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Java Iteration
looping
Options while dowhile for Allow programs to control how many times a statement list is executed
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Averaging
Problem Extract a list of positive numbers from standard input and produce their average Numbers are one per line A negative number acts as a sentinel to indicate that there are no more numbers to process Observations Cannot supply sufficient code using just assignments and conditional constructs to solve the problem Don't how big of a list to process Need ability to repeat code as needed
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Averaging
Algorithm Prepare for processing Get first input While there is an input to process do { Process current input Get the next input } Perform final processing
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Averaging
Problem Extract a list of positive numbers from standard input and produce their average Numbers are one per line A negative number acts as a sentinel to indicate that there are no more numbers to process Sample run Enter positive numbers one per line. Indicate end of list with a negative number. 4.5 0.5 1.3 1 Average 2.1
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public class NumberAverage { // main(): application entry point public static void main(String[] args) { // set up the input // prompt user for values // get first value // process values onebyone while (value >= 0) { // add value to running total // processed another value // prepare next iteration get next value } // display result if (valuesProcessed > 0) // compute and display average else // indicate no average to display
}
}
int valuesProcessed = 0; double valueSum = 0;
// set up the input
Scanner stdin = new Scanner (System.in);
// prompt user for values
notice the format!
System.out.println("Enter positive numbers 1 per line.\n" + "Indicate end of the list with a negative number.");
// get first value
double value = stdin.nextDouble();
// process values onebyone
while (value >= 0) { valueSum += value; ++valuesProcessed; value = stdin.nextDouble(); }
// display result
if (valuesProcessed > 0) { double average = valueSum / valuesProcessed; System.out.println("Average: " + average); } else { System.out.println("No list to average"); }
While syntax and semantics
while ( Expression ) Action Logical expression that determines whether Action is to be executed Action is either a single statement or a statement list within braces
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While semantics for averaging problem
Test expression is evaluated at the start of each iteration of the loop. // process values onebyone while ( value >= 0 ) { // add value to running total valueSum += value; // we processed another value ++valueProcessed; // prepare to iterate get the next input value = stdin.nextDouble();
}
If test expression is true, these statemen are executed. Afterward, the test express is reevaluated and the process repeats
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While Semantics
Expression is evaluated at the start of each iteration of the loop
Expression
If Expression is true, Action is executed
true
false
Action
If Expression is false, program execution continues with next statement
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Suppose input contains: 4.5 0.5 1.3 1
Execution Trace
int valuesProcessed = 0; double valueSum = 0;
valuesProcessed valueSum value
3 2 1 0 4.5 6.3 5.0 0 1.3 0.5 4.5 1
double value = stdin.nextDouble(); while (value >= 0) { valueSum += value; ++valuesProcessed; value = stdin.nextDouble(); } average 2.1
if (valuesProcessed > 0) { double average = valueSum / valuesProcessed; System.out.println("Average: " + average); } else { System.out.println("No list to average"); }
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Converting text to strictly lowercase
public static void main(String[] args) { Scanner stdin = new Scanner (System.in); System.out.println("Enter input to be converted:"); String converted = ""; String currentLine = stdin.nextLine(); while (currentLine != null) { String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n"); currentLine = stdin.nextLine(); } } System.out.println("\nConversion is:\n" + converted);
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Sample run
An empty line was entered A Ctrl+z was entered. I the t is Windows escape sequence for indicating end-of-file
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Program trace
public static void main(String[] args) { Scanner stdin = new Scanner (System.in); System.out.println("Enter input to be converted:"); String converted = ""; String currentLine = stdin.nextLine(); while (currentLine != null) { String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n"); currentLine = stdin.nextLine(); } } System.out.println("\nConversion is:\n" + converted);
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Program trace
The append assignment operator updates the representation of converted to include the current input line
onverted += (currentConversion + "\n"); Representation of lower case conversion of current input line Newline character is needed because method nextLine() "strips" them from the input
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Converting text to strictly lowercase
public static void main(String[] args) { Scanner stdin = new Scanner (System.in); System.out.println("Enter input to be converted:"); String converted = ""; String currentLine = stdin.nextLine(); while (currentLine != null) { String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n"); currentLine = stdin.nextLine(); } } System.out.println("\nConversion is:\n" + converted);
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All your base are belong to us
Flash animation
Reference: http://en.wikipedia.org/wiki/All_your_base_are_belong_to_us
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Loop design
Questions to consider in loop design and analysis
What initialization is necessary for the loop's test expression? What initialization is necessary for the loop's processing? What causes the loop to terminate? What actions should the loop perform? What actions are necessary to prepare for the next iteration of the loop? What conditions are true and what conditions are false when the loop is terminated? When the loop completes what actions are need to prepare for subsequent program processing?
