Lecture#2 Computer languages computer system and Programming EC-105

Computer System & Programming
Department of Computer Engineering (DCE)
College of E&ME
NUST

Machine Languages, Assembly Languages,
and High-level Languages
 Three types of computer languages
1. Machine language
• Only language computer directly understands
• “Natural language” of computer
• Defined by hardware design
– Machine-dependent
• Generally consist of strings of numbers
– Ultimately 0s and 1s
• Instruct computers to perform elementary operations
– One at a time
• Cumbersome for humans
• Example:
+1300042774
+1400593419
+1200274027

2. Assembly language
• English-like abbreviations representing elementary computer
operations
• Clearer to humans
• Incomprehensible to computers
– Translator programs (assemblers)
• Convert to machine language
• Example:
LOAD PURCHASE PRICE
ADD SALES TAX
STORE RETAIL PRICE

3. High-level languages
• Similar to everyday English, use common mathematical
notations
• Single statements accomplish substantial tasks
– Assembly language requires many instructions to
accomplish simple tasks
• Translator programs (compilers)
– Convert to machine language
• Example:
RETAIL PRICE = PURCHASE PRICE + SALES TAX

Basics of a Typical C++ Environment
 C++ systems
– Program-development environment
– Language
– C++ Standard Library

Basics of a Typical C++ Environment
Phases of C++ Programs:
1. Edit
2. Preprocess
3. Compile
4. Link
5. Load
6. Execute
Loader
Primary
Memory
Program is created in
the editor and stored
on disk.
Preprocessor program
processes the code.
Loader puts program
in memory.
CPU takes each
instruction and
executes it, possibly
storing new data
values as the program
executes.
Compiler
Compiler creates
object code and stores
it on disk.
Linker links the object
code with the libraries,
Creates an executable
file and stores it on disk
Editor
Preprocessor
Linker
CPU
Primary
Memory
.
.
.
.
.
.
.
.
.
.
.
.
Disk
Disk
Disk
Disk
Disk

A Simple Program:
Printing a Line of Text
 Input/output
– cin
• Standard input stream
• Normally keyboard
– cout
• Standard output stream
• Normally computer screen

A Simple Program:
 Comments
– Document programs
– Improve program readability
– Ignored by compiler
– Single-line comment
• Begin with //
 Preprocessor directives
– Processed by preprocessor before compiling
– Begin with #

A Simple Program:
 Standard output stream object
– “Connected” to screen
– <<
• Stream insertion operator
• Value to right (right operand) inserted into output stream
 Escape characters
–
– Indicates “special” character output

1
2 // A first program in C++.
3 #include <iostream.h>
4
5 // function main begins program execution
6 int main()
7 {
8 cout << "Welcome to C++!n";
9
10 return 0; // indicate that program ended successfully
11
12 } // end function main
Welcome to C++!
Single-line comments.
Preprocessor directive to
include input/output stream
header file <iostream>.
Function main appears
exactly once in every C++
program..
Function main returns an
integer value.Left brace { begins function
body.
Corresponding right brace }
ends function body.
Statements end with a
semicolon ;.
Name cout belongs to
namespace std.
Stream insertion operator.
Keyword return is one of
several means to exit
function; value 0 indicates
program terminated
successfully.

1
2 // Printing a line with multiple statements.
4
6 int main()
7 {
8 cout << "Welcome ";
9 cout << "to C++!n";
10
12
Welcome to C++!
Multiple stream insertion
statements produce one line of
output.

1
2 // Printing multiple lines with a single statement
4
6 int main()
7 {
8 cout << "WelcomentonnC++!n";
9
11
Welcome
to

C++!
Using newline characters to
print on multiple lines.

Another Simple Program:
Adding Two Integers
 Variables
– Location in memory where value can be stored
– Common data types
• int - integer numbers
• char - characters
• double - floating point numbers
– Declare variables with name and data type before use
int integer1;
int integer2;
int sum;
– Can declare several variables of same type in one declaration
• Comma-separated list
int integer1, integer2, sum;

Adding Two Integers
 Variables
– Variable names
• Valid identifier
– Series of characters (letters, digits, underscores)
– Cannot begin with digit
– Case sensitive

Adding Two Integers
 Input stream object
– >> (stream extraction operator)
• Waits for user to input value, then press Enter (Return) key
• Stores value in variable to right of operator
– Converts value to variable data type
 = (assignment operator)
– Assigns value to variable
– Binary operator (two operands)
– Example:
sum = variable1 + variable2;

1
2 // Addition program.
4
6 int main()
7 {
8 int integer1; // first number to be input by user
9 int integer2; // second number to be input by user
10 int sum; // variable in which sum will be stored
11
12 cout << "Enter first integern"; // prompt
13 cin >> integer1; // read an integer
14
15 cout << "Enter second integern"; // prompt
16 cin >> integer2; // read an integer
17
18 sum = integer1 + integer2; // assign result to sum
19
20 cout << "Sum is " << sum << endl; // print sum
21
23
Declare integer variables.
Use stream extraction
operator with standard input
stream to obtain user input.
Stream manipulator
std::endl outputs a
newline, then “flushes output
buffer.”
Concatenating, chaining or
cascading stream insertion
operations.

