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Java Basics:Data Types, Variables, and Loops
“ If debugging is the process of removing software bugs, then programming must be the process of putting them in. ”
- Edsger Dijkstra Plan for the DayVariables - symbols or names that represent valuesData Types - the kind of value does a variable represent
int - integers, such as 1024, -77, 2, 0double - real numbers, or actually (good) approximations of
real numbers. For example, 3.14159, -0.023343char - single characters, which are enclosed in single quotes in
Java: ‘$’, ‘a’, ‘*’, ‘R’boolean - logical values: true or false
Arithmetic operators (and another use of +)+ represents addition in Java, but also the concatenation or
appending of two strings
Based on slides at buildingjavaprograms.com
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Data typestype: A category or set of data values.
Constrains the operations that can be performed on dataMany languages ask the programmer to specify types
// Variable myNumber stores integer values, and currently // has the value 3 int myNumber = 3;
Internally, computers store everything as 1s and 0s104 01101000"hi" 01101000110101
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Java's primitive types
primitive types: Java’s built-in data types for numbers, text characters and logic values.
8 primitive types in Java
Non-primitive types are object types
We'll use these four primitive types in this class:
Name Description Examplesint integers (whole numbers) 42, -3, 0, 926394
double real numbers 3.14, -0.25, 9.0char single text characters 'a', 'X', '?', '\n'
boolean logical values true, false
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Expressionsexpression: A value or operation that computes a
value.Example: 3 + 9 * 2The simplest expression is a literal value.
84-2.11
A complex expression can use operators and parentheses.(-4.5 + 2.11) * 8 // * represents multiplication
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Arithmetic operatorsOperators we will use often in cs 305j:
+ addition - subtraction (or negation)* multiplication/ division% modulus (a.k.a. remainder)
As a program runs, its expressions are evaluated.1 + 1 evaluates to 2System.out.println(3 * 4); //prints 12How would we print the text 3 * 4 on the screen?
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Integer division with /When we divide integers, the quotient is also an integer.
14 / 4 is 3, not 3.5 3 4 4 ) 14 10 ) 45 12 40 2 5 More examples:
32 / 5 is 684 / 10 is 8156 / 100 is 1
Dividing by 0 causes an error when your program runs.
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Integer remainder with %The % operator computes the remainder from integer
division.14 % 4 is 2, since 14 divided by 4 is 3, with remainder 2
33 % 5 is 3, since 218 divided by 5 is 6, with remainder 3
What is the result of the following operations?
45 % 62 % 28 % 2011 % 4
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Applications of % Operator
How do we…Obtain last (ones) digit of a number.
Ex: From 432115, get 5Obtain last 4 digits of a SSN.
Ex: For SSN 552689321, get 9321Determine whether a number is odd.
Exercise: Suppose some unknown integer (greater than 100) is stored in variable n. How do you obtain the tens digit?
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Operator Precedenceprecedence: Order in which operators are evaluated.
Operators of same precedence level evaluate left-to-right.1 - 2 - 3 is (1 - 2) - 3 which is -4
*/% have a higher precedence than +-
1 + 3 * 4 is 136 + 8 / 2 * 36 + 4 * 36 + 12 is 18
Parentheses can force a certain order of evaluation:(1 + 3) * 4 is 16
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Precedence examples
1 * 2 + 3 * 5 % 4 \_/ | 2 + 3 * 5 % 4
\_/ | 2 + 15 % 4
\___/ | 2 + 3
\________/ | 5
1 + 8 % 3 * 2 - 9 \_/ |1 + 2 * 2 - 9
\___/ |1 + 4 - 9
\______/ | 5 - 9
\_________/ | -4
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Precedence questionsWhat values result from the following expressions?
