CSEN 202 Introduction to Computer Programming

Transcription

1 CSEN 202 Introduction to Computer Programming Lecture 4: Iterations Prof. Dr. Slim Abdennadher and Dr Mohammed Abdel Megeed Salem, German University Cairo, Department of Media Engineering and Technology February 24 - March 1, Synopsis 1.1 What you learned so far Previous topics What is Java, how to compile and run Hello Wld! Primitive datatypes and their properties (byte, sht, int, long, float, double, boolean, char) Literals and special values Type compatibilities (explicit and implicit cast) Simple expressions (+, -, *, /, %, &,,, >>,... ), their properties, etc. Operand and result types, precedence, etc. Assignments (expression with essential side effect), memy changing expressions (++, --, +=,... ) Previous topics Blocks ({... ) f structuring the program The break-instruction f leaving a Branching and decision constructs if (), switch () { s,? expression : expression. 1

2 2 Loop constructs 2.1 Overview Today s topic iterative constructs What is life? Life is just one damn thing after another. Mark Twain Life isn t just one damn thing after another... it is the same damn thing over and over again. Looping Looping causes computer to execute section of code repeatedly Edna St. Vincent Millay We use boolean expressions ( and ) as loop ; when boolean is, loop equals exit and loop is terminated As with als, this section of code can be single multiple s enclosed in curly braces (s) We call the section of code executed the loop s body The loops Java offers three different iterative constructs: The while-loop, The do-while-loop, and The f-loop They differ in the relation between loop and loop body 2

3 3 While and do 3.1 While The while loop Fmat while () A while-loop executes the loop body (a a ) as long as the loop is. The must be of type boolean Befe every execution of the loop body, the loop is evaluated. As soon as the evaluates to, the loop terminates. Note: The loop body may not be executed at all. The while loop schema while 3

4 while while while 4

5 while How to construct a while loop 1. Fmulate the test which tells you whether the loop needs to be run again count <= 3 2. Fmulate the actions f the loop body which take you one step closer to termination { System.out.println( "count is: " + count ); count = count + 1; // add one to count 3. In general, initialization is required befe the loop and some postprocessing after the loop int count = 1; How to construct a while loop class WhileExample { public static void main (String[] args ) { int count = 1; // start count out at one while ( count <= 3 ) // loop while count is <= 3 { System.out.println( "count is: " + count ); count = count + 1; // add one to count System.out.println( "Done with the loop" ); 5

6 while loop example Example 1 (Investment with compound interest). Invest 10000C with 5% interest compounded annually: Year Balance 0 10, , , , , Question. When will the balance be at least Euro? while loop example while while balance< targetbalance increment years add interest to balance while loop example class InvestmentTest { public static void main (String[] args ) { double balance = 10000; 6

7 double rate = 5; double targetbalance = 20000; int year = 0; while (balance < targetbalance) { year++; double interest = balance * rate / 100; balance = balance + interest; System.out.println("The investment doubled after"+ year +"years"); 3.2 Do The do loop Fmat do while (); A do-while-loop executes the loop body ( ) once and then repeats as long as the is. The must be of type boolean The do loop Fmat do while (); After every execution of the loop body, the loop is evaluated. As soon as the evaluates to, the loop terminates. Note: The loop body is executed at least once. The do loop schema 7

8 do while do while do while 8

9 do while do while do while do loop example Example 2 (Validating an input). Task: Accept only a positive integer 9

10 do loop example do while do while read input value value <= 0 do loop example class ValidateInput { public static void main (String[] args ) throws IOException { BufferedReader userin = new BufferedReader (new InputStreamReader(System.in)); String inputdata; int value; // data entered by the user do { System.out.println( "Please enter a positive number: " ); inputdata = userin.readline(); value = Integer.parseInt( inputdata ); while (value >= 0); System.out.println( "Entered negative number: " + value ); 3.3 Comparison Comparing while and do loops 10

