CSS161 Fundamentals#of#Compu3ng# Flow#control#(4)#courses.washington.edu/css161/umurthy/learning-resources/slides/c… · CSS161 Fundamentals#of#Compu3ng# Flow#control#(4)# October17,2012

CSS 161 Fundamentals of Compu3ng

Flow control (4) October 17, 2012

Instructor: Uma Murthy

Announcements and reminders Announcements

–  HW 1 grades delayed – will be out today

–  Website update –  HW1 solu3on (check homeworks and solu3ons page)

–  Lecture 7 slides (check updated schedule page) Reminders

–  All homeworks will be due before class at 11am, unless otherwise specified

–  Prac3ce using Prac3ce it, self-‐test exercises, and programming exercises

–  Midterm 1 on Wed, Oct 24

CSS 161: Fundamentals of Compu3ng 2

Outline today

•  Review loops •  Complete Chapter 3

•  Do problems from book and Prac3ce it


Acknowledgments

•  A subset of the following slides have been either directly taken from or derived from the supplements of the book: Building Java Programs: A Back to Basics Approach, 2nd edi(on by Stuart Reges and Marty Stepp

hVp://www.buildingjavaprograms.com/supplements.shtml


Review

•  Boolean expressions – Short circuit evalua3on – Side-‐effects of Boolean expressions – Boolean expressions outside of condi3onals and loops

– Comparing Strings


Comparing Strings

•  Comparison operators only work on primi%ve types – boolean, char, byte, short, int, long, float, double

•  To compare other types, need other methods – compareTo() – equals() – equalsIgnoreCase() [for Strings only]


Comparing Strings example

String s1 = "tintin"; String s2 = "snowy"; String s3 = "Tintin";

System.out.println(s1.equals(s2)); System.out.println(s1.equals("tintin")); System.out.println(s1.equals(s3)); System.out.println(s1.equalsIgnoreCase(s3)); System.out.println(s1.compareTo(s2)); System.out.println(s2.compareTo(s1)); System.out.println(s2.compareTo("snowy"));


Comparing Strings example String s1 = "tintin"; String s2 = "snowy"; String s3 = "Tintin";

System.out.println(s1.equals(s2)); System.out.println(s1.equals("tintin")); System.out.println(s1.equals(s3)); System.out.println(s1.equalsIgnoreCase(s3)); System.out.println(s1.compareTo(s2));

System.out.println(s2.compareTo(s1)); System.out.println(s2.compareTo("snowy"));

false

true

false

true

1

-‐1

0


Loops

•  Mechanism for repea3ng block of statements – Do we know how many 3mes to repeat?

– Do we want to execute at least once? •  3 types of loops:

  while (boolean expression) statement

  do statement while (boolean_expression)

  for (initialization; stop_condition; update) statement


Loops

•  Loops in Java are similar to those in other high-‐level languages

•  Java has three types of loop statements: the while, the do-while, and the for statements –  The code that is repeated in a loop is called the body of the loop

–  Each repe33on of the loop body is called an itera%on of the loop

3-‐10 CSS 161: Fundamentals of

Compu3ng

while (Boolean_Expression) Statement Or while (Boolean_Expression) { Statement_1 Statement_2

Statement_Last }

while Syntax

. . .


Compu3ng

do Statement while (Boolean_Expression); Or do { Statement_1 Statement_2

Statement_Last } while (Boolean_Expression);

do-while Syntax

. . .


Compu3ng

The for loop

The for Statement Syntax

for (Initializing; Boolean_Expression; Update) Body

•  The Body may consist of a single statement or a list of statements enclosed in a pair of braces ({ })

•  Note that the three control expressions are separated by two, not three, semicolons

•  Note that there is no semicolon aeer the closing parenthesis at the beginning of the loop


Compu3ng

for loop syntax for (ini(aliza(on; test; update) { statement; statement; ... statement; }

–  Perform ini(aliza(on once.

–  Repeat the following: •  Check if the test is true. If not, stop. •  Execute the statements.

•  Perform the update.

body

header

Seman3cs of the for Statement


Compu3ng

Repe33on with for loops

•  So far, repea3ng a statement is redundant: System.out.println("Homer says:"); System.out.println("I am so smart"); System.out.println("I am so smart"); System.out.println("I am so smart"); System.out.println("I am so smart"); System.out.println("S-M-R-T... I mean S-M-A-R-T");

•  Java's for loop statement performs a task many 3mes. System.out.println("Homer says:");

for (int i = 1; i <= 4; i++) { // repeat 4 times System.out.println("I am so smart"); }

System.out.println("S-M-R-T... I mean S-M-A-R-T");

Ini3aliza3on for (int i = 1; i <= 6; i++) { System.out.println("I am so smart"); }

