Introduction to C++

Introduction to C++
Computer Science I

Q: What is C++
 C++ is a compiled, object-oriented language
 It is the “successor” to C, a procedural
language
 (the “++” is called the successor operator in C++)
 C was derived from a language called B which
was in turn derived from BCPL
 C was developed in the 1970’s by Dennis
Ritchie of AT&T Bell Labs
 C++ was developed in the early 1980’s by
Bjarne Stroustrup of AT&T Bell Labs.
 Most of C is a subset of C++

People & Programs
 User: an individual who runs, or
executes, a program
 Programmer: an individual who creates,
or writes, a program

C++ Program
Consists of…
 Declarations
 Define the use of various identifiers, thus
creating the elements used by the program
(computer)
 Statements
 Or executable statements, representing
actions the computer will take on the user’s
behalf

Identifiers
 Names for various entities used in a program;
used for...
 Variables: values that can change frequently
 Constants: values that never changes
 Functions: programming units that represents
complex operations
 Parameters: values that change infrequently

Simple C++ Program
#include <iostream.h>
int main()
{
// Declarations
// Statements
return 0;
}
 Compiler
directive:
Tells the
compiler
what to do
before
compiling
 This one
includes
source code
from another
file

Simple C++ Program
int main()
{
// Declarations
// Statements
return 0;
}
 Main function

Simple C++ Program
int main()
{
// Declarations
// Statements
return 0;
}
 Header for
main function
 States…
 data type
for the
return value
 identifier for
function
 list of
arguments
between
parenthesis
(none for
this
function)

Simple C++ Program
int main()
{
// Declarations
// Statements
return 0;
}
 Braces
enclose the
body of the
function
 They
represent the
start and end
of the
function

Simple C++ Program
int main()
{
// Declarations
// Statements
return 0;
}
 Declarations
and
statements
 Main body of
function (or
main part)
 “//”
represents
the start of a
comment

Simple C++ Program
int main()
{
// Declarations
// Statements
return 0;
}
 Return
statement
 specifies the
value the
function
returns
 All (almost)
declarations
and
statements
end with a
semi-colon
“;”

Simple C++ Program
int main()
{
// Declarations
// Statements
return 0;
}
 This
program
doesn’t
do
anything
!

Sample C++ Program
void main()
{
int number;
cout << “Enter a number” <<
endl;
cin >> number;
cout << “You entered: “ <<
number << endl;
}
 Variable
declaration
 The identifier
number is
declared as
being of data
type int, or
integer

Sample C++ Program
void main()
{
int number;
endl;
cin >> number;
number << endl;
}
 cout
the output
statement for
C++
 Note the
direction of
“<<“
 endl
represents an
end-of-line

Sample C++ Program
void main()
{
int number;
endl;
cin >> number;
number << endl;
}
 cin
the input
statement for
C++
 Note the
direction of
“>>”

Sample C++ Program
void main()
{
int number;
endl;
cin >> number;
number << endl;
}
 Did you
copy this
down?
 You had
better,
since this
will be the
first
program
you’ll try!

Sample C++ Program
void main()
{
int number;
endl;
cin >> number;
number << endl;
}
That
mea
ns
right
now!

Assignment
 Assignment is an operation that assigns
the value of an expression to a variable
 Ex.
Total = 2 + 3 + 5
 First, the expresssion “2 + 3 + 5” is
evaluated
 Then, this value is assigned to the
variable “Total”

Assignment
 When a variable is declared, space is
allocated in the computer’s memory for the
variable
 Each data type requires a different number of
bytes in memory for storing a variable
int - 2
float - 4
double - 8
char, bool - 1

Assignment
 When a variable is
assigned a value,
the value is placed
into the variable’s
memory location
10
Total
Total = 2 + 3 + 5;

Arithmetic Operations
 Addition: 2 + 3
 Subtraction: 5 - 2
 Multiplication: 10 * 4
 Division: 12 / 3

Order of Operations
 Arithmetic expressions are evaluated
according to the following order of
operations
 At each level, operations are evaluated
left to right
 (1) Parenthesis, Functions
(2) Multiplication, Division
(3) Addition, Subtraction

Parenthesis
 Parenthesis are used to alter the order
with which operations are evaluated
 Ex.
4 + 5 * 2 equals 14
(4 + 5) * 2 equals 18

Here we go!
 Problem: To determine the average of three
numbers
 Task: Request, from the user, three numbers,
compute the average and the three numbers,
and print out the original values and the
computed average
 Do it!
 You have 20 minutes!

