2.overview of c++ ________lecture2

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A C++ program
//if necessary include headers
//#include <foo.h>
void main() {
//variable declaration
//read values input from user
//computation
//print output to user
}
Notes:
 what follows after // on the same line is considered comment
 indentation is for the reader; compiler ignores all spaces and new
line ; the delimiter for the compiler is the semicolon
 all statements ended by ;

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Identifiers
 Rules:
 Names begin with alphabetic character
including an underscore e.g. get_Name,
_Name, a_type
 Variables- Case sensitive
 Lower vs. upper case matters!!
 Void is different than void
 Main is different that main
Initialization can be done at the beginning e.g.
int m=10;

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Example
When learning a new language, the first program people
usually write is one that salutes the world :)
Here is the Hello world program in C++.
#include <iostream.h>
int main() {
cout << “Hello world!”;
return 0;
}

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Variable declaration
type variable-name;
Meaning: variable <variable-name> will be a variable of type
<type>
Where type can be:
 int //integer
 double //real number
 char //character
 Long int // 32 bits
 Float // floating point numbers
Example:
int a, b, c;
double x;
int sum;
char my-character;

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Input statements
cin >> variable-name;
Meaning: read the value of variable <variable-name>
from the user
Example:
cin >> a;
cin >> b >> c;
cin >> x;
cin >> my-character;

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Output statements
cout << variable-name;
Meaning: print the value of variable <variable-name> to the user
cout << “any message “;
Meaning: print the message within quotes to the user
cout << endl;
Meaning: print a new line
Example:
cout << a;
cout << b << c;
cout << “This is my character: “ << my-character << “ he he he”<< endl;

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If statements
if (condition) {
S1;
}
else {
S2;
}
S3;
True False
condition
S1 S2
S3

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Boolean conditions
..are built using
 Comparison operators
== equal
!= not equal
< less than
> greater than
<= less than or equal
>= greater than or equal
 Boolean operators
&& and
|| or
! not

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Examples
Assume we declared the following variables:
int a = 2, b=5, c=10;
Here are some examples of boolean conditions we
can use:
 if (a == b) …
 if (a != b) …
 if (a <= b+c) …
 if(a <= b) && (b <= c) …
 if !((a < b) && (b<c)) …

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If example
void main() {
int a,b,c;
cin >> a >> b >> c;
if (a <=b) {
cout << “min is “ << a << endl;
}
else {
cout << “ min is “ << b << endl;
}
cout << “happy now?” << endl;
}

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While statements
while (condition) {
S1;
}
S2;
True False
condition
S1
S2

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While example
//read 100 numbers from the user and output their sum
void main() {
int i, sum, x;
sum=0;
i=1;
while (i <= 100) {
cin >> x;
sum = sum + x;
i = i+1;
}
cout << “sum is “ << sum << endl;
}

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Arrays
Used to store a collection of elements (variables)
type array-name[size];
Meaning:
This declares a variable called <array-name> which contains
<size> elements of type <type>
The elements of an array can be accessed as: array-name[0],
…array-name[size-1]
Example:
int a[100]; //a is a list of 100 integers, a[0], a[1], …a[99]
double b[50];
char c[10];

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Array example
//Read 100 numbers from the user
void main() {
int i, a[100], n;
i=0; n=100;
while (i<n) {
cout << “Input element “ << i << “: ”;
cin >> a[i];
i = i+1;
}
//do somehing with it ..
}

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Exercises
Write a C++ program to read a sequence of (non-negative)
integers from the user ending with a negative integer and
write out
 the average of the numbers
 the smallest number
 the largest number
 the range of the numbers (largest - smallest)
 Example:
 The user enters: 3, 1, 55, 89, 23, 45, -1
 Your program should compute the average of {3, 1, 55, 89, 23,
45} etc

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Exercises
 Write a program that asks the user
 Do you want to use this program? (y/n)
 If the user says ‘y’ then the program
terminates
 If the user says ‘n’ then the program asks
 Are you really sure you do not want to use this program? (y/n)
 If the user says ‘n’ it terminates, otherwise it prints
again the message
 Are you really really sure you do not want to use this program?
(y/n)
 And so on, every time adding one more “really”.

