Control & Arrays 15-121 Fall 2020 Margaret Reid-Miller Today - - PowerPoint PPT Presentation

Control & Arrays

15-121 Fall 2020 Margaret Reid-Miller

Today

• No in class Quiz. Quiz will be asynchronous.
• Strings continued
• Java naming conventions
• Flow Control: conditionals, loops, scope
• Arrays

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Basic types

What is the difference between a char and a String variable? char is a primitive type for a single character. String is an object type for a sequence of characters. How are char and String literals different? char uses a pair of single quotes (e.g., 'x'). String uses a pair of double quotes (e.g., "x"). What is the difference between a null String and an empty String? null is when a String variable is undefined. an empty no has characters in it (i.e., "").

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Python vs Java

How does LetterCounter1.java compare to LetterCounter1.py? In Java

• Must declare types of variables
• To read input, need a Scanner object
• Statements terminated with ;
• Uses {} to delimit blocks of code
• Cannot use list(array) indexing on Strings
• for loops are different

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Identifiers and Keywords

• Identifiers are names that specify different elements
• f a program such as class, method, or variable
• can be any combination of letters, digits, _ or $
• the first character must NOT be a digit
• case-sensitive (total is different from Total)

Examples: main method1 maxCount TUESDAY $amount Puzzle

• Keywords are a set of predefined identifiers that are

reserved for special uses. Examples: public static void class

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Java naming conventions readily distinguish various Java elements

• Class: Starts with a capital letter, same name as file name
• A class names should be a noun that describe an
• bject type.

e.g., String, Dice, Radio

• Method: Starts with a lower case letter, camelCase
• A method name should start with a verb and

describe what the method does. e.g., displayQuestion, getName, computeTax

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Java Naming Conventions (cont’d)

• Variable: Starts with a lower case letter, camelCase
• A variable name should be a noun that describes

what data it holds. e.g., favoriteFood, name

• Constant: All upper case with underscores
• Constants are declared with the keyword final
• A constant name should be a noun that describes

what data it holds. e.g., PI, TAX_RATE

Flow Control

Conditionals, Loops, Scope

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Conditionals

if (boolean_expression) statement if (boolean_expression) statement1 else statement2

A statement can be any Java statement:

• A simple statement
• A compound statement, such as an if statement
• A block statement, a group of statements enclosed in

braces {}

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The Dangling else Problem

• When an if statement is nested inside an if clause,

its else clause is paired with the closest if statement that does not have an else clause.

if (x > 0) if (y > 0) color = “red”; else color = “blue”;

Misleading indentation Recall: Indentation is not semantic!

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Each else is paired with closest if that has no else

• In reality it is

if (x > 0) if (y > 0) color = “red”; else color = “blue”; y x y x

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Always use {} with if statements

• Use braces to pair else

with the outer if

if (x > 0) { if (y > 0) color = “red”; } else { color = “blue”; } y x

(if or else on a single line without braces is OK.)

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Multiple Alternatives

• Determine if a number is positive, negative, or zero:

if (value < 0) { System.out.println(“Value is negative.”); } else if (value == 0) { System.out.println(“Value is zero.”); } else if (value > 0) { // Redundant test! System.out.println(“Value is positive.”); } At most one statement is executed. Each choice, however, is at same indentation.

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Multiple Alternatives

• Determine if a number is positive, negative, or zero:

if (value < 0) { System.out.println(“Value is negative.”); } else if (value == 0) { System.out.println(“Value is zero.”); } else { // value must be positive System.out.println(“Value is positive.”); } It is clear, exactly one statement is executed.

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The switch statement

• If an if/else statement with multiple alternatives

compares an int or char expression against several constants you can use a switch statement. Example:

switch (suitAsChar) { case ‘C’: suitAsName = “Clubs”; break; case ‘D’: suitAsName = “Diamonds”; break; case ‘H’: suitAsName = “Hearts”; break; case ‘S’: suitAsName = “Spades”; break; default: suitAsName = “Unknown”; }

• WARNING: You must include break to avoid executing the

the body of the following case!

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Java has 3 kinds of Loops

initialize while (test) { do_stuff update } for (initialize; test; update) { do_stuff } initialize do { do_stuff update } while (test);

Note: do-statement executes loop at least

• nce – not often used

While Loop

int i = 0; while (i < n) { System.out.print(“*”); i++; } System.out.println();

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for Loop

for (int i = 0; i < n; i++) { System.out.print(“*”); } System.out.println();

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Scope

• The scope of a variable is the area within a program

that can reference the variable.

• The scope depends on where the variable is

declared.

int sum = 0;

for (int i = 1; i <= n; i++) { sum += i*i; }

System.out.println(sum);

Scope of variable i

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Scope

int sum = 0; int i;

for (i = 1; i <= n; i++) { sum += i*i; }

System.out.println(“Sum of first “ + (i-1) + “ integers squared is “ + sum); }

Scope of variable i ends at the enclosing brace

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Use a do-while if you must execute the body of the loop at least once.