9 / 5695 % 207 + 6 * 57 * 6 + 5248 % 100 / 56 * 3 - 9 / 4(5 - 7) * 46 + (18 % (17 - 12))
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Real numbers (type double)Java can manipulate real numbers (numbers with a decimal point): 6.022 , -42.0 , 2.143e17
The operators +-*/%() all still work with double./ produces a more precise answer:
15.0 / 2.0 is 7.5% is not typically used on real numbersPrecedence is the same: () before */% before +-
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Real number example2.0 * 2.4 + 2.25 * 4.0 / 2.0 \___/ | 4.8 + 2.25 * 4.0 / 2.0
\___/ | 4.8 + 9.0 / 2.0
\_____/ | 4.8 + 4.5
\____________/ | 9.3
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Precision Issues with RealsComputations with real numbers are not always as precise
as we expect (though the answer we get is close):System.out.println ( 11.0 - 10.91);We expect: 0.09What’s printed: 0.08999999999999999
Computers represent some real numbers in an imprecise way internally, so some calculations with them are off by a very slight amount.We can usually tolerate the precision errors, but later we will
learn some ways to produce a better output for examples like above.
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Mixing typesWhen int and double are mixed, the result is a double.
4.2 * 3 is 12.6
The conversion is per-operator, affecting only its operands. 7 / 3 * 1.2 + 3 / 2 \_/ | 2 * 1.2 + 3 / 2
\___/ | 2.4 + 3 / 2
\_/ | 2.4 + 1
\________/ | 3.4
3 / 2 is 1 above, not 1.5.
2.0 + 10 / 3 * 2.5 - 6 / 4 \___/ |2.0 + 3 * 2.5 - 6 / 4
\_____/ |2.0 + 7.5 - 6 / 4
\_/ |2.0 + 7.5 - 1
\_________/ | 9.5 - 1
\______________/ | 8.5
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String concatenation string concatenation: Using + between a string and
another value to make a longer string.
"hello" + 42 is "hello42"1 + "abc" + 2 is "1abc2""abc" + 1 + 2 is "abc12"1 + 2 + "abc" is "3abc""abc" + 9 * 3 is "abc27""1" + 1 is "11"4 - 1 + "abc" is "3abc"
Use + to print a string and an expression's value together.
System.out.println("Grade: " + (95.1 + 71.9) / 2);
• Output: Grade: 83.5
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Receipt exampleWhat's bad about the following code?public class Receipt { public static void main(String[] args) { // Calculate total owed, assuming 8% tax / 15% tip System.out.println("Subtotal:"); System.out.println(38 + 40 + 30);
System.out.println("Tax:"); System.out.println((38 + 40 + 30) * .08); System.out.println("Tip:"); System.out.println((38 + 40 + 30) * .15); System.out.println("Total:"); System.out.println(38 + 40 + 30 + (38 + 40 + 30) * .08 + (38 + 40 + 30) * .15); }}
The subtotal expression (38 + 40 + 30) is repeatedSo many println statements
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Variablesvariable: A piece of the computer's memory that is given a
name and type, and can store a value.Like preset stations on a car stereo, or cell phone speed dial:
We use variables to store the result of a calculation, so we can use it later
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Variable Declarationvariable declaration: Sets aside memory for storing a value
of some specified type.Variables must be declared before they can be used.A declaration specifies the variable’s name and type. The
name is an identifier.
Declaration Syntax:
<type> <name>;
-- Example: int x;
-- Example: double myGPA;
Multiple variable declarations on same line:double sideA, sideB, sideC;
x
myGPA
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AssignmentDeclaring a variable: sets aside a chunk of memory for
storing valuesassignment: Stores a value in a variable.
The value can be an expression; the variable stores its result.Syntax:
<name> = <expression>;int x;x = 3;
double myGPA;myGPA = 1.0 + 2.25;
x 3
myGPA 3.25
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Using variablesOnce given a value, a variable can be used in expressions:
int x;x = 3;System.out.println("x is " + x); // x is 3
System.out.println(5 * x - 1); // 5 * 3 - 1
You can assign a value more than once:
int x;x = 3;System.out.println(x + " here"); // 3 here
x = 4 + 7;System.out.println("now x is " + x); // now x is 11
x 3x 11
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Declaration/initializationA variable can be declared/initialized in one statement.
Syntax:<type> <name> = <value>;
double myGPA = 3.95;
int x = (11 % 3) + 12;
x 14
myGPA 3.95
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Assignment and algebraAssignment uses = , but it is not an algebraic equation.