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Reading a file
Background
Same Scanner class!
filename is a String
canner fileIn = new Scanner (new File (filename) );
The File class allows access to files It's in the java.io package
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Reading a file
Class File Allows access to files (etc.) on a hard drive Constructor File (String s) Opens the file with name s so that values can be extracted Name can be either an absolute pathname or a pathname relative to the current working folder
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Reading a file
Scanner stdin = new Scanner (System.in); System.out.print("Filename: "); String filename = stdin.nextLine(); Scanner fileIn = new Scanner (new File (filename)); String currentLine = fileIn.nextLine(); while (currentLine != null) { System.out.println(currentLine); currentLine = fileIn.nextLine(); }
Get next line Get first line Process lines one by one Set up file stream Display current line Close the file stream Determine file name Make sure got a line to process Set up standard input stream If not, loop is done 21
The For Statement
The body of the loop iterates while the test expression is true
Initialization step is performed only currentTerm = 1; After each iteration of the once -- just priorint body of the loop, the update to the first expression is reevaluated evaluationfor ( int i = 0; i < 5; ++i ) { of the test expression System.out.println(currentTerm); currentTerm *= 2; The body of the loop displays the } current term in the number series. It then determines what is to be the new current number in the series
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Evaluated once at the beginning of the for statements's execution
ForInit
The ForExpr is evaluated at the start of each iteration of the loop
If ForExpr is true, Action is executed After the Action has completed, the PostExpression is evaluated
ForExpr true false
Action
If ForExpr is false, program execution continues with next statement
PostExpr
After evaluating the PostExpression, the next iteration of the loop starts
for statement syntax
Logical test expression that determines whether the action and update step are executed Initialization step prepares for the first evaluation of the test expression
Update step is performed after the execution of the loop body
ForInit ; ForExpression ; ForUpdate ) Action for ( The body of the loop iterates whenever the test expression evaluates to true
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for vs. while
A for statement is almost like a while statement for ( ForInit; ForExpression; ForUpdate ) Action
is ALMOST the same as: ForInit; while ( ForExpression ) { Action; ForUpdate; }
This is not an absolute equivalence! We'll see when they are different below
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Variable declaration
You can declare a variable in any block:
Variable n gets created (and initialized) each time Thus, println() always prints out 1 Variable n is not defined once while loop ends
while ( true ) { int n = 0; n++; System.out.println } (n); System.out.println (n);
As n is not defined here, this causes an error
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Variable declaration
You can declare a variable in any block:
if ( true ) { int n = 0; n++; System.out.println (n); } System.out.println (n);
Only difference from last slide
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End of lecture on 1 November 2004
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Execution Trace
for ( int i = 0; i < 3; ++i ) {
i
3 2 1 0
System.out.println("i is " + i); } System.out.println("all done");
i is 0 i is 1 i is 2 all done
Variable i has gone out of scope it is local to the loop
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for vs. while
An example when a for loop can be directly translated into a while loop: int count; for ( count = 0; count < 10; count++ ) { System.out.println (count); }
Translates to: int count; count = 0; while (count < 10) { System.out.println (count); count++; }
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for vs. while
An example when a for loop CANNOT be directly translated into a while loop: only difference for ( int count = 0; count < 10; count++ ) { System.out.println (count); }
Would translate as:
count is NOT defined here
int count = 0; while (count < 10) { System.out.println (count); count++; }
count IS defined here
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for loop indexing