Q: What is a function?
 A programming unit
 Similar to mathematical functions
 Example:
 f(x) = x2 + 5x + 7
 For x = 2, f(2) = (2)2 =5(2) + 7 = 21

Q: What is a function?
 It has...
 ... arguments
 ... a name
(identifier)
 ... a value it
returns
 ... a bodyint foo(int x)
{
int result;
result = x*x + 5*x + 7;
return result;
}

Procedural Abstraction
 Think “Black Box” !
 When using a function, you only need
to be concerned with what it does, not
how it does it
 When writing a function, you need to
be concerned with the how

Example: Cube it!
int cubeIt(int x)
{
int result;
result = x*x*x;
return result;
}
int cubeIt(int x)
{
int result;
result = x;
result = x*result;
result = x*result;
return result;
}

Pseudocode
 Looks like a programming language
 Has all the structure of a programming
language
 Has a verrrrrry loose syntax

Pseudocode
 Example:
function foo (x)
result  x2 + 5x + 7
return result
 That’s it!
 Sloppy, ain’t it?

Computer Science I
Decision Statements

Q: What is a decision?
 Something that represents a branching
point in a solution
 Outcomes are often dependent on initial
conditions

Decisions in Programs
 Without decision statements (or other
dynamic control structures), programs
are static
 Static programs do exactly the same
things each time they are executed
 Dynamic programs do not

Boolean Algebra
 Based on values that are either True or
False
 True and False values are often
represented by 1’s and 0’s, respectively

Logical Operations: And
 A  B
 Expression is True
iff A and B are
both true
T F
T T F
F F F

Logical Operations: Or
 A  B
 Expression is True
if either A or B are
True
 Note: Also True
when A and B are
both True
T F
T T T
F T F

Logical Operations: Exercises
A = True, B = True, C = False
1. A  B
2. A  C
3. A  B  C
4. (A  B)  (A  C)

Relational Operations
 A < B “A less than B”
 A > B “A greater than B”
 A = B “A equal to B”
 A  B “A less than or equal to B”
“A not greater than B”
 A  B “A greater than or equal to B”
“A not less than B”
 A  B “A not equal to B”
“A less than or greater than B”

Relational Operations:
Exercises
A = 5, B = 3, C = -7
1. A < B
2. A  C
3. (A < C)  (B < C)

Boolean Operations: C++
 A  B
 A  B
 A < B
 A > B
 A = B
 A  B
 A  B
 A  B
 A && B
 A | | B
 A < B
 A > B
 A = = B
 A > = B
 A < = B
 A < > B

Try this!
Problem:
 You’d like to go see a movie.
 The movie costs $8.00, a soda costs $2.50
and a large popcorn costs $4.50.
 Based on the amount of money in your
pocket, determine whether you could...
(a) See the movie and buy a soda,
(b) See the movie, and buy soda and
popcorn, or
(c) Stay home

Know?
 Movie costs $8.00
 Soda costs $2.50
 Popcorn costs $4.50
 How much money I have in my pocket

Need?
 Cost of movie and soda
 Cost of movie, soda and popcorn
 Way to select one of the three options
(that is, make a decision!)

Do?
 Option (a) costs $10.50
 Option (b) costs $15.00
 Option (c) costs nothing
 What next?

How about a diagram?
 This is
called a
flowchart
Stay home Money < $15.00
Movie & soda
Movie, soda
& popcorn
Money < $10.50

 Boxes
represent
actions
Movie & soda
Movie, soda
& popcorn
Money < $10.50

 Diamonds
represent
decision
points
Movie & soda
Movie, soda
& popcorn
Money < $10.50

 Arrows
show flow
Movie & soda
Movie, soda
& popcorn
Money < $10.50

 The arrow
at the top
tells us
there were
previous
steps
 The arrow
at the
bottom
tells us
there are
subsequen
t steps
Movie & soda
Movie, soda
& popcorn
Money < $10.50

How would I write this?
 Using Pseudocode
 Wait!
 What the CENSORED is Pseudocode?