•  Tells Java what variable to use in the loop –  Performed once as the loop begins

–  The variable is called a loop counter

•  can use any name, not just i

•  can start at any value, not just 1

Test for (int i = 1; i <= 6; i++) { System.out.println("I am so smart"); }

•  Tests the loop counter variable against a limit

–  Uses comparison operators:

< less than <= less than or equal to

> greater than

>= greater than or equal to

Repe33on over a range System.out.println("1 squared = " + 1 * 1); System.out.println("2 squared = " + 2 * 2); System.out.println("3 squared = " + 3 * 3); System.out.println("4 squared = " + 4 * 4); System.out.println("5 squared = " + 5 * 5); System.out.println("6 squared = " + 6 * 6);

–  Intui3on: "I want to print a line for each number from 1 to 6"

•  The for loop does exactly that! for (int i = 1; i <= 6; i++) { System.out.println(i + " squared = " + (i * i)); }

–  "For each integer i from 1 through 6, print ..."

Loop walkthrough

for (int i = 1; i <= 4; i++) { System.out.println(i + " squared = " + (i * i)); } System.out.println("Whoo!");

Output:

1 squared = 1 2 squared = 4 3 squared = 9 4 squared = 16 Whoo!

1

1

2

2

3

3

4

4

5

5

Increment and decrement shortcuts to increase or decrease a variable's value by 1

Shorthand Equivalent longer version variable++; variable = variable + 1; variable--; variable = variable - 1;

int x = 2; x++; // x = x + 1; // x now stores 3

double gpa = 2.5; gpa--; // gpa = gpa - 1; // gpa now stores 1.5

Modify-‐and-‐assign

shortcuts to modify a variable's value

Shorthand Equivalent longer version variable += value; variable = variable + value; variable -= value; variable = variable - value; variable *= value; variable = variable * value; variable /= value; variable = variable / value; variable %= value; variable = variable % value;

x += 3; // x = x + 3;

gpa -= 0.5; // gpa = gpa - 0.5;

number *= 2; // number = number * 2;

The Comma in for Statements

•  A for loop can contain mul3ple ini3aliza3on ac3ons separated with commas –  Use cau3on when combining a declara3on with mul3ple ac3ons

•  A for loop can contain mul3ple update ac3ons, separated with commas, also –  It is even possible to eliminate the loop body in this way

•  However, a for loop can contain only one Boolean expression to test for ending the loop


Compu3ng

Infinite Loops

•  A while, do-while, or for loop should be designed so that the Boolean expression is changed in a way that eventually makes it false, and terminates the loop

•  If the Boolean expression remains true, then the loop will run forever, resul3ng in an infinite loop –  Loops that check for equality or inequality (== or !=) are especially prone to this error and should be avoided if possible


Compu3ng

Nested Loops

•  Loops can be nested, just like other Java structures –  When nested, the inner loop iterates from beginning to end for each

single itera3on of the outer loop

int rowNum, columnNum; for (rowNum = 1; rowNum <=3; rowNum++) { for (columnNum = 1; columnNum <=2; columnNum++) System.out.print(" row " + rowNum + " column " + columnNum); System.out.println(); }


Compu3ng

Algorithms and Pseudocode

•  The hard part of solving a problem with a computer program is coming up with the underlying solu3on method

•  An algorithm is a set of precise instruc3ons that lead to a solu3on –  An algorithm is normally wriVen in pseudocode, which is a mixture of programming language and a human language, like English

–  Pseudocode must be precise and clear –  However, pseudocode is much less rigid than code


Compu3ng

Example: Drawing complex figures • Use nested for loops to produce the following output.

• Why draw ASCII art? – Real graphics require a lot of finesse – ASCII art has complex paVerns – Can focus on the algorithms

#================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================#

Development strategy •  Recommenda3ons for managing complexity: 1. Design the program (think about steps or methods needed -‐ pseudocode).

•  write an English descrip3on of steps required •  use this descrip3on to decide the methods

2. Create a table of paVerns of characters •  use table to write your for loops

#================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================#

1. Pseudo-‐code

•  pseudo-‐code: An English descrip3on of an algorithm.

•  Example: Drawing a 12 wide by 7 tall box of stars

print 12 stars. for (each of 5 lines) {

print a star.

print 10 spaces.

print a star.

}

print 12 stars.