X = … ; means “store the value of the expression on the right in the memory set aside for x”
x = 3; means “store 3 in the memory allocated for x”
What happens here?
int x = 3;x = x + 2; // ??? x 3x 5
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Assignment and types A variable can only store a value of its own type.
int x = 2.5; // ERROR: incompatible types
An int value can be stored in a double variable. The value is converted into the equivalent real number.
double myGPA = 4;
double avg = 11 / 2;
Why does avg store 5.0and not 5.5 ?
myGPA 4.0
avg 5.0
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Compiler errorsA variable can't be used until it is assigned a value.
int x;
System.out.println(x); // ERROR: x has no value
You may not declare the same variable twice.
int x;int x; // ERROR: x already exists
int x = 3;int x = 5; // ERROR: x already exists
How can this code be fixed?
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Printing a variable's valueUse + to print a string and a variable's value on one line.
double grade = (95.1 + 71.9 + 82.6) / 3.0;System.out.println("Your grade was " + grade);
int students = 11 + 17 + 4 + 19 + 14;System.out.println("There are " + students + " students in the course.");
• Output:
Your grade was 83.2There are 65 students in the course.
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Receipt questionImprove the receipt program using variables.
public class Receipt { public static void main(String[] args) { // Calculate total owed, assuming 8% tax / 15% tip System.out.println("Subtotal:"); System.out.println(38 + 40 + 30);
System.out.println("Tax:"); System.out.println((38 + 40 + 30) * .08);
System.out.println("Tip:"); System.out.println((38 + 40 + 30) * .15);
System.out.println("Total:"); System.out.println(38 + 40 + 30 + (38 + 40 + 30) * .15 + (38 + 40 + 30) * .08); }}
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Receipt answerpublic class Receipt { public static void main(String[] args) { // Calculate total owed, assuming 8% tax / 15% tip int subtotal = 38 + 40 + 30; double tax = subtotal * .08; double tip = subtotal * .15; double total = subtotal + tax + tip;
System.out.println("Subtotal: " + subtotal); System.out.println("Tax: " + tax); System.out.println("Tip: " + tip); System.out.println("Total: " + total); }}
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ExercisesWhat is the result?1. int x = 4; x = x + 3;
2. 5 = 1 + 4;
3. Double y = 6; y = 2 * y;
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What is the Output?1. int average = (9 + 8)/2;System.out.println(average);
average = (average * 2 + 10 + 4)/4;System.out.println(average);
2. System.out.println(3 + 2 + “friends are coming over”);
3. System.out.println(“Call “ + 9 + 1 + 1);
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Real or Integer? Categorize each of the following quantities by whether an int or double
variable would best to store it:
1. Temperature in degrees 5. Number of rainy days this month2. Population of lemmings 6. Number of miles traveled today3. Your grade point average 7. A person’s weight in pounds4. The average of a collection of exam scores
credit: Kate Deibel, http://www.cs.washington.edu/homes/deibel/CATs/
Integer (int) Real Number (double)
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ExerciseWrite a program that assigns the length and width of a
rectangle to two variables, and then prints the area of the rectangle.
Sample Output:Length = 4Width = 3The triangle’s area is 12.
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Exercise using VariablesFor student Jane Doe:
Exam 1 score: 95Exam 2 score: 89Exam 3 score: 93
Write a program that displays Jane Doe’s exam average.
Output:On exam 2, Jane Doe made 89.
Her exam average is 92.333333.
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Increment and Decrement Operators
The increment operator ++ adds 1 to a numberresult ++; // The value stored in result is increased
by 1
The decrement operator -- subtracts 1 from a number result --; // value stored in result is decreased by 1
These operators can come before or after the variable/value: ++count; --count;
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Example - Increment/Decrement
Output?int count = 1;int n = 2 + count++;System.out.println(“count = “ + count);System.out.println(“n = “ + n);
What about now:int count = 1;int n = 2 + ++count;
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Assignment OperatorsCombine assignment with an operationUse whenever you want to apply some operation to the
value of a variable, and store the result in that variableExample:
count += 5;is equivalent to
count = count + 5;
Example:int result = 2;result *= 4; // result = ?
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Assignment OperatorsExample:
int count1 = 3;int count2 = 6;int product *= count1 + count2; // what is product?
equivalent toint product = product * (count1 + count2);
Example:String s = “hello”;S += “ world”; // what is s?
equivalent toS = s + “ world”;