Transcript nested loops, choosing loops, break, files

QMCS 230: Today in Class
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Homework
Nested Loops
Other things
Group work if time
Start working with files?
Lab 8
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Homework
• Typical weekly assignments
– 1 “informal” lab
• Hand in source code ONLY
• Less credit than the “formal” lab
• Checked in general for content and form
– 1 “formal” lab
• E-mail source code ONLY
– Checked carefully
• Grading
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Check plus plus – a few labs – for special work – think A+
Check plus – almost no problems – think A
Check – Some problems – think of a B
Check Minus – many or major problems – think of a C
No Credit – major parts of the assignment missing
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On the Exam
• Chapter 2, all sections except
– 2.6, 2.8
• Chapter 3, all sections except
– 3.8, 3.9
• Chapter 4, Sections 4.1-4.6
• Study the ‘Checkpoints’
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Reference Materials
• I will provide reference guides
– Java syntax basics
– Reserved words you might use
– Tables of details, if needed
• Closed book closed notes
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Nested Loops
• How would we do hh:mm:ss ?
• What does “nesting” mean?
– Nothing to do with eggs.
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Break and Continue (Sec 4.8)
• Loops do all statements in order
• Break and Continue change this.
– Cause ‘surprising’ behavior
• They exist, but we will NOT use them
– Makes programs harder to understand
– And harder for ME to read
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Picking a loop
• Always do it at least once?
• Maybe skip it all sometimes?
• A fixed number of times?
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Picking a loop
• Always do it at least once?
– Do - While
• Maybe skip it all sometimes?
– While
• A fixed number of times?
– For loop
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Group Work
• 8 groups - 4 problems
• ALL: average 4 numbers read from
keyboard
• Different problems
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Use 4 separate statements
Use ‘for’ loop
Use ‘do while’ loop
Use ‘while’ loop
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Working with Files
• We use the same methods as
System.in/out
– println, print, printf, Scanner class
• We must “open” and “close” files
– “Open” when we create the file object
– “close()” method when done
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Output Files
• PrintWriter class
– “new” takes file name as an argument
– Creates that file
• Use the object with ‘print’ methods
• Write to a file
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import java.io.*;
public static void main(String[] args) throws IOException
PrintWriter pwa;
pwa = new PrintWriter(“existingfile.txt”);
• Example: write student names
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Append to Output File
• Use an extra object for the existing file
• “FileWriter” object
• FileWriter fw;
• PrintWriter pwa;
// to define the existing file
// to open for print functions
• fw = new FileWriter(“existingfile.txt”, true);
• pwa = new PrintWriter(fw);
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Input Files
• Use Scanner class and File class
• import java.util.Scanner;
• import java.io.*;
• public static void main(String[] args) throws IOException
• fn;
// identify the file to read
• Scanner sc; // for reading the data
• fn = new File(“filename.txt”);
• sc = new Scanner(fn);
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That’s it.
• Questions?
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