************ * * * * * * * * * * ************

Pseudo-‐code algorithm 1. Line

•  # , 16 =, # 2. Top half

•  | •  spaces (decreasing) •  <> •  dots (increasing) •  <> •  spaces (same as above) •  |

3. BoVom half (top half upside-‐down) 4. Line

•  # , 16 =, #

#================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================#

Methods from pseudocode public class Mirror { public static void main(String[] args) { line(); topHalf(); bottomHalf(); line(); }

public static void topHalf() { for (int line = 1; line <= 4; line++) { // contents of each line } }

public static void bottomHalf() { for (int line = 1; line <= 4; line++) { // contents of each line } }

public static void line() { // ... } }

2. Tables •  A table for the top half:

– Compute spaces and dots expressions from line number

line spaces dots

1 6 0

2 4 4

3 2 8

4 0 12

line spaces line * -2 + 8 dots 4 * line - 4

1 6 6 0 0

2 4 4 4 4

3 2 2 8 8

4 0 0 12 12

#================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================#

3. Wri3ng the code • Useful ques3ons about the top half:

– What methods? (think structure and redundancy)

– Number of (nested) loops per line? #================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================#

Par3al solu3on // Prints the expanding pattern of <> for the top half of the

figure. public static void topHalf() { for (int line = 1; line <= 4; line++) { System.out.print("|");

for (int space = 1; space <= (line * -2 + 8); space++) { System.out.print(" "); }

System.out.print("<>");

for (int dot = 1; dot <= (line * 4 - 4); dot++) { System.out.print("."); }

System.out.print("<>");

for (int space = 1; space <= (line * -2 + 8); space++) { System.out.print(" "); }

System.out.println("|"); } }

The break and continue Statements

•  The break statement consists of the keyword break followed by a semicolon –  When executed, the break statement ends the nearest

enclosing switch or loop statement •  The continue statement consists of the keyword continue followed by a semicolon –  When executed, the continue statement ends the current

loop body itera3on of the nearest enclosing loop statement –  Note that in a for loop, the continue statement transfers

control to the update expression

•  When loop statements are nested, remember that any break or con3nue statement applies to the innermost, containing loop statement


Compu3ng

The Labeled break Statement

•  There is a type of break statement that, when used in nested loops, can end any containing loop, not just the innermost loop

•  If an enclosing loop statement is labeled with an Iden%fier, then the following version of the break statement will exit the labeled loop, even if it is not the innermost enclosing loop: break someIdentifier;

•  To label a loop, simply precede it with an Iden%fier and a colon: someIdentifier:


Compu3ng

The exit Statement

•  A break statement will end a loop or switch statement, but will not end the program

•  The exit statement will immediately end the program as soon as it is invoked: System.exit(0);

•  The exit statement takes one integer argument –  By tradi3on, a zero argument is used to indicate a normal ending of the program


Compu3ng

Loop Bugs

•  The two most common kinds of loop errors are unintended infinite loops and off-‐by-‐one errors –  An off-‐by-‐one error is when a loop repeats the loop body one too many or one too few 3mes

•  This usually results from a carelessly designed Boolean test expression

–  Use of == in the controlling Boolean expression can lead to an infinite loop or an off-‐by-‐one error

•  This sort of tes3ng works only for characters and integers, and should never be used for floa3ng-‐point


Compu3ng

Tracing Variables

•  Tracing variables involves watching one or more variables change value while a program is running

•  Many IDEs have a built-‐in Debugging u3lity that allows variables to be traced

•  Another way to trace variables is to insert temporary output statements in a program System.out.println("n = " + n); // Tracing n –  When the error is found and corrected, the trace statements can

simply be commented out

•  A good way to test your program is to manually plug-‐in values and walk through your code –  Common prac3ce in interviews


Compu3ng

General Debugging Techniques (1) •  Examine the system as a whole – don’t assume the bug occurs in one par3cular place – Manually tracing helps significantly and is good prac%ce for exams and interviews

•  Try different test cases and check the input values


Compu3ng

General Debugging Techniques (2) •  Comment out blocks of code to narrow down the offending code

•  Check common pinalls

•  Take a break and come back later •  DO NOT make random changes just hoping that the change will fix the problem!


Compu3ng

Debugging Example (1 of 9) •  The following code is supposed to present a menu and get user input un3l either ‘a’ or ‘b’ is entered.


Compu3ng

String s = ""; char c = ' '; Scanner keyboard = new Scanner(System.in);

do { System.out.println("Enter 'A' for option A or 'B' for option B."); s = keyboard.next(); s.toLowerCase(); c = s.substring(0,1); } while ((c != 'a') || (c != 'b'));

Debugging Example (2 of 9)

•  Using the “random change” debugging technique we might try to change the data type of c to String, to make the types match

•  This results in more errors since the rest of the code treats c like a char


Compu3ng

Result: Syntax error:

c = s.substring(0,1); : incompatible types found: java.lang.String required: char

Debugging Example (3 of 9) •  First problem: substring returns a String, use charAt to get the first character:


