CSE 114 Computer Science I

Transcription

1 CSE 114 Computer Science I Iteration Cape Breton, Nova Scotia

2 What is Iteration? Repeating a set of instructions a specified number of times or until a specific result is achieved How do we repeat steps? Imagine 3 instructions A, B, & C Instruction C can be jump A, meaning go back to A Iteration might result in: Execute A Execute B Execute C Execute A

3 Why use iteration? To make our code more practical and efficient To make our code more flexible and dynamic

4 For Example How would we write code to print N! (factorial), where N is a number entered by the user? Without iteration (or recursion) this would be impractical! We do not know N, when we are about to write the program.

5 Is this practical? Without iteration or recursion: System.out.print("Enter N: "); int N = Keyboard.readInt(); int factorial = 1; if ((N == 1) (N == 0)) factorial = 1; else if (N == 2) factorial = 2 * 1; else if (N == 3) factorial = 3 * 2 * 1; else if (N == 4) factorial = 4 * 3 * 2 * 1; else if (N == 5) factorial = 5 * 4 * 3 * 2 * 1; System.out.println(factorial); Inefficient coding!

6 import java.util.scanner; public class FactorialPrinter public static void main(string[] args) Scanner input = new Scanner(System.in); System.out.print("Enter N: "); int N = input.readint(); int factorial = calculatefactorial(n); System.out.println(factorial); return type Using Iteration and Methods Method argument type and name public static int calculatefactorial(int n) int count = n; int fact = 1; while (count > 0) fact *= count--; return fact; return value Helper method

7 Can Anyone guess what 1000! is? Impossible to calculate using Java s primitive data types, instead we would have to use the BigInteger class double type can estimate up until 170! ( E306)

8 Any exceptions? All programs need iteration one and done programs Why do they all need iteration? because the start up, then wait for the user to act Most programs work as follows: 1. Initialize variables 2. Present options to user 3. Wait for user input 4. Respond to user input 5. Go back to step 2

9 GUIs and Iteration Think of a GUI. How does it work? 1. Draw the GUI 2. Check for input from user 3. Respond to user input, if any 4. Go back to Step 1

10 Java and iteration We have 3 types of iterative statements a while loop a do while loop a for loop All 3 can be used to do similar things Which one should you use? a matter of individual preference/convenience

11 The while Statement Syntax Diagram: while ( boolean Expr ) Statement Statement Flowchart: false true boolean Expr Statement(s) Loop Condition Loop Body

12 Bracketing is again important Use either: while (boolean expression) statement; Or: Be careful NOT to put a semicolon, ;, here while (boolean expression) statement; statement;

13 while statement example What does this method do? public static String whatdoesthisdo(int x, char c) // null character "" String result = ""; while (x > 0) result = result + c; x--; return result; What do we get if we call: String s; s = whatdoesthisdo(5,'a'); System.out.println(s); s = whatdoesthisdo(0,'a'); System.out.println(s); AAAAA

14 while statement example 2 What does this method do? public static String whatdoesthisdo2(int x, char c) String result = ""; while (x > 0) result = result + c; x--; c++; return result; What do we get if we call: String s; s = whatdoesthisdo2(5, 'A'); System.out.println(s); ABCDE

15 while statement example 3 What does this method do? public static String whatdoesthisdo3(int x, char c) String result = ""; while (x > 0) result = c + result; x--; c++; return result; What do we get if we call: String s; s = whatdoesthisdo3(5, 'A'); System.out.println(s); EDCBA

16 Using a flag A flag is a boolean loop control boolean flag = true; How does it work? flag used as loop condition inside the loop, test for ending condition when condition is reached, turn flag off once turned off, loop ends

17 flag example public static int factorial(int N) boolean moreworkflag = true; int factorial = 1; while (moreworkflag) factorial *= N; N--; if (N == 1) moreworkflag = false; return factorial; What do we get if we call: int fact; fact = factorial(5); System.out.println(fact); 120

18 The do-while Statement Syntax Diagram: do Statement Statements while ( boolean Expr ) ; Flowchart: Loop Body Statement(s) Loop Condition boolean Expr false true

19 Bracketing is again important Use either: do statement; while (boolean expression); Or: do You MUST put a semicolon, ;, here statement; statement; while (boolean expression); NOTE: though they can be used to do similar operations, while statements and do-while statements are different Java constructs.

20 do while statement example What does this method do? public static String whatdoesthisdowhiledo( String result = ""; do result += c; x--; while (x > 0); return result; int x, char c) What do we get if we call: String s; s = whatdoesthisdowhiledo(5,'a'); System.out.println(s); s = whatdoesthisdowhiledo(0,'a'); System.out.println(s); AAAAA A

21 How could we fix this? public static String whatdoesthisdowhiledo String result = ""; if (x > 0) do result += c; x--; while (x > 0); return result; (int x, char c) Now what s the output? String s; s = whatdoesthisdowhiledo(5,'a'); System.out.println(s); s = whatdoesthisdowhiledo(0,'a'); System.out.println(s); AAAAA

22 Why use do while? So what s the point? For when you have a loop body that must execute at least once Example: a program menu

23 Console Program Menu Example String selection; PrintStream out = System.out; Scanner in = new Scanner(System.in); int counter = 0; do out.println("choose a Menu Option:"); out.println("p) Print Counter"); out.println("q) Quit"); out.print("enter: "); selection = in.nextline(); if (selection.touppercase().equals("p")) out.println("counter: " + counter++); while(!selection.touppercase().equals("q")); out.println("goodbye!");