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Pointers
int *intPtr;
intPtr = new int;
*intPtr = 6837;
delete intPtr;
int otherVal = 5;
intPtr = &otherVal;
Create a pointer
Allocate memory
Set value at given address
*intPtr 6837
intPtr 0x0050
Change intPtr to point to
a new location
*intPtr 5
intPtr 0x0054
otherVal
&otherVal
Deallocate memory

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Arrays
Stack allocation
int intArray[10];
intArray[0] = 6837;
Heap allocation
int *intArray;
intArray = new int[10];
intArray[0] = 6837;
...
delete[] intArray;

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Strings
A string in C++ is an array of characters
char myString[20];
strcpy(myString, "Hello World");
Strings are terminated with the NULL or '0' character
myString[0] = 'H';
myString[1] = 'i';
myString[2] = '0';
printf("%s", myString);
output: Hi

Prevents multiple references
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Class Basics
#ifndef _IMAGE_H_
#define _IMAGE_H_
#include <assert.h>
#include "vectors.h“
class Image {
public:
...
private:
...
};
#endif
Include a library file
Include a local file
Variables and functions
accessible from anywhere
Variables and functions accessible
only from within this class

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C++ Functions
Predefined Functions
 C++ comes with libraries of predefined
functions
 Example: sqrt function
 the_root = sqrt(9.0);
 returns, or computes, the square root
of a number
 The number, 9, is called the argument
 the_root will contain 3.0
3.2

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Function Calls
 sqrt(9.0) is a function call
 It invokes, or sets in action, the sqrt function
 The argument (9), can also be a variable or an
expression
 A function call can be used like any
expression
 bonus = sqrt(sales) / 10;
 Cout << “The side of a square with area “ <<
area
<< “ is “ << sqrt(area);

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Function Call Syntax
 Function_name (Argument_List)
 Argument_List is a comma separated list:
(Argument_1, Argument_2, … , Argument_Last)
 Example:
 side = sqrt(area);
 cout << “2.5 to the power 3.0 is “ << pow(2.5, 3.0);

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Function Libraries
 Predefined functions are found in libraries
 The library must be “included” in a program
to make the functions available
 An include directive tells the compiler which
library header file to include.
 To include the math library containing sqrt():
#include <cmath>
 Newer standard libraries, such as cmath, also
require the directive using namespace std;

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Other Predefined Functions
 abs(x) --- int value = abs(-8);
 Returns absolute value of argument x
 Return value is of type int
 Argument is of type x
 Found in the library cstdlib
 fabs(x) --- double value = fabs(-8.0);
 Returns the absolute value of argument x
 Return value is of type double
 Argument is of type double
 Found in the library cmath

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Type Casting
 Look at this problem with integer division:
int total_candy = 9, number_of_people = 4;
double candy_per_person;
candy_per_person = total_candy / number_of_people;
 candy_per_person = 2, not 2.25!
 A Type Cast produces a value of one type
from another type
 static_cast<double>(total_candy) produces a double
representing the integer value of total_candy

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Type Cast Example
 int total_candy = 9, number_of_people = 4;
double candy_per_person;
candy_per_person = static_cast<double>(total_candy)/
number_of_people; candy_per_person now is 2.25!
 This would also work:
candy_per_person = total_candy /
static_cast<double>( number_of_people);
 This would not!
candy_per_person = static_cast<double>( total_candy /
number_of_people);
Integer division occurs before type cast

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Exercise
 Can you
 Determine the value of d?
double d = 11 / 2;
 Determine the value of
pow(2,3) fabs(-3.5), sqrt(pow(3,2))
7 / abs(-2),ceil(5.8),floor(5.8)
 Convert the following to C++
x + y xy+7
b b ac
- + 2 - 4
a
2

