Variables: Named Storage Locations
Variables must be defined or declared before they
can be used so that appropriate memory storage can
be allocated for them.
Examples of declaration (or definition) statements
int number_of_tests;
float test_average;
int grade_test1, grade_test2, grade_test3;
char letter_grade;
Assigning Values to Variables
• =
is the assignment operator
o number_of_tests = 3;
o letter_grade = ‘B’;
o test_average = 88.56;
• Type of literal assigned should match the type
declaration/definition of the variable
• If a floating point value is assigned to a variable of
type ‘int’ only the integer part will be stored
(fractional part is truncated NOT rounded)
• Care should be taken to not exceed the maximum
storage value of the data type
Note: Until an assignment statement is executed, the
contents of the storage area for a variable are
garbage (unpredictable)!
Initializing Variables in the Definition
When a variable is declared (defined), it can also be initialized
with an assignment statement. You can also initialize it with an
assignment statement that is not part of the declaration. Both
examples below accomplish the same task. However, when
possible, it is best to initialize a variable as close to where it is used
in the source code. This practice is a matter of style and will be
required in this course.
Examples:
int number_of_tests = 3;
or
int number_of_tests;
number_of_tests = 3:
Arithmetic Operators
Arithmetic expressions can be used in assignment statements.
Below is a table of the arithmetic operators we will be using:
Operator
Functionality
Example
+
addition
final_price = tag_price + sales_tax
-
subtraction
discount_price = tag_price – coupon_discount
*
multiplication
sales_tax = tag_price * tax_rate
/
division
test_average = sum_of_tests/number_of_tests
%
modulus
remainder = number % 2
-
negation
x = -y
Evaluating Arithmetic Expressions
Precedence
•
•
•
•
•
Evaluate left to right (except unary negation)
() contents of parenthesis are evaluated first
- when used as unary negation is next
*, / and % are then next in order of appearance
+ and – are last (the – here is the binary subtraction
operator)
Examples:
4 + 3 * 2 = 10
(4 + 3) * 2 = 14
6+8%3=8
Exponents
C++ has no exponent operators so a library function is required.
A library in C++ and other languages is a group of predefined
functions that perform usually common tasks.
If you need to raise a value to the 5th power you could write:
value_raised = value * value * value * value * value;
Imagine doing this to raise a number to the 20th or even a higher
power.
Instead we use the pow(number, exponent) function. number
and exponent are called arguments.
value_raised = pow(value,5.0);
Note: To use any predefined C++ mathematical function you
must include the cmath header file.
#include <cmath>
More on Assignment
Multiple Assignment: Assign the same value to several variables in a
single assignment statement (Do not use this feature in this class!)
Example: The statements below initializes all 3 variables to zero
int number1, number2, number3;
number1 = number2 = number3 = 0;
Combined Assignment: The variable name can appear on BOTH
sides of the assignment operator. NOTE: The variable must have a
valid value BEFORE this kind of assignment statement is used.
Example: The statement below increments the number by 2
int number = 2;
number = number + 2;
Note: The resulting value stored in memory for number is now 4
These two kinds of assignment statements are very common in
programming. However, as beginners, we will not use multiple
assignment in this class.
Combined Assignment Operators
Special operators are available in C++ to be used for combined
assignment statements. In the examples below we will assume
that Number has been initialized to 10
Operator
+=
-=
*=
/=
%=
Example
number += 2;
number -= 2;
number *= 2;
number /= 2;
number %= 2;
Resulting Value of Number
12
8
20
5
0
Mixing Data Types in Arithmetic Expressions
• Data Type Ranking – A data type out ranks another data
type if it can hold a larger number
o long double > double > float > unsigned long > long > unsigned
int > int
o Exception: if int and long are same size then unsigned int > long
• char, shorts and unsigned shorts used in mathematical
expressions are promoted to int
o Exception: if short is same size as int then unsigned short is
promoted to unsigned int
• When an operator is used with two values of differing
types then the lower ranking type is promoted to the
same type as the higher ranking type
• When the result of an expression evaluation is assigned
to a variable it will be converted to the type of the
variable
Integer Division
If both operands of a division statement are integers
the result will be an integer with any fractional
component being discarded.
In an expression the literal 18 is an integer but 18.0 is a
floating point number.
Be VERY careful and mindful when using mixed mode
arithmetic.
Overflow and Underflow
Based on the storage size of data types in any
particular system there is a fixed range of values that
can be stored in a variable of a give data types.
Overflow: The assignment of a value LARGER than the
storage capacity of the data type of the variable can
hold
Underflow: The assignment of a value SMALLER than
the storage capacity of the data type of the variable
can hold
This is a danger when working at the limits of a data
type’s storage capacity.
Type Casting: Control Type Promotion
Manually
static_cast<data type>(value)
For example: Type casting can be used to avoid
integer division
int test1, test2, test3, test_sum;
float test_average;
… (input values for the three tests)
test_sum = test1 + test2 + test3;
test_average = static_cast<float>(test_sum) / 3;
Watch Out!
int
float
test1,
test2;
avg;
test1 = 100;
test2 = 83;
//What value does avg have after each of these
//assignment statements are executed?
avg = test1 + test2 / 2;
avg = (test1 + test2) / 2;
avg = static_cast<float>(test1 + test2) / 2;
avg = static_cast<float>((test1 + test2) / 2);
avg = (test1 + test2) / 2.0;
Expression Evaluation Practice