24 Choose a Menu Option: P) Print Counter Q) Quit ENTER: P Counter: 0 Choose a Menu Option: P) Print Counter Q) Quit ENTER: A Choose a Menu Option: P) Print Counter Q) Quit ENTER: P Counter: 1 Choose a Menu Option: P) Print Counter Q) Quit ENTER: Q An Example Session

25 The for Statement Syntax Diagram: for ( Init. Operation ; boolean Expr ; Update Operation ) Statement Statement

26 Flowchart: The for Statement Initialization false Boolean Expr Update true Statement(s)

27 A for loop example public static int indexof(string s, char c) for (int i = 0; i < s.length(); i++) if (s.charat(i) == c) return i; return -1; What s the output? String word = "London Calling"; int index = indexof(word, 'd'); System.out.println(index); index = indexof(word, 'x'); System.out.println(index); index = indexof(word, 'n'); System.out.println(index);

28 for loops and counting for loops are popular for counting loops through the indices of a string through the indices of an array (later) through iterations of an algorithm Good for algorithms that require a known number of iterations counter-controlled loops

29 Do you understand the order of operations? for (int i = 0; i < 10; i++) System.out.print(i); 1) int i = 0; 2) i < 10; 3) System.out.print(i); 4) i++; 5) i < 10; 6) System.out.print(i); 7) i++; 8) i < 10; 9) System.out.print(i); 10) i++; 11) i is 0 i is 1 i is 2 i is 3

30 Examples of for Statements int sum = 0; for (int i=1; i<=4; i++) sum = sum + i; sum i int sum = 0; for (int j=1; j<=4; j++) sum = sum + j; j++; sum j Be careful not to double the update of your counting variable

31 Examples of for Statements int product = 1; for (int i=1; i<4; i+=1) product *= i; Can be used equivalently in many cases int product = 1; for (int j = 1; j<= 3; j+=1) product *= j; product i product j

32 Equivalence of Loops for ( INIT ; BOOL_EXPR; UPDATE ) LOOP_BODY INIT if ( BOOL_EXPR ) INIT while ( BOOL_EXPR ) LOOP_BODY UPDATE do LOOP_BODY UPDATE while ( BOOL_EXPR );

33 Are these loops equivalent? int sum = 0, count = 0; while (count < x) sum += y; count++; System.out.println("sum is " + sum); int sum = 0, count = 0; for (count = 0; count < x; count++) sum += y; System.out.println("sum is " + sum); int sum = 0, count = 0; do sum += y; count++; while (count < x); System.out.println("sum is " + sum); Test using: x = 5, y = 3 OUTPUT: Sum is 15 Sum is 15 Sum is 15 x = 0, y = 3 OUTPUT: Sum is 0 Sum is 0 Sum is 3 Not Equivalent!

34 Syntax Diagram: WATCH OUT! ; The empty statement if (score > 21); System.out.println("Bust! You Lose"); if (score > 21 ) System.out.println("Bust! You Lose"); else; System.out.println("Hit or Stay?"); ; This is a valid line of code that does nothing, but can create unexpected problems.

35 The empty statement (cont d) int sum = 0; for (int i = 1; i <= 4; i++); sum = sum + i; int sum = 0, i = 1; while (i <= 4); Infinite loop! sum = sum + i; i = i + 1; Compiler Error

36 Nested Loops for (int i = 1; i <= 10; i++) for (int j = 1; j <= 10; j++) int product = i*j; System.out.print(product + ); System.out.print( \n );

37 Beware Improper Nesting int sum = 0, count = 0; if (x > 0) do sum += y; count++; while (count < x); System.out.println(sum); IS NOT int sum = 0, count = 0; if (x > 0) do sum += y; count++; while (count < x); System.out.println(sum); This loop is regarded as a single statement

38 Advice: Align your brackets Indenting properly is really important: if (flag) if (flag) if (y > 0) if (y > 0) x = 0; x = 0; GOOD BAD

39 Local Variables and Blocks A block (a compound statement) is the set of statements between a pair of matching braces (curly brackets) A variable declared inside a block is known only inside that block it is local to the block, therefore it is called a local variable when the block finishes executing, local variables disappear references to it outside the block cause a compiler error

40 Local Variables and Blocks (cont d) Some programming languages (e.g., C and C++) allow the variable name to be reused outside the local block it is confusing and not recommended, nevertheless, it is allowed However, a variable name in Java can be declared only once for a method although the variable does not exist outside the block, other blocks in the same method cannot reuse the variable's name

41 When and Where to Declare Variables Declaring variables outside all blocks but within the method definition makes them available within all the blocks Good programming Practice: declare variables just before you use them initialize variables when you declare them do not declare variables inside loops it takes time during execution to create and destroy variables, so it is better to do it just once for loops) OK to declare loop counters in the Init field of for loops, e.g. for(int i=0; i < 10; i++) the Init field executes only once, when the for loop is first entered

42 Java Comments Important for you and for others wishing to use your code. Always specify author, version. For classes, explain what the class is and how it is to be used. For methods, explain what it does. For programs, explain the input and output. 2 ways to add comments // Everything after the double slashes is ignored /* Everything inside the slash star is ignored */