Java (and C and C++) indexes everything from zero Thus, a for loop like this: for ( int i = 0; i < 10; i++ ) { ... }
Will perform the action with i being value 0 through 9, but not 10 To do a for loop from 1 to 10, it would look like this: for ( int i = 1; i <= 10; i++ ) { ... }
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Nested loops
int m = 2; int n = 3; for (int i = 0; i < n; ++i) { System.out.println("i is " + i); for (int j = 0; j < m; ++j) { System.out.println(" j is " + j); } i is 0 } j is 0 j is 1 i is 1 j is 0 j is 1 i is 2 j is 0 j is 1
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Nested loops
int m = 2; int n = 4; for (int i = 0; i < n; ++i) { System.out.println("i is " + i); for (int j = 0; j < i; ++j) { System.out.println(" j is " + j); } } i is 0 i is 1 j is 0 i is 2 j is 0 j is 1 i is 3 j is 0 j is 1 j is 2
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The dowhile statement
Syntax do Action while (Expression) Semantics Execute Action If Expression is true then execute Action again Repeat this process until Expression evaluates to false Action is either a single statement or a group of statements within braces
Action
true Expression false
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Picking off digits
Consider System.out.print("Enter a positive number: "); int number = stdin.nextInt(); do { int digit = number % 10; System.out.println(digit); number = number / 10; } while (number != 0); Sample behavior Enter a positive number: 1129 9 2 1 1
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while vs. dowhile
If the condition is false: while will not execute the action dowhile will execute it once while ( false ) { System.out.println ("foo"); } do { System.out.println ("foo"); } while ( false );
never executed
executed once
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while vs. dowhile
A dowhile statement can be translated into a while statement as follows: do { Action; } while ( WhileExpression );
can be translated into: boolean flag = true; while ( flag || WhileExpression ) { flag = false; Action; }
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End of lecture on 3 November 2004
We watched the Kerry concession speech and the Bush acceptance speech in class today, so after the announcements, we only got to about 2025 minutes of actual lecture time
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A digression: Perl again
Consider the statement: if ( !flag ) { ... } else { ... }
Perl has a command unless: unless ( flag ) { ... } else { ... }
An unless command is a if statement with a negated condition It can get a bit confusing, though The else of an unless...
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A digression: Perl again
Consider the statement: while ( !flag ) { ... }
Perl has a command until: until ( flag ) { ... }
An until command is a while loop with a negated condition As most people are quite used to ifelse and while, unless and until are rarely used 41
Problem solving
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Data set manipulation
Often five values of particular interest Minimum Maximum Mean Standard deviation Size of data set Let's design a data set representation The data set represents a series of numbers Note that the numbers themselves are not remembered by the DataSet Only properties of the set (average, minimum, etc.)
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Implication on facilitators
public double getMinimum() Returns the minimum value in the data set. If the data set is empty, then Double.NaN is returned, where Double.NaN is the Java double value representing the status notanumber public double getMaximum() Returns the maximum value in the data set. If the data set is empty, then Double.NaN is returned
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Implication on facilitators
public double getAverage() Returns the average value in the data set. If the data set is empty, then Double.NaN is returned public double getStandardDeviation() Returns the standard deviation value of the data set. If the data set is empty, then Double.NaN is returned
Left to the interested student
public int getSize() Returns the number of values in the data set being represented
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Constructors
public DataSet() Initializes a representation of an empty data set public DataSet(String s) Initializes the data set using the values from the file with name s public DataSet(File filep) Initializes the data set ...