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Programmer-Defined Functions
 Two components of a function definition
 Function declaration (or function prototype)
 Shows how the function is called
 Must appear in the code before the function can be called
 Syntax:
Type_returned Function_Name(Parameter_List);
//Comment describing what function does
 Function definition
 Describes how the function does its task
 Can appear before or after the function is called
 Syntax:
Type_returned Function_Name(Parameter_List)
{
//code to make the function work
}
;
3.3

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Function Declaration
 Tells the return type
 Tells the name of the function
 Tells how many arguments are needed
 Tells the types of the arguments
 Tells the formal parameter names
 Formal parameters are like placeholders for the actual
arguments used when the function is called
 Formal parameter names can be any valid identifier
 Example:
double total_cost(int number_par, double price_par);
// Compute total cost including 5% sales tax on
// number_par items at cost of price_par each

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Function Definition
 Provides the same information as the declaration
 Describes how the function does its task
 Example:
function header
double total_cost(int number_par, double price_par)
{
const double TAX_RATE = 0.05; //5% tax
double subtotal;
subtotal = price_par * number_par;
return (subtotal + subtotal * TAX_RATE);
}
function body

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The Return Statement
 Ends the function call
 Returns the value calculated by the function
 Syntax:
return expression;
 expression performs the calculation
or
 expression is a variable containing the
calculated value
 Example:
return subtotal + subtotal * TAX_RATE;

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The Function Call
 Tells the name of the function to use
 Lists the arguments
 Is used in a statement where the
returned value makes sense
 Example:
double bill = total_cost(number, price);

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Function Call Details
 The values of the arguments are plugged into
the formal parameters (Call-by-value
mechanism with call-by-value parameters)
 The first argument is used for the first formal
parameter, the second argument for the second
formal parameter, and so forth.
 The value plugged into the formal parameter is used
in all instances of the formal parameter in the
function body

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Alternate Declarations
 Two forms for function declarations
1. List formal parameter names
2. List types of formal parmeters, but not names
 First aids description of the function in comments
 Examples:
double total_cost(int number_par, double price_par);
double total_cost(int, double);
 Function headers must always list formal
parameter names!

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Order of Arguments
 Compiler checks that the types of the arguments
are correct and in the correct sequence.
 Compiler cannot check that arguments are in the
correct logical order
 Example: Given the function declaration:
char grade(int received_par, int min_score_par);
int received = 95, min_score = 60;
cout << grade( min_score, received);
 Produces a faulty result because the arguments are not in
the correct logical order. The compiler will not catch this!

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Function Definition Syntax
 Within a function definition
 Variables must be declared before they
are used
 Variables are typically declared before
the executable statements begin
 At least one return statement must end
the function
 Each branch of an if-else statement might
have its own return statement

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Placing Definitions
 A function call must be preceded by either
 The function’s declaration
or
 The function’s definition
 If the function’s definition precedes the call, a
declaration is not needed
 Placing the function declaration prior to the
main function and the function definition
after the main function leads naturally to
building your own libraries in the future.

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Parameter Passing
pass by value
int add(int a, int b) {
return a+b;
}
int a, b, sum;
sum = add(a, b);
pass by reference
int add(int *a, int *b) {
return *a + *b;
}
int a, b, sum;
sum = add(&a, &b);
Make a local copy of a & b
Pass pointers that reference
a & b. Changes made to a
or b will be reflected
outside the add routine

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Parameter Passing
pass by reference – alternate notation
int add(int &a, int &b) {
return a+b;
}
int a, b, sum;
sum = add(a, b);

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Exercise
 Can you
 Write a function declaration and a function definition
for a function that takes three arguments, all of type
int, and that returns the sum of its three arguments?
 Describe the call-by-value parameter mechanism?
 Write a function declaration and a function definition
for a function that takes one argument of type int and
one argument of type double, and that returns a value
of type double that is the average of the two
arguments?

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Good sites
 http://www.cs.ucr.edu/cs10/cs10_03fal/slides/

2.overview of c++ ________lecture2

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