3, then abs (Num) stands for 3.


• There are functions which get non-numeric values and/or return non-numeric values such as those involved in the ASCII code conversion.

• e.g. int('A') returns 65.• char(65) returns ‘A’

• You can add more functions to the programming language by defining them yourself. We will cover this later.


Lines and Statements

Syntax and

Semantics


#include <iostream>using namespace std

- is a request that source file iostream, that is a standard routine written in visual C++ can be used by the programmers.

- Again, handled by the preprocessor- Contains code to handle, among other things, simple

input and output.- Your “Hello World!” program will turnout, in reality,

to be quite large once all the library code has been inserted.


{…}

- Logical blocks of code are referred to as blocks and are delimited by an open and close brace.

- In case of “Hello World!”, {…} delimit the beginning and ending of executable C++ code.

- You will be seeing a lot of {…}- They are very important


cout <<“Hello World!”;

- cout is a special variable that refers to the default output device.

- In our case, the default output device is the screen.

- Defined in iostream file.


cout << “Hello World!”;

- << is called the insert operator.

- Sends the character string “Hello World!” to the screen.

- Character strings are delimited by double quotation marks.

- C++ statements are terminated by a semicolon.


- C++ programs can be very complex, but are based upon a small number of principles that are rigorously adhered to.

- Understand the principles and you will understand C++ and programming better.



- “Statements” in C++ are not related to the lines in the source file on which they are written. “white-space” (blanks, tabs and linefeeds) are ignored!

- For example for code:cout << “This is some output ”;cout << “and some more output”;

- The output is:This is some output and some more output



On code:

cout << “This is some output ”;cout <<“and some more output.”;

The output is:This is some output and some more output.



• The only place that white-space matters inside double quotes “…”

• For example on code:cout << “This is some output”;

• The output is: This is some output.


Writing a Program

- Several steps

- Understand the problem- Solve the problem yourself- Tell the computer how to do it- Find and fix any errors- Which is the most important thing to learn?- What takes the most practice?- How can you tell the computer what to do when you

don’t know yourself?


Writing a Program

- You will find “telling the computer what to do” (i.e., writing C++) difficult at first.

- Lots of typing mistakes and syntax errors, like learning any language.

- In the long run, what are we teaching you is (very formal) problem solving.

- This is the hard part

- Sitting and typing away in front of the machine won’t do you any good if you don’t know what you’re doing!


Writing a Program

- Know what you’re doing first!

- Construct a step by step solution you can write out without worrying about the computer at all!

- E.g., calculating x to the power of y

- Take x; multiply it by itself over and over; count the number of times as you do this until the number is equal to y.


Writing a Program

- Now that I have that solution, I can implement it as a computer program in any language I like!

- Independent of any details of language or machine! I can use this any time.

- An Algorithm - a finite series of statements that describe how to solve a problem.

- Algorithms are independent of computer programs and computers.


C++: Syntax and Semantics

Syntax:

- Rules governing how valid instructions are written.

- Defines what is a valid program

- Syntactically correct program will compile correctly

- No guarantee it will run correctly

For example:cout >> “This is bad!”// syntactically incorrect: compiler error.



Semantics:

- Rules which determine the meaning of a syntactically correct program.

- What the computer is expected to do when it runs your program.

- A syntactically correct program may not do what we want.

- It always does exactly what you programmed it to do.



- C++ programs consist of a series of functions.

- A function is a piece of code which returns a value.

- We have already seen one special function.- “main” is a function

- takes no arguments- returns nothing (it can return an int)- computer starts to execute at the beginning of function

“main”


- Generally we want multiple lines of output:


int main(){ cout<<“This is line 1”<<endl;

cout<<“This is line 2”;cout<<“Is this on line 3?”;

}

- The output from this segment is:This is line 1This is line 2ls this line 3?


- We can put more than one insertion (<<) operator in a single C++ statement

#include <iostream>using namespace std;int main(){

cout<<“This is line 1”<<“ and more. ”; cout<<“This is still line 1.”<<endl <<“is this on line 2?”<<endl;

}

- The output from this segment is:This is line 1 and more. This is still line 1. is this on line 2?


