C++ Basics 02

Slide 2- 5
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Display 2.1
Variables and Assignments

Variables are like small blackboards

We can write a number on them

We can change the number

We can erase the number

C++ variables are names for memory locations
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We can write a value in them

We can change the value stored there

We cannot erase the memory location

Some value is always there
Slide 2- 6
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Identifiers
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Variables names are called identifiers
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Choosing variable names
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Use meaningful names that represent data to
be stored

First character must be

a letter

the underscore character
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Remaining characters must be

letters

numbers

underscore character
Slide 2- 7
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Keywords
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Keywords (also called reserved words)
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Are used by the C++ language

Must be used as they are defined in
the programming language

Cannot be used as identifiers
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Declaring Variables (Part 1)

Before use, variables must be declared

Tells the compiler the type of data to store
Examples: int number_of_bars;
double one_weight, total_weight;
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int is an abbreviation for integer.

could store 3, 102, 3211, -456, etc.

number_of_bars is of type integer

double represents numbers with a fractional
component

could store 1.34, 4.0, -345.6, etc.

one_weight and total_weight are both of type double
Slide 2- 9
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Declaring Variables (Part 2)

Immediately prior to use
int main()
{
…
int sum;
sum = score1 + score 2;
…
return 0;
}

At the beginning
int main()
{
int sum;
…
sum = score1 +
score2;
…
return 0;
}
Two locations for variable declarations
Slide 2- 10
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Declaring Variables (Part 3)

Declaration syntax:

Type_name Variable_1 , Variable_2, . . . ;

Declaration Examples:

double average, m_score, total_score;

double moon_distance;

int age, num_students;

int cars_waiting;
Slide 2- 11
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Assignment Statements

An assignment statement changes the value of a variable

total_weight = one_weight + number_of_bars;

total_weight is set to the sum one_weight + number_of_bars

Assignment statements end with a semi-colon

The single variable to be changed is always on the left
of the assignment operator ‘=‘

On the right of the assignment operator can be

Constants age = 21;

Variables my_cost = your_cost;

Expressions circumference = diameter * 3.14159;
Slide 2- 12
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Assignment Statements and Algebra

The ‘=‘ operator in C++ is not an equal sign

The following statement cannot be true in
algebra

number_of_bars = number_of_bars + 3;

In C++ it means the new value of
number_of_bars
is the previous value of number_of_bars plus 3
Slide 2- 13
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Initializing Variables

Declaring a variable does not give it a value

Giving a variable its first value is initializing the variable

Variables are initialized in assignment statements
double mpg; // declare the variable
mpg = 26.3; // initialize the variable

Declaration and initialization can be combined
using two methods

Method 1
double mpg = 26.3, area = 0.0 , volume;

Method 2
double mpg(26.3), area(0.0), volume;
Slide 2- 14
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Section 2.1 Conclusion

Can you

Declare and initialize two integers variables to zero?
The variables are named feet and inches.

Declare and initialize two variables, one int and one
double?
Both should be initialized to the appropriate form of 5.

Give good variable names for identifiers to store

the speed of an automobile?

an hourly pay rate?

the highest score on an exam?

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