Basics of program writing (Variables and operators)

Test Program

• #include<iostream>
• #include <conio.h>
• using namespace std;
• void main()
• {
• cout << “Hello World” << endl;
• getch();
• }

Compilation

• Performed by a program called the compiler
• Translates the preprocessor-modified source code into object code (machine code)
• Checks for syntax errors and warnings
• Saves the object code to a disk file, if instructed to do so.
• If any compiler errors are received, no object code file will be generated.
• An object code file will be generated if only warnings, not errors, are received.

Program Development

• Editor produces Source File

program.cpp

• Preprocessor Modified Source Code

not saved on disk

• Compiler produces Object Code File

program.obj

• Linker links Object Code File and Other Object Code Files (if any) and produces Executable File

program.exe

Basic Structure of C++ programs

#include <iostream>

Using namespace std;

void main ( )

{

cout << “Hello World”;

}

void main ( )

• Every C++ program consists of one or more functions.
• main ( ) is a function. It is the first function to which control is passed from OS when a program is executed
• void indicates that the main function does not return any value

Basic Structure of C++ programs

#include <iostream>

Using namespace std;

void main ( )

{

cout << “Hello World”;

}

Syntax

• cout is a statement.
• A statement in C++ is terminated by a semi colon ( ; ) .

Delimiters

Opening and Closing brace or Curly brackets marks the start and end of the code

The start and end of a block ( such as loop, if else statement, switch statement) is also identified by the start and end of curly brackets.

Syntax

• C++ pays no attention to the “whitespace” characters encountered in a program like space, carriage return(newline) and tab
• You can put as many whitespaces in your program as you like; they will be invisible to the compiler

Indentation

Stretching the code vertically makes it more readable

Indentation of blocks of code enclosed in braces is an important aspect for making the programs readable

e.g. the printf statement that is indented in the previous example

The same program below will execute perfectly

Variable Names:

Name assigned to a memory location

• Variables are like containers in your computer’s memory
• you can store values in them and retrieve or modify them when necessary

Assignment Operator

3 = 5  In correct

X + 1 = y + 2   In correct

x + 5 = Y   Incorrect

Z = y + 4  Correct

When l-value has assigned a r-value to it, the current l-value is overwritten with r-value

Variable names are called identifiers

But

all the identifiers are not variable names!!!

Identifiers Refer to the  Names of:

• variables
• data types
• Constants
• functions

Identifier: Variable Names

Any combination of letters, numbers, and underscore (_)

• Case sensitive

“sum” is different than “Sum“ or “SUM”

• Cannot begin with a digit

1stName ——  illegal

devideby10 —-  legal

• Usually, variables beginning with underscore are used only in special library routines.

_validsystemcall  —-  legal

Identifier: Variable Names

• Usually only the first 32 characters are significant

aReally_longName_moreThan31chars

aReally_longName_moreThan31characters

(both of the above identifiers are legal and  the same)

• There can be no embedded blanks or special character

gross salary  (  is illegal because it contains space )

%rate  (  is illegal because it contains special      character )

• Keywords or Reserve words cannot be used as identifiers

float   ( is illegal because it contains a key word )

Keywords / Reserved Words

• Some words may not be used as identifiers
• These words have special meaning in C and C++

Keywords

auto break case char const continue default do double else

enum extern float for goto if int long register return short

signed sizeof static struct switch typedef union unsigned void volatile while

Data types in C/ C++

• Integer declared as int
• float declared as float
• double declared as double
• Character declared as char
• Void declared as void

Size and Types of Data Types

Int Integer stores an integer value e.g. 125

  int  (at least 16 bits, commonly 32 bits)

  long (at least 32 bits)

short (at least 8 bits)

float floating point (at least 32 bits)

stores decimal value  (e.g. 125.1234567)

double floating point (commonly 64 bits bits)

stores decimal value

(e.g. 125.123456789012345)

double the precision of a float

char character (at least 8 bits)

stores an integer representing a character   (e.g. ‘A’)

different codes of 8 bit and 15 bit

ANSCII – American National Standard Code        for Information Interchange (8 bits)

EBCDIC – Extended Binary Coded Decimal

interchange Code (8 bits)

UNICODE – 15 bit code

void means nothing ( zero bytes)

used for the function return nothing.

e.g. void main ( void)

Size of Data Types

Exact size can vary, depending on processor

• int is supposed to be “natural” integer size;

for older processors, it’s 16 bits

for most modern processors  it’s 32 bits

How can you know the size?

• Use function called sizeof.

e.g. sizeof(int),  it returns answer in bytes

Additional to Data Type

Like constants but is preprocessor directive

Literal

• Unnamed constants that appear literally in source code.

Constants

• Variables whose values do not change during the execution of the program
• This done by appending ‘const” before the data type

Integer

• 125 ( positive decimal )
• -125 ( negative decimal )
• 0x125 ( treated as octal)

Character

• ‘s‘ (character s )
• ‘\t’ ( tab )
• ‘\xC’ (ASCII 12 (0xC) )

Floating point

• 6.023 ( real value)
• 6.023e23 ( 6.023 x 1023 )
• 5E12  ( 5.0 x 1012 )

Constants and Symbol

#define RADIUS 15.0   //symbol

int main( )

{

const double pi = 3.14159;  //constant  //literal

double area;

double circum;

area = pi * RADIUS * RADIUS;

circum = 2 * pi * RADIUS;

}

Operators

An operator has:

• Function

What it does?

Example: + , -, *, / or others

• Precedence

In which order it will execute

Example:   “a * b + c * d”

will be executed as : “(a * b) + (c * d)”

because multiply (*) has a higher precedence   than addition (+)

Operators

• Associativity

Order In which the operators of the   same precedence combined?