Other output issues

\n Position the screen cursor to the beginning of the next line

\t Horizontal tab

\a Alert. Sound the system bell

\\ Backslash. Used to print backslash character

\” Double quote. Used to print double quote character

\r Puts the cursor at the beginning of the line


Basic C++ Input

Expressions

The contents of this lecture prepared by the instructors at the University of Manitoba in Canada and modified by Dr. Ahmad Reza Hadaegh


Basic C++ Input

- Computers often require input from the program user

- interactively (such as query to the user) from a file

- In C++, we get input in a manner similar to print output


Basic C++ Input

#include<iostream>


int main()

{

float x;

cout<<“Enter x ”;

cin>>x;

cout<<“The square of x is ”<<x*x <<endl;

}


Basic C++ Input

- cin is the standard input stream

- like cout, it is defined in iostream

- Input to a variable (like x) is accomplished by the extraction operator >>

- Basically, we think of the insertion (<<) and extraction (>>) operators as pointing in the direction that the data is flowing

- Input is flowing from cin to x


Basic C++ Input

- Stream input reads the next item(s) of the desired type from the input stream. For example,

float x,y;

cin>>x; //reads one float into x

cin>>x>>y; //reads a float into x and another float into y

reads floats from the standard input stream

- It does nothing if the next input is not a float, or if something else goes wrong (like an “end-of-life”)

- More on this later...


Basic C++ Input

- C++ input ignores whitespace

- It reads the next item on the input stream, skipping any leading whitespace

- If the C++ statements

char onechar; float onefloat; int oneint;

cin>>onefloat>>oneint; cin>>onechar;

are executed in input

3.14 25

x

- the variables have values

onefloat = 3.14 oneint = 25 onechar = ‘x’


The type bool

- Boolean data & variables can have two possible values: true & false

- Such variables are typically used to represent the state of the computation at a particular time. For example,

- If we are looking for something in a database, we might have a boolean variable “found” which is false until we find what we are looking for


The type bool

- In C++, Boolean variables are of type bool and have possible values as true or false

- bool is a type

- true & false are literals

int main()

{

bool a,b;

a = true; b = false;

cout<<“True is”<<a<<endl;

cout<<“False is”<<b<<endl;

}


The type bool

- The output is:

True is 1

False is 0

- Note that by default, true & false are represented by the integers 0 and 1

- In general, false is represented by 0 and true is represented by any non-zero integer

- This is an example of a coercion

- You generally don’t need to think about this coercion


The type bool

- We can also do I/0 on type bool with “English” names use the “boolalpha” manipulator. For example,

int main()

{

bool a,b;

a = true; b = false;

cout<<boolalpha; //insert manipulator

cout<<“True is”<<a<<endl;

cout<<“False is”<<b<<endl;

}

- The output is:True is true

False is false


Boolean Expressions

- bool values are generally obtained from comparing variables

of other types:

- For example:

float x,y;

bool c;

c = (x<=y) // c is “true” if x is less than or equal to y.

// c is “false” otherwise


Boolean Expressions

- There are a number of basic comparisons that C++ defines between 2 variables or constants:

= = Equal to (note: different from =)

!= Not equal to

> Greater than

< Less than

>= Greater than or Equal to

<= Less than or Equal to


Boolean Expressions

- For type int, the result of such comparisons is relatively clear

- For type float, be careful of round off:

1.333333333333333 = = (4.0/3.0) // ??

- For type char, order consistent with alphabet of same case

‘a’<’z’ true

‘A’<=‘B’ true

‘a’<‘B’ false (!!)


Logical Operators

- In mathematical logic, we combine logical expressions with AND, OR, and NOT to form more complex expressions

- C++ offers these operators as well:

AND is represented by && in C++

OR is represented by || in C++

NOT is represented by ! in C++


Logical Operators

- For bool variables a,b:

a && b true only when a and b are

both true, false otherwise

a || b true when one or more of a and b

are true, false otherwise

!a true when a is false, false

when a is true


Logical Operators

#include <iostream> using namespace std;int main() {

float x,y,z;bool c;cout<<“Enter x,y,z:”;cin>>x>>y>>z;c = (x<=y) && (y<=z);cout<<"It is " << boolalpha<<c

<<" that x, y, and z are in ascending order?" << endl;

}

< See: Example 1>


Logical Operators

#include <iostream>


int main()

{

float x,y,z;

bool c;

cout<<“Enter x,y,z”;

cin>>x>>y>>z;

c=((x<=y) && (y<=z)) || ((x>=y) && (y>=z))

cout<<“It is ” << boolalpha << c << “ that x, y, and z are in ascending or descending order?” << endl;

}


C++ Input from a File

- we can obtain C++ input from a file rather than getting it from the screen:

#include<iostream>#include<fstream> //input or output stream declarationsusing namespace std;

int main() {

float x;ifstream fin; //input stream named finfin.open(“myinputfile.txt”); //attach to file “myinputfile”fin>>x; //get x from input filecout<<“The input is”<<x<<endl;

}

< See: Example 2>


C++ Output from a File

- we can also put C++ output into a file rather than showing it on the the screen:

#include<iostream>#include<fstream> //input/output stream declarationsusing namespace std;int main() {

ofstream fout; //output stream named foutfout.open(“myoutputfile”); //open a file called myoutputfile or

