CHAPTER THREE

MORE ON:

INPUT AND OUTPUT,

INCREMENT AND DECREMENT

OPERATORS,

PREPROCESSOR DIRECTIVES,

INPUT OUTPUT MANIPULATION,

DEBUGGING (FIXING SYNTAX ERRORS)

LECTURER: Samson Mekbib

INPUT AND

OUTPUT

SUMMARY AND FEW MORE

POINTS

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INTRODUCTION

The keywords for input and output operations are stored in the standard library called iostream

Input = cin, output = cout

To use the cout and cin commands in C++, you need to include the following two lines:

#include <iostream>

Using namespace std;

The first line which start with # is a preprocessor directive

Lines that begin with # are preprocessed by the preprocessor before the program is compiled

The second line using namespace tells C++ to use the standard names cout and cin

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STRING DATA TYPE

To use the string type in C++, we need to access its definition from

the header file string

To process strings effectively, C++ provides the type string

Include the following preprocessor directive:

#include <string>

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CONSTANTS

Some data must stay the same throughout a program

In C++, you can use a named constant to instruct the program to mark those

memory locations in which data is fixed throughout program execution.

Named constant: A memory location whose content is not allowed to change during

program execution

The syntax to declare a named constant is:

Const dataType identifier;

Identifier = value;

An equivalent form of the above syntax is:

Const dataType identifier = value;

Examples:

const int No_of_students = 54;

const float Pay_Rate = 15.75 5

INPUT (READ) STATEMENT

Syntax

cin >> variable >> variable …;

cin is used with >> to gather input

This is called an input (read) statement

Using more than one variable in cin allows more than one value to be read at a time

Example:

cin >> feet >> inches;

This will input tow values from the keyboard (specified by the user) and places them in variables feet and inches respectively.

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OUTPUT

Syntax:

cout << expression or manipulator << expression or manipulator …;

Is call an output statement

The stream operator is <<

Text to be displayed must be typed in between double quotations

“””

To print out values of identifiers like variables and constants we

write them without “”

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MANIPULATOR

A manipulator is used to format the output

Example: endl causes insertion point to move to the beginning of

the next line.

Instead of endl the new line character ‘\n’ could be used

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COMMONLY USED ESCAPE

SEQUENCE MANIPULATORS

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SPECIAL STATEMENTS IN C++

A C++ statement such as

num = num + 2;

means “evaluate whatever is in num, and 2 to it, and assign the

new value to the memory location num.”

The expression on the right side will be evaluated first and then

that value will be assigned to the variable.

Example:

num = 6;

num = num + 2;

After running the second statement C++ will assign 8 to the

variable num.

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SPECIAL STATEMENTS CONTD.

Example: Suppose that num1, num2 and num3 are int variables and the following statements are executed in sequence

1. num1 = 18;

2. num1 = num1 + 27;

3. num2 = num1;

4. num3 = num2/5;

5. num3 = num3 / 4; The output at the end of each step will be as follows:

Before statement 1 no value was assigned to num1, num2, num3

After statement 1 num1 equals 18 and no value assigned to num2 and num3

After statement 2 num2 equals 45 and no value assigned to num2 and num3

After statement 3 num1 equals 45, num2 equals 45 and no value assigned to num3

After statement 4 num1=45, num2=45, num3 = 9

After statement 5 num1 = 45, num2 = 45, num3 = 2 11

INCREMENT AND DECREMENT

OPERATORS

Often used by programmers and they are useful programming tools

Suppose count is an int variable. The statement

count = count + 1;

Increments the value of count by 1.

Such statements are frequently used to keep track of how many times certain things have happened

To simplify such statements C++ provides the increment operator, ++, which increases the value of a variable by 1, and the decrement operator, --, which decreases the value of a variable by 1

Syntax of pre-increment: ++variable

Syntax of post-increment: variable++

Syntax of pre-decrement: --variable

Syntax of post-decrement: variable--

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EXAMPLE - 1 ON PRE AND POST

- INCREMENT

Consider the following statement:

x = 5;

y = ++x;

The first statement assigns the value 5 to x. To evaluate the second statement, which uses the pre-increment operator, first the value of x is incremented to 6, and then this value 6 is assigned to y. After second statement execute both x and y will have the value 6.

Now consider the following statements:

x=5;

y = x++;

As before the 1st statement assigns 5 to x. In the 2nd statement since post-increment is used, the value 5 before increment is assigned to y, and x is incremented to 6. Hence after the second statement x = 6 and y= 5.

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EXAMPLE - 2 ON PRE AND POST

- INCREMENT

Consider the following statement:

a = 5;

b = 2 + (++a);

The first statement assigns the value 5 to a. To evaluate the second

statement, which uses the pre-increment operator, first the value of a

is incremented to 6, then 2 is added to it and the value 8 is assigned

to b. Therefore, after the second statement executes, a is 6 and b is

8.

Now consider the following statements:

a = 5;

b = 2 + (a++);

After the second statement is executed the value of a is 6 while the

value of b is 7.

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MORE PREPROCESSOR

DIRECTIVES

In a C++ system, a preprocessor program executes automatically

before the compiler's translation phase begins

Preprocessor directive statements will not end with a semicolon (;)

So far we have seen the preprocessor directives #include

<iostream> and #include <string> that allows us to handle input out

operations and using the string variables respectively in C++

Other preprocessor directive types are #include<filename>, which

are used to include saved C++ files with extension either .cpp or .h

Another preprocessor directive type is #include<iomanip> that

allows some C++ output manipulation commands like setw().

The manipulator set width is used for setting field output in C++

Output manipulators we saw so far (\n, \t, ‘\b’ etc.) do not need a

preprocessor directive. But to run the manipulator setw() we need to

write #include<iomanip> at the start of the program 15

SET WIDTH MANIPULATOR

Syntax setw(num)

setw() is library function in C++

setw() is declared inside #include<iomanip>

setw() will set field width

Setw() sets the number of characters to be used as the field width for the next insertion operation

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DEBUGGING: UNDERSTANDING

AND FIXING SYNTAX ERRORS

Compile a program

• Compiler will identify the syntax errors

• Specifies the line numbers where the error occur

Examples:

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SYNTAX

Syntax rules- indicate what is legal and what is not legal

Error in syntax are found in compilation

Errors and debugging is common in programming

Examples

int x; //Line 1

Int y; // Line 2: error

float b // line 3: error

double z = w + x; // line 4: error, identifier w not defined

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USE OF BLANKS

In C++, you use one or more blanks to separate

numbers when data is input

Blanks are used to separate reserved words and

identifiers from each other and from other symbols

Blanks must never appear within a reserved word or

identifier

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USE OF SEMICOLONS,

BRACKETS, AND

COMMAS

All C++ statements end with a semicolon

Semicolons are also called statement terminator

Brackets like { indicate start of main function and a

closing bracket } indicates end of function main

Commas are used to separate items in a list

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SEMANTICS

Semantics: set of rules that gives meaning in a language

• It is possible to remove all syntax errors in a program and still

not able to run it

• Even if it runs, it may still not do what we expect it to do

Example: 2 + 3 * 5 and (2 + 3) * 5

are both syntactically correct expressions, but have different

meanings.

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NAMING IDENTIFIERS

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PROMPT LINES

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DOCUMENTATION

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FORM AND STYLE

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