Enter first integer
45
Enter second integer
72
Sum is 117

Memory Concepts
 Variable names
– Correspond to actual locations in computer's memory
– Every variable has name, type, size and value
– When new value placed into variable, overwrites previous
value
– Reading variables from memory nondestructive

Memory Concepts
cin >> integer1;
– Assume user entered 45
cin >> integer2;
– Assume user entered 72
sum = integer1 + integer2;
integer1 45
integer1 45
integer2 72
integer1 45
integer2 72
sum 117

Arithmetic
 Arithmetic calculations
– *
• Multiplication
– /
• Division
• Integer division truncates remainder
– 7 / 5 evaluates to 1
– %
• Modulus operator returns remainder
– 7 % 5 evaluates to 2

Arithmetic
 Rules of operator precedence
– Operators in parentheses evaluated first
• Nested/embedded parentheses
– Operators in innermost pair first
– Multiplication, division, modulus applied next
• Operators applied from left to right
– Addition, subtraction applied last
• Operators applied from left to rightOperator(s) Operation(s) Order of evaluation (precedence)
() Parentheses Evaluated first. If the parentheses are nested, the
expression in the innermost pair is evaluated first. If
there are several pairs of parentheses “on the same level”
(i.e., not nested), they are evaluated left to right.
*, /, or % Multiplication Division
Modulus
Evaluated second. If there are several, they re
evaluated left to right.
+ or - Addition
Subtraction
Evaluated last. If there are several, they are
evaluated left to right.

Decision Making: Equality and Relational
Operators
 if structure
– Make decision based on truth or falsity of condition
• If condition met, body executed
• Else, body not executed
 Equality and relational operators

Operators
Standard algebraic
equality operator or
relational operator
C++ equality
or relational
operator
Example
of C++
condition
Meaning of
C++ condition
Relational operators
> > x > y x is greater than y
< < x < y x is less than y
≥ >= x >= y x is greater than or equal to y
≤ <= x <= y x is less than or equal to y
Equality operators
= == x == y x is equal to y
≠ != x != y x is not equal to y

Operators
 if ( x == y )
cout << x << " is equal to " << y << endl;

1
2 // Using if statements, relational
3 // operators, and equality operators.
5
6
7
8
9
11 int main()
12 {
13 int num1; // first number to be read from user
14 int num2; // second number to be read from user
15
16 cout << "Enter two integers, and I will tell youn"
17 << "the relationships they satisfy: ";
18 cin >> num1 >> num2; // read two integers
19
20 if ( num1 == num2 )
21 cout << num1 << " is equal to " << num2 << endl;
22
23 if ( num1 != num2 )
24 cout << num1 << " is not equal to " << num2 << endl;
25
Can write cout and cin
without std:: prefix.
Declare variables.
if structure compares values
of num1 and num2 to test for
equality.
If condition is true (i.e.,
values are equal), execute this
statement.if structure compares values
of num1 and num2 to test for
inequality.
If condition is true (i.e.,
values are not equal), execute
this statement.

26 if ( num1 < num2 )
27 cout << num1 << " is less than " << num2 << endl;
28
29 if ( num1 > num2 )
30 cout << num1 << " is greater than " << num2 << endl;
31
32 if ( num1 <= num2 )
33 cout << num1 << " is less than or equal to "
34 << num2 << endl;
35
36 if ( num1 >= num2 )
37 cout << num1 << " is greater than or equal to "
38 << num2 << endl;
39
41
Enter two integers, and I will tell you
the relationships they satisfy: 22 12
22 is not equal to 12
22 is greater than 12
22 is greater than or equal to 12
Statements may be split over
several lines.

Enter two integers, and I will tell you
the relationships they satisfy: 7 7
7 is equal to 7
7 is less than or equal to 7
7 is greater than or equal to 7

Algorithms
 Before writing a program
– Have a thorough understanding of problem
– Carefully plan your approach for solving it

Algorithms
 Computing problems
– Solved by executing a series of actions in a specific order
 Algorithm a procedure determining
– Actions to be executed
– Order to be executed
– Example: recipe
 Program control
– Specifies the order in which statements are executed

Pseudocode
 Pseudocode
– Artificial, informal language used to develop algorithms
– Similar to everyday English
 Not executed on computers
– Used to think out program before coding
• Easy to convert into C++ program
– Only executable statements
• No need to declare variables

Lecture#2 Computer languages computer system and Programming EC-105

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Lecture#2 Computer languages computer system and Programming EC-105