Transcript Week 3 Power Point Slides

Expressions and Order of Operations
Definition: An expression is a sequence of operators and operands
• Operators
– There are the standard operators: add, subtract, divide, multiply
– Note that * means multiply? (No times symbol on the keyboard)
– Java has many other operators (Next slide)
• Operands
– Can be a literal (A constant that we don’t have to declare)
– Can be a variable
– Can be a call to a method call that returns a value
(Ex: int x = 3 + Math.round(4.4); )
• Order of operations (The text has a complete list)
– Multiply and division happen before add and subtract
– Use parentheses to alter the order
Important Note: Equal does NOT mean equal, it means assign
Other Operators in Java
Definition: Precedence is a number indicating which operators get applied first
• Prefix ++: Add one, very high precedence
int x = 3; int y = ++x; // x and y becomes a 4.
• Postfix ++: Add one, very low precedence
Int x = 3; int y = x++; // x becomes 4, y becomes 3.
• +=, -=, *=, /=: Adjust the previous value
int x = 3; x += 4; // x becomes 7
Int x = 6; x /= 5; // x becomes 1
• %: Compute the remainder
int x = 7 % 4; // x becomes a 3 (remainder of 7/4)
int x = 7; x %=4; // x becomes 3 (remainder of x/4)
Assignment Operator
• Java evaluates what is on the right of the equal sign, and then
stores into the variable on the left
• You can have more than one assignment in a statement
• Examples
1. x = 3 + 4; // Add 3 + 4 and store the result in x
2. int x = 3; y = x + 4; // Add 4 to what’s in x and store in y.
3. x = x + 3; // Add 3 to x and store in x
4. x = y + Math.round(3.4); // Round 3.4 and add to y and then
store in x.
* Equal does not mean equal; it means assign
* The call to the Math.round() method returns a value
Bitwise Operators
&(and), |(or), ^(xor), !(not)
Consider a byte: b = 10110100, c = 00000110
1. byte d = b & c puts a 4 in d
(one bit only if both corresponding bits on)
2. byte d = c | 5 puts a 7 in d
(one bit if either of the corresponding bits on)
3. byte d = c ^ 5 puts a 3 in c
(one bit if corresponding bits not equal)
4. ! Flips the bits (!b becomes 75)
We’ll discuss these examples in class
Ternary operator
• Most operators use two operands, one to the left of the
operator and one to the right (ex: a+b)
• Java has one ternary operator
– Example: (The following statement stores a 10 into x)
int x = (5<6)? 10 : 20;
– Example:
int x = 5;
System.out.println( (x++<6)? "hello" : "goodbye");
System.out.println((x<6)? "hello": "goodbye");
(The first line above prints "hello", second prints "goodbbye")
Logical operators
And (&&), OR (||), Not (!)
Int x = 3; double y = 3.1, z = 3.0;
String s="abc", t="abcdef", u="abb"
Examples
1.
2.
3.
4.
5.
6.
boolean b = (x<y);
boolean b = (x<y && x<=z);
boolean b = (x>y || x<z);
boolean b = !(x>y || y<z);
boolean b = (s<t);
boolean b = (s<u);
Note: s<t and s<u will not
work in practice because
we are comparing object
addresses and not contents
Answers: true, true, false, true, true, false
Basic Control
Structures
All computer processing can be done this way
Sequence Structure
1.
2.
3.
4.
5.
Do this
Do that
Do something else
Do that again
Do another thing
Condition or Transfer Structure
IF this THEN do that
ELSE do the other thing
Iteration or Loop
•
•
•
WHILE this true DO something
DO something UNTIL this false
FOR many times DO something
Condition Statement
Do one thing if an expression is true, and another if it is false
• Syntax (Java Grammar)
If (boolean expression)
{ ** true part: any number of Java statements ** }
else { ** false part: any number of java statements ** }
• Notes
1. The braces are not needed if there is only one
statement in the block
2. The else part is optional
Condition Statement Example
// Convert military time to standard time
// Assume hours has the current military hours
String str;
hours = hours %12;
if (hours == 0) hours = 12;
if (hours <=12)
str = " a.m. ";
else { str = " p.m. "; } // {} not required here
System.out.println("It is " + hours + str);
There is a bug in this logic; do you see it?
Nested If
/* Assume month has the current month stored in it
and years has the current year */
int days;
if (month == 9 || month == 4
|| month == 6 || month == 11)
days = 30;
else
{
if (month == 2)
{
if (year %4 == 0) days = 29;
else
days = 28;
}
else days = 31;
}
System.out.println("month " + month + " has "
+ days + " days");
Another Example
• Find maximum of five numbers if it is greater than 10
• Variables we will use are: first, second, third, fourth, fifth
maximum = first;
if (second > maximum) maximum = second;
if (third > maximum) maximum = third;
if (fourth > maximum) maximum = fourth;
if (fifth > maximum) maximum = fifth;
if (maximum > 10)
System.out.println(maximum);
else System.out.println("Maximum less than 10");
Review
• What are the three structures that we can use to program any
application, no matter how complicated?
• What are the following operators for (%, ?:, and !)?
• What is the difference between x++ and ++x?
• What is the difference between || and |?
• What is the ternary operator?
• What does the term precedence mean?
• What is the difference between = and ==?
• What is a condition statement?
• When are braces needed in a condition statement?
• What is a bitwise operator?