Example: “a – b – c”

will be executed in the order “(a – b) – c”

because add/sub associate left-to-right

Assignment Operator

• l-value equals to r-value

  x = x + 5;

Evaluate right-hand side (x + 5 )   called r-value

Set l-value ( of left-hand side variable) to r-value.

• All expressions evaluate to a value with the assignment operator

Example:     y = x = 5;

the result is the value assigned

• Assignment is associated from right to left.

  y = x = 5;

y gets the value 5,

because (x = 5) evaluates to the value 5

Arithmetic Operators

Symbol  Operation  Example  Association

• * multiply x * y   Left to Right
• / divide x / y   Left to Right
• % modulo x % y   Left to Right
• + addition x + y   Left to Right
• – subtraction x – y  Left to Right

* / %   have same precedence

whichever comes from left to right

have higher precedence than + –

+  –  have same precedence

whichever comes from left to right

have lower precedence than + –

• Addition, subtraction, and multiplication work as you would expect.
• Division (/) returns the whole part of the division (the quotient)

12 / 3 = 4   15 / 2 = 7

• Modulus (%) returns the remainder

12 % 3 = 0   15 % 2 = 1

• Example of precedence

2 + 3 * 4   equals  14

where as  (2 + 3) * 4  equals  20

Arithmetic Expressions

• If mixed types, smaller type is “promoted” to larger

Example: x + 4.3

if x is int, converted to double and result is double

• Integer division — fraction is dropped

Example: x / 3

if x is int and x=5, result is 1 (not 1.666666…)

• Parentheses ( ) can be used to force a different order of evaluation:

12 – 5 * 2 = 2

But  (12 – 5) * 2 = 14

Arithmetic Operator Precedence

• Precedence rules specify the order in which operators are evaluated
• Associativity determines from Left-to- Right order
• Remember for arithmetic operators precedence

PMDAS

Parentheses, Multiplication, Division, Addition, Subtraction

Bitwise Operators

~   bitwise NOT   ~x

<<   shift left    x << y

>>   shift right  x >> y

&   bitwise  AND   x & y

^   bitwise  XOR   x ^ y

|   bitwise  OR   x | y

Will be taught in future

Logical Operators

Symbol  Operation  Example  Association

!   Logical NOT   !x   Right to Left

&&   logical AND   x && y   Right to Left

||   logical OR  x || y  Right to Left

Treats entire variable (or value) as:

TRUE (non-zero), or    FALSE (zero)

Result is 1 (TRUE) or 0 (FALSE)

Relational Operators

Symbol  Operation  Example  Association

>   greater than   x > y   R-to-L

>=   greater than or equal   x >= y  R-to-L

<   less than   x < y  R-to-L

<=   less than or equal   x <= y  R-to-L

==   equal   x == y  R-to-L

!=   not equal   x != y  R-to-L

Result is 1 (TRUE) or 0 (FALSE)

Assignment ( =) vs. Equality ( ==)

Be Careful using equality (==) and assignment (=) operators:

int x = 5;

int y = 7;

if (x == y) {

printf(“ will not be printed\n”);

}

if (x = y) {

printf(“x = %d y = %d”, x, y);

}

Result: “x = 7 y = 7” is printed.

Explain?

Special Operators: ++ and —

Increment and decrement  the value of variable before or after its value is used in an expression.

Symbol  Example  Operation

++   x++    Increment after (Post increment)

—   x–    decrement after (Post decrement)

++   ++x    Increment before (Pre increment)

—   –x    decrement before (Pre decrement)

Examples Using ++ and —

Example-1:

int x = 4;

y = x++;

printf ( “X = %d  \t “Y = %d “,  x, y )

will print  X = 5  Y = 4

as x is incremented after assignment operation.

Example-2:

x = 4;

y = ++x;

printf ( “X = %d  \t “Y = %d “,  x, y )

will print  X = 5  Y = 5

as x is incremented before assignment operation.

Special Operators: +=, *=, etc.

Arithmetic operators and bitwise operators can be combined with assignment operator

Equivalent assignment statement:

x += y;   Equivalent to  x = x + y;

x -= y;   Equivalent to   x = x – y;

x *= y;   Equivalent to   x = x * y;

x /= y;   Equivalent to   x = x / y;

x %= y;   Equivalent to   x = x % y;

x &= y;   Equivalent to   x = x & y;

x |= y;   Equivalent to   x = x | y;

x ^= y;   Equivalent to   x = x ^ y;

x <<= y;   Equivalent to   x = x << y;

x >>= y;   Equivalent to   x = x >> y;

Special Operator: Conditional

Symbol  Operation  Example

?:   conditional   x?y:z

Explanation: x ? y : z

If x is non-zero,(true) then result of expression is y

If x is zero, (false) then result of expression is z

Practice Quiz

Question -1: Solve the following expressions

a).  3 – 8 / 4  b).    3 * 4 + 18 / 2

Solution:

• / has the highest precedence, so we

compute 8 / 4 first, then subtract the result

from 3 the answer is 1

• 3 * 4 + 18 / 2

12  +   9

The answer is 21

Question – 2:  If  a=2, b=4, c=6, d=8, then what will be the result of the following expression?

x = a * b + c * d / 4;

Will be solved as:

x = (a * b) + ((c * d) / 4);

x = ( 2 * 4) + ((6 * 8) / 4);

x =  ( 8 )    +   (12 )

x =         20

Question – 3: Put the parentheses in the order of execution of the following expression.

5 – 3 + 4 + 2 – 1 + 7

• + and – have equal precedence, so this

expression is evaluated left to right:

(((((5 – 3) + 4) + 2) – 1) + 7)

• Innermost parentheses are evaluated first