Scanner console = new Scanner(System.in); int month; do { System.out.print( “Please enter the month [1-12]: ”); month = console.nextInt(); } while ( ); month < 1 || month > 12 Must be declared

• utside the loop

Outside the scope

• f the loop

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Arrays

• How do we say/pronounce this?

int[]

• What is poor about this code?

int[] count;

• What does this code do?

int[] counts;

• How do I create an array?

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Array of int or int Array Use plural names Declares counts to refer to an array of ints (but doesn’t yet) new [ ] type how many

Java arrays use special syntax

• Declare and create:

int[] counts; counts = new int[6];

• Shortcut:
• Arrays are automatically filled with default

values when created:

• int[], double[], char[] is
• boolean[] is
• String[] is

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int[] counts = new int[6]; false null (as with all objects)

Creating an array

must match

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Initializer List

• You can declare, create, and initialize an array with a

list of values or expressions. int[] counts = {12, 22, 17, 16, 4, 11};

• You can use an initializer list only when the array is

first declared.

• Each value must match the type of the array.
• The values go into the array in the order given and

determine the length of the array.

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Arrays are uniform, fixed length, and have no methods

• Every element of an array must be the same type.
• We will see how to get around that restriction later
• You can make arrays of primitives values or objects.
• The length of an array is determined when it is first

created and cannot grow or shrink

E.g., counts.length

• Unlike Python, there is no slicing (e.g., arr[2:5])

and no negative indices (e.g., arr[-1]).

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Note no ()! It’s a value, not a method.

Array Demo

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• When you copy an array variable x to a variable y, you

copy the reference not the values in the array.

int[] x = {1, 3, 7, -4, 2, 8, 5, 0, 4, 6} int[] y = x

• Changes made to x or y are seen by both variables.
• We say x and y are aliases (two names for the same

array)

Aliasing

x

f2e23 f2e23

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y f2e23

1 3 7

• 4

2 8 5 4 6

Pass by Value

• When you call a method with an argument, the

method parameter variable receives a copy of the argument value.

• The argument and parameter variables are in

different scopes.

• When the method returns, what happens to the

parameter variable?

• It is destroyed. Recall the scope (end-of-the-life)
• f a variable.
• What is copied when the argument is an array?
• The reference to the array.

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int[] values = new int[10]; fillAllSums(values); public static void fillAllSums(int[] vals) { vals[0] = 0; for (int i = 1; i < vals.length; i++) { vals[i] = vals[i-1] + i; } }

Arrays as Parameters

values

ae2f3 ae2f3

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In main, for example

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int[] values = new int[10]; fillAllSums(values); public static void fillAllSums(int[] vals) { vals[0] = 0; for (int i = 1; i < vals.length; i++) { vals[i] = vals[i-1] + i; } }

The array reference is copied to the parameter

values

ae2f3 ae2f3

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vals ae2f3

int[] values = new int[10]; fillAllSums(values); public static void fillAllSums(int[] vals) { vals[0] = 0; for (int i = 1; i < vals.length; i++) { vals[i] = vals[i-1] + i; } }

1 3 6 10 15 21 28 36 45

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An array parameter is an alias

values

ae2f3 ae2f3

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vals ae2f3

1 3 6 10 15 21 28 36 45

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int[] values = new int[10]; fillAllSums(values); public static void fillAllSums(int[] vals) { vals[0] = 0; for (int i = 1; i < vals.length; i++) { vals[i] = vals[i-1] + i; } }

Changes to an array in a method are visible outside the method!

values

ae2f3 ae2f3

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Returning an Array

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int[] values = getAllSums( 8 ); … } public static int[] getAllSums(int n) { int[] vals = new int[n]; vals[0] = 0; for (int i = 1; i < n; i++) { vals[i] = vals[i-1] + i; } return vals; }

What happens when we swap values?

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An array variable holds null if no array is created (distinct from an array with 0 elements.)

int array[] counts; public static int[] copy(int[] data){ int[] data2 = new int[data.length]; for (int i = 0; i < data.length; i++){ data2[i] = data[i]; } return data2; }

The null reference

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counts null

Causes a nullPointerException if data is null

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public static int[] copy(int[] data){ if (data == null) return null; int[] data2 = new int[data.length]; for (int i = 0; i < data.length; i++){ data2[i] = data[i]; } return data2; }

Be sure to test for null!

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Arrays class (part 1)

What happens if we just print an array variable (using toString)? Some useful methods from java.util.Arrays

• Arrays.toString(arr)
• Arrays.equals(arr1, arr2);
• Arrays.sort(arr)

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