11 In both loops Stops executing body if loop is you must make sure loop becomes by some computations Infinite loop means your loop is such that it will never turn (i. e., the exit never occurs) do-while while body always executed at least once loop tested at bottom of loop may not execute at all loop tested befe body; loop variables must be set befe loop entry Comparing while and do loops while-loops and do-while-loops can be transfmed to each other do-while to while do { 1; 2; while (); 1; 2; while () { 1; 2; Comparing while and do loops while-loops and do-while-loops can be transfmed to each other do-while to while while () { 1; 2; if () do { 1; 2; while (); 4 A me comftable loop 4.1 F The f loop Fmat f (initialization; ; update) Most common loop construct: just repeats a f a fixed number of times (counting loop) 11

12 The initialization is an expression f setting initial value of the loop counter. The must be of type boolean The update expression modifies the loop counter Purpose: To execute an initialization, then keep executing and updating an expression while a is. The f loop schema f initialization update f initialization update 12

13 f initialization update f initialization update f initialization update 13

14 f initialization update f initialization update f loop example Example 3 (Investment with compound interest). Invest Euro with 5% interest compounded annually. Question: What will be the balance after n years? f loop example 14

15 f initialization update f i = 0 i < n i++ add interest to balance f loop example class Balance { public static void main (String[] args ) { double balance = 10000; double rate = 5; int year = 15; f (int i = 0; i < year; i++) { double interest = balance * rate / 100; balance = balance + interest; System.out.println("The investment after"+ year + "will be" + balance); Another f loop example Print a string backwards Recall what you have learned about strings String is not a primitive type, it is a class. The instances of a class are called objects 15

16 Objects provide their own functionality We can use the dot-operat. and the methods length (), and charat (position) Another f loop example Print a string backwards public class Reverse { public static void main (String[] args) { String wd = "Slim"; if (wd == null) { return; int max = wd.length (); f (int i=max-1; i >=0; i--) { System.out.print (wd.charat (i)); System.out.println (""); 4.2 Comparison Comparing while and f loops In general a while-loop has the fm initialization; while () { ce loop body update/advancement This is exactly matched by the f-loop f (initialization; ; update/advancement) { ce loop body 5 Summary 5.1 General comparison Choosing the right loop The f-loop is called definite loop because you can typically predict how many times it will loop. while- and do-loops are indefinite loops, as you do not know a prii when they will end. The f-loop is typically used f math-related loops like counting finite sums. while-loop is good f situations where the could turn at any time. 16

17 do is used in same kind of situation as while loop, but when the body of the loop should execute at least once. When me than one type of loop will solve problem, use cleanest, simplest one 6 Nested loops 6.1 Multiplication table Task Write an algithm that will print the multiplication table f the numbers from 1 to n. F example let n = 4: Solution Loop over all rows: int i = 1; while (i <= n) {... i++; Build an individual row: int j = 1; while (j <= n) { System.out.print (i * j + " "); j++; System.out.println (""); Solution Putting the elements together: public class MultTable { public static void main(string[] args) { int n = 5; int i = 1; while (i <= n) { int j = 1; while (j <= n) { System.out.print (i * j + " "); j++; System.out.println (""); i++; 17

18 Solution An alternative solution: public class MultTable2 { public static void main(string[] args) { int n = 5; f (i = 1; i <= n; i++) { f (j = 1; j <= n; j++) System.out.print (i * j + " "); System.out.println (""); 6.2 Triangle pattern Task Print a triangle pattern using an increasing number of brackets. F example (5 rows): [] [][] [][][] [][][][] [][][][][] Solution Loop over all rows:... f (int i = 1; i <= n; i++) { Build an individual triangle-row: f (int j = 1; j <= i; j++) r = r + "[]"; r = r + "\n"; Solution Putting the elements together: class Triangle { public static void main (String[] args ) { String r = ""; int n = 10; f (int i = 1; i <= n; i++) { f (int j = 1; j <= i; j++) r = r + "[]"; r = r + "\n"; System.out.print (r); 18

19 7 Coming up 7.1 Next week Next week s events The next topic will be the concept of procedures and methods. 19