Pseudocode
 Example:
get x
result <- x2 + 5x + 7
print result
 That’s it!
 Sloppy, ain’t it?

One more time!
 Pseudocode...
If (Money < $10.50) then
Stay home
else If (Money < $15.00) then
Movie, soda
else Movie, soda, popcorn

 First, we need to decide how to
organize our solution
 Should we “hard code” the costs of the
movie, soda and popcorn into the
algorithm?
 Should we input these values?
 Let’s take another look at that problem!

 The problem
statement tells us
the individual costs
 So, let’s assume
they’re fixed or
constant
 No need to ask the
user for them
Problem:
 You’d like to go see a movie.
 The movie costs $8.00, a soda
costs $2.50 and a large popcorn
costs $4.50.
 Based on the amount of money
in your pocket, determine
whether you could...
(a) See the movie and buy a
soda,
(b) See the movie, and buy
soda and popcorn, or
(c) Stay home

 Another question: Should we pre-compute
the cost of each option?
 Or, should we let the program do this?
 Since we’ve already stated that the item costs
are fixed, it would seem logical to pre-compute
the cost of each option
 Movie: $8.00
 Movie & soda: $10.50
 All three: $15.00

 Next, we need to make sure we have a
complete algorithm
Input Money
If (Money < $10.50) then
Display “Stay home.”
else If (Money < $15.00) then
Display “Go to a movie;buy a soda.”
else Display “Go to a movie; buy a
soda and
popcorn.”
 Almost done!

 Determine how we wish to organize our
program
 Do we want one function?
 Or, should we create a few functions?
 Let’s two functions: One to input Money
from the user
 And a second to determine the outcome

 Here’s the prototypes for the functions
int getMoney()
void showResults(int myMoney)

Program
 Okay, now we get to use our algorithm
and program design to create a
program
 Well, what are you waiting for?
Do It!!!

Multiway Branching
 If statements can be used for multiway
branching
 That is, choosing one of n mutually
exclusive outcomes
 But what about n outcomes that are not
totally unique?

Multiway Branching
 Consider the following problem:
Each year, a local middle school
requires students to purchase supplies
based on their grade level. 6th graders
need pencils and five notebooks. 7th
graders also need a calculator. 8th
graders add to this a 3-ring binder with
loose leaf paper.

Multiway Branching
 We could use a nested If statement to handle
this, but there is an alternative
 Whenever we need to represent a decision
step, with n possible outcomes, where
 the outcomes form subsets of each other,
and/or
 the outcomes are chosen based upon
unique scalar values for a control
expression,
 we can use a Case (switch) structure

Multiway Branching
 Case
When Grade = 8th
3-ring binder
loose leaf paper
When Grade = 7th
calculator
When Grade = 6th
pencils
5 notebooks

Multiway Branching
 In C++ ...
switch (grade){
case 8:
cout << “3-ring binder, loose leaf, “;
case 7:
cout << “calculator, “;
case 6:
cout << “5 notebooks, & pencils.” << endl;
}
 When the switch is encountered, control jumps to the
matching case statement and continues until either a break is
found or the end of the switch

Multiway Branching
 Here’s an example with a few break’s
cout << “Your lunch period comes “;
switch (grade) {
case 8:
cout << “first.” << endl;
break;
case 7:
cout << “second.” << endl;
break;
case 6:
cout << “third.” << endl;
} No final break

Exercise:
 Create a program that will inform the
user which advisor they should go to
based on their major code number
Well? Get started!

Q: What is a Loop?
 A control structure that allows for a
sequence of steps to be repeated a
certain number of times
 This sequence of steps is called the
body of the loop

Q: What is a Loop?
 There are three basic loop structures in
programming:
 For
 While
 Repeat

While loop
Condition
T
Body
F
Look familiar?
What if you
added a change
step to the end
of the body?