// create the file if it does not exitfout <<“-----------------------------------------------” << endl;fout<< “Name : Joe Anderson” << endl;fout << “Assignment #: 1 ” << endl;fout << “ Date: Oct 20, 2002 ” << endl;fout <<“-----------------------------------------------” << endl;

}

< See: Example 3>


If Statement

and

Flow of Control


Flow Control

- Program statements are executed in a particular order

- Flow of control

- Flow is sequential unless altered by control structures

- Control structures are used to alter the normally sequential flow of control


If

If statement- Based upon a logical expression, segments of code can

be executed or not executed during run-time

if (grade>=80)cout<<“You get an A ”<<endl;


If

- The logical expression is evaluated

- If true, statement that follows is executed

- If false, statement that follows will not be executed

- Regardless, flow will continue with statement following if construct


If else

- If else statement

if (grade>=50) cout<<“You pass - barely!”<<endl;

else cout<<“See you next year!”<<endl;

- Although mutually exclusive, one of the above statements must execute


If else

if (grade>=50)cout<<“You pass - barely!”<<endl;

elsecout<<“Oops…you fail!”<<endl’cout<<“See you next year!”<<endl;

- “See you next year!” is not part of the if construct and will always print


Blocking

Blocks- The if construct requires, in effect, a single statement for

the true & false conditions.

- However, it is possible to bind many statements into one by blocking them

{stmt1; stmt2; stmt3; … ; stmtn;

}


Blocking

So…

if (grade>=50)cout<<“You pass - barely!”<<endl;else {cout<<“Oops…you fail!”<<endl;cout<<“See you next year!”<<endl;}


Blocking

if (grade>=50) { cout<<“You pass - barely!”<<endl; cout<<“Congratulations!”<<endl;}else {

cout<<“Oops…you fail!”<<endl;cout<<“See you next year!”<<endl;

}


Nested ifs

- Many if versus nested ifs

- It is often necessary to have many ifs in order to solve a problem


Nested ifs

if (month = = 1)cout<<“January”;

if (month = = 2)cout<<“February”;

if (month = = 3)cout<<“March”;

- Even if month == 1, program will continue to check if month == 2, 3, 4, 5, …

- Will work, but better to nest the ifs


Nested ifs

if (month = = 1)

cout <<“January”;else

if (month = =2) cout<<“February”;else if (month = = 3) cout<<“March”;

- Will leave nested if construct as soon as one condition is found to be true


Nested ifs

if (month == 1)cout<<“January”;

elseif (month == 2)

cout<<“February”;else

if (month == 3) cout<<“March”;

- Will leave nested if construct as soon as one condition is found to be true


Nested ifs

if (grade>=90)

cout <<“A+”; if (grade>=80)

cout<<“A”; if (grade>=75)

cout<<“B+”;

- If grade == 85, “A”, “B+”, …will print

- Presumable not what is desired

< See: Example 1>


Nested ifs

if (grade>=90)

cout<<“A+”;else if (grade>=80)

cout<<“A”;else if (grade>=75)

cout<<“B+”;

- If grade = = 85, only “A” will print


Nested ifs

if (false) if (true)

cout<<“This will never print!”<<endl; else

cout<<“Goodbye World!”<<endl;

- You might be under the impression that the else binds with the first if, but you’d be wrong

- This code results in nothing


Another Common Trap

int i=25, j=50;

if (i=j) {

cout<<“The if is true”<<endl; cout<<i<<j<<endl;

}else {

cout<<“The if is false”<<endl;cout<<i <<“ ” << j <<endl;

}

< See: Example 2>

This is what will execute Why ?


Trap

- Operators = and == are not the same

- if (i = j) will…

- Assign j to i- They’re now equal

- Determine that i is not 0- As far as if is concerned, if it’s not 0, it’s true!

- Since the If is true it prints out:50 50


Assignment Operator

- The assignment operator is unusual in that it has a value and a side effect

int x=0, y=1;cout<<(x=y)+10;cout<<“ ”<<x<<endl;

- The output is:11 1

- The value of (x=y) is the value of y- The side effect of (x=y) is to assign x to be y- This is an extremely important decision

< See: Example 3>

Do not do this


Assignment Operator

- The assignment operator has lower precedence than any of the other operators that we have seen

- This means that when we execute

x = 3*z+5;

- the expression 3*z+5 is evaluated first and the value is then assigned to x


Assignment Operator

- The assignment operator has lower precedence than any operator we have seen. This means it is executed last

++, --, unary - , ! Highest Precedence*, /, %+, -<, <=, >, >== =, !=&&||= Lowest Precedence

sahar mosleh california state university san marcospage 1 introduction to c++

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