Compu3ng

String s = ""; char c = ' '; Scanner keyboard = new Scanner(System.in);

do { System.out.println("Enter 'A' for option A or 'B' for option B."); s = keyboard.next(); s.toLowerCase(); c = s.charAt(0); } while ((c != 'a') || (c != 'b'));

Now the program compiles, but it is stuck in an infinite loop. Employ tracing:



Compu3ng

do { System.out.println("Enter 'A' for option A or 'B' for option B."); s = keyboard.next(); System.out.println("String s = " + s); s.toLowerCase(); System.out.println("Lowercase s = " + s); c = s.charAt(0); System.out.println("c = " + c); } while ((c != 'a') || (c != 'b'));

Sample output: Enter 'A' for option A or 'B' for option B. A String s = A Lowercase s = A c = A Enter 'A' for option A or 'B' for option B.

From tracing we can see that the string is never changed to lowercase. Reassign the lowercase string back to s.

Debugging Example (5 of 9) •  The following code is supposed to present a menu and get user input un3l either ‘a’ or ‘b’ is entered.


Compu3ng

do { System.out.println("Enter 'A' for option A or 'B' for option B."); s = keyboard.next(); s = s.toLowerCase(); c = s.charAt(0); } while ((c != 'a') || (c != 'b'));

However, it’s still stuck in an infinite loop. What to try next?

Debugging Example (6 of 9) •  Could try the following “patch”


Compu3ng

do { System.out.println("Enter 'A' for option A or 'B' for option B."); s = keyboard.next(); s = s.toLowerCase(); c = s.charAt(0); if ( c == 'a') break; if (c == 'b')

break; } while ((c != 'a') || (c != 'b'));

This works, but it is ugly! Considered a coding atrocity, it doesn’t fix the underlying problem. The boolean condition after the while loop has also become meaningless. Try more tracing:



Compu3ng

do { System.out.println("Enter 'A' for option A or 'B' for option B."); s = keyboard.next(); s = s.toLowerCase(); c = s.charAt(0); System.out.println("c != 'a' is " + (c != 'a')); System.out.println("c != 'b' is " + (c != 'b')); System.out.println("(c != 'a') || (c != 'b')) is "

+ ((c != 'a') || (c != 'b'))); } while ((c != 'a') || (c != 'b'));

Sample output: Enter 'A' for option A or 'B' for option B. A c != 'a' is false c != 'b' is true (c != 'a') || (c != 'b')) is true

From the trace we can see that the loop’s boolean expression is true because c cannot be not equal to ‘a’ and not equal to ‘b’ at the same time.

Debugging Example (8 of 9) •  Fix: We use && instead of ||


Compu3ng

do { System.out.println("Enter 'A' for option A or 'B' for option B."); s = keyboard.next(); s = s.toLowerCase(); c = s.charAt(0); } while ((c != 'a') && (c != 'b'));

Debugging Example (9 of 9) •  Even beVer: Declare a boolean variable to control the do-‐while loop. This makes it clear when the loop exits if we pick a meaningful variable name.


Compu3ng

boolean invalidKey; do { System.out.println("Enter 'A' for option A or 'B' for option B."); s = keyboard.next(); s = s.toLowerCase(); c = s.charAt(0); if (c == 'a')

invalidKey = false; else if (c == 'b')

invalidKey = false; else

invalidKey = true; } while (invalidKey);

Preven3ve Coding

•  Incremental Development – Write a liVle bit of code at a 3me and test it before moving on

•  Code Review – Have others look at your code

•  Pair Programming – Programming in a team, one typing while the other watches, and periodically switch roles


Compu3ng

Loops prac3ce

•  See conversa3on on discussion board on catalyst

53 CSS 161: Fundamentals of Compu3ng

Asser3on Checks

•  An asser%on is a sentence that says (asserts) something about the state of a program –  An asser3on must be either true or false, and should be true if a

program is working properly –  Asser3ons can be placed in a program as comments

•  Java has a statement that can check if an asser3on is true assert Boolean_Expression; –  If asser3on checking is turned on and the Boolean_Expression

evaluates to false, the program ends, and outputs an asser%on failed error message

–  Otherwise, the program finishes execu3on normally


Compu3ng

Asser3on Checks

•  A program or other class containing asser3ons is compiled in the usual way

•  Aeer compila3on, a program can run with asser3on checking turned on or turned off –  Normally a program runs with asser3on checking turned off

•  In order to run a program with asser3on checking turned on, use the following command (using the actual ProgramName): java –enableassertions ProgramName


Compu3ng

Documents

CSS161 Fundamentals#of#Compu3ng# Flow#control#(4)#courses.washington.edu/css161/umurthy/learning-resources/slides/c… · CSS161 Fundamentals#of#Compu3ng# Flow#control#(4)# October17,2012