For loop
Condition
T
Body
F

While loop
Condition
T
Body
F
A while loop is
a control
structure where
the body is
repeated as long
as the condition
is true

While loop
Condition
T
Body
F
When the
condition is
false, the body
is bypassed, and
flow continues
with the next
part of the
algorithm

Example: Sequential search
k  0
found  False
while (k<size)  (found)
do if A[k] = target
then found 
True
else k = k +
1
(k<size)  (found)
T
if A[k] = target
F
then found  True
else k = k + 1

Example: Sequential search
k = 0;
found = False;
while ((k<size) &&
(!found))
if (A[k] == target)
found = True;
else k = k +
1;
(k<size)  (found)
T
if A[k] = target
F
then found  True
else k = k + 1

Exercise
 Strings are arrays of characters
 How would you determine if a string
fragment is part of a larger string?
(needle in a haystack?)

Q: What is an array?
 An array is a data structure consisting
of one or more indexed members
 An array is like a row of mailboxes at
the post office
 Each box is numbered in sequence
(indices), and …
 Each box contains the same type of
stuff (datatype)

An array could be drawn like
…
g d a f c z l
0 1 2 3 4 5 6

…
g d a f c z l
0 1 2 3 4 5 6
Each box is numbered in sequence

…
Each box has the same datatype
g d a f c z l
0 1 2 3 4 5 6

In pseudocode we use …
 X[j]
 Where X is the name of the array
(identifier), and …
 j is an index indicating which position
in the array we wish to address

An array is declared by …
int X[10];
 Where int is the common datatype for all
elements in the array,
 X is the name of the array (identifier), and …
 10 is the size of the array, or how many
elements are in the array
 Indices for a C++ array always begin with 0

Example: Student Grades
// Declare array
double stGrades[7];
:
// Assign value to array element
stGrades[5] = 87;
:
// Display array element
cout << stGrades[3] << endl;

Problem:
 Create a program that will ask the user
for three (3) numbers, determine the
average, and then display the original
numbers and the average
 Hint: You might wish to use a loop as
well as an array!

What is a string?
 A string is a sequence of characters
 Example:
nc9*hNB98B&^v*&G
 Blank spaces are characters
 Each character requires one byte of storage
in memory
 Each character is represented by a one byte
character code, usually an ASCII code

Strings
 A literal is a string bounded by
quotation marks
 Example:
“nc9*hNB 98B&^v*&G”
 Notice the blanks spaces in the
sequence - they are characters!

Strings
 In C++, we declare string variables as
follows
 char string_identifier[length];
 string_identifier is the name of the
string variable
 length represents the length of the
string, or how many characters are in
the sequence

Strings
 Example:
void main()
{
char name[24];
cout << “Enter your name: “;
cin >> name;
cout << “Your name is “ << name <<
endl;
}

Streams
A stream …
 Is a flow of characters
 Is an object that represents either on input
or an output to a program
 Can be associated with the keyboard,
monitor, or files
 cout is an output stream
 cin is an input stream

How about file I/O?
#include <fstream.h>
:
ifstream in_stream;
ofstream out_stream;
:
Where definitions for
datatypes ifstream
and ofstream are
located
Input stream declaration
Output stream declaration

How about file I/O?
#include <fstream.h>
:
ifstream in_stream;
ofstream out_stream;
:
in_stream.open(“infile.txt”);
out_stream.open(“outfile.txt”);
instream >> number;
outstream << number << endl;
instream.close();
outstream.close();
:
Associates files
with streams &
opens files for
use
Stream use
Closes files

What if a file doesn’t exist?
In_stream.open(“notthere.txt”);
If (in_stream.fail())
{
cout << “Input file opening failed” << endl;
exit(1);
}

How about formatting?
out_stream.setf(ios::fixed);
out_stream.setf(ios::showpoint);
out_stream.precision(2);
out_stream.width(10);
:
Set flag:
fixed/scientific
showpoint
showpos
right/left
Number of
decimal places
Number of digits

More member functions
 get()
 put()
 eof()

Introduction to C++

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