1. Lecture 3
Expressions
Richard Gesick

2. Topics
Expressions
Data Conversion

3. Expression Combination of one or more operators and operands
Has both data type & value
Operands may be
literals
constants
variables

4. Arithmetic Operators Operator Operation + addition - subtraction *
multiplication /
division %
modulus (remainder after division)

5. Integer Division & Modulus
When dividing two integers:
the quotient is an integer
the remainder is truncated (discarded)
To get the remainder, use the modulus operator with the same operands

6. Examples 8
25 / 3
25 % 3
3 / 25
3 % 25
25.0 / 5
10 / 3.0 1 3
5.0

8. Operator Precedence
What is the order of evaluation in the following expressions?
a + b + c + d + e
a + b * c - d / e 1 2 3 4 3 1 4 2
a / (b + c) - d % e 2 1 4 3
a / (b * (c + (d - e))) 4 3 2 1

9. Assignment Revisited
The assignment operator has a lower precedence than the arithmetic operators
First the expression on the right hand
side of the = operator is evaluated
answer = sum / 4 + MAX * lowest; 4 1 3 2
Then the result is stored in the
variable on the left hand side

10. Assignment Revisited First the expression on the right hand
side of the = operator is evaluated
Expression has data type and value
variable = expression;
Then the result is stored in the
variable on the left hand side (has data type),
if types are compatible

11. Assignment Revisited
The right and left hand sides of an assignment statement can contain the same variable
First, one is added to the
original value of count
count = count + 1;
Then the result is stored back into count
(overwriting the original value)

12. Shortcut Operators ++ increment by 1 -- decrement by 1 Example:
count++; // count = count + 1;
count--; // count = count - 1;

13. Increment and Decrement
The increment and decrement operators can be applied in postfix form:
count++
or prefix form:
++count
When used as part of a larger expression, the two forms can have different effects
Because of their subtleties, the increment and decrement operators should be used with care

14. Which form to use?
when the increment (or decrement) operator is used in a “stand alone” statement solely to add one (or subtract one) from a variable’s value, it can be used in either prefix or postfix form
USE EITHER
dogs-- ; dogs;

15. BUT...
when the increment (or decrement) operator is used in a statement with other operators, the prefix and postfix forms can yield different results
LET’S SEE HOW. . .

16. PREFIX FORM “First increment, then use ”
int alpha ; int num ; num = 13; alpha = ++num * 3; 13
num 14
alpha 42

17. POSTFIX FORM “Use, then increment ”
int alpha ; int num ; num = 13; alpha = num++ * 3; 13
num
alpha 13 39
num
alpha 14
num

18. More Shortcut Operators
Example
Equivalent +=
a += 3;
a = a + 3; -=
a -= 10;
a = a - 10; *=
a *= 4;
a = a * 4; /=
a /= 7;
a = a / 7; %=
a %= 10;
a = a % 10;

19. Assignment Operators is equivalent to
The right hand side of an assignment operator can be a complex expression
The entire right-hand expression is evaluated first, then the result is combined with the original variable
Therefore
result /= (total-MIN) % num;
is equivalent to
result = result / ((total-MIN) % num);

20. Common Error Trap
No spaces are allowed between the arithmetic operator and the equals sign
Note that the correct sequence is +=, not =+
Example: add 2 to a
// incorrect
a =+ 2; // a = +2; assigns 2 to 2
// correct
a += 2; // a = a + 2;

21. Assigning the Values of Other Variables
Syntax:
dataType variable2 = variable1;
Rules:
1. variable1 needs to be defined before this statement appears in the source code
2. variable1 and variable2 need to be compatible data types; in other words, the precision of variable1 must be lower than or equal to that of variable2.

22. Mixed-Type Arithmetic
When performing calculations with operands of different data types:
Lower-precision operands are promoted to higher-precision data types, then the operation is performed
Promotion is effective only for expression evaluation; not a permanent change
Called "implicit type casting"
Bottom line: any expression involving a floating-point operand will have a floating-point result.

23. Two types of data conversion
Widening conversion (promotion)
safe, do not lose data
goes to a “wider” (more bits) data type
may lose precision (long to float)
Narrowing conversion (demotion)
may lose data
may lose precision
should be avoided
compiler error unless specific cast done

26. Conversion Techniques
assignment conversion
assign an int to a long
promotion
divide an int by a double
casting

27. Explicit Type Casting Syntax: (dataType)( expression )
Note: parentheses around expression are optional if expression consists of 1 variable
Useful for calculating averages
double result = (double) 25 / 3;
double result = (double) total / count;

28. Summary
What did you learn?