COP3502 Programming Fundamentals for CIS Majors 1 Instructor: Parisa Rashidi Agenda Sakai In case of any problems, contact TA before due date! Previously ... Chapter 3 Conditional statements if if..else else if switch Logical &&, operators ||, ^, ! Objectives Chapter 4 Loops while do-while for Input from file Loops Motivation Suppose we want to show “Java is fun!” 100 times. How do we do that? Repeat the following statement 100 times!? System.out.println("Welcome to Java!"); Naïve Solution Naïve solution System.out.println("Welcome System.out.println("Welcome System.out.println("Welcome System.out.println("Welcome System.out.println("Welcome System.out.println("Welcome 100 times! to to to to to to Java!"); Java!"); Java!"); Java!"); Java!"); Java!"); … … … System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); System.out.println("Welcome to Java!"); Better Solution Better solution Using loop int count = 0; while (count < 100) { System.out.println("Welcome to Java"); count++; } Why Loops? 1. To automate the repetition of calculations E.g. compute the profit for “a number of different months” of a company's sale 2. To iterate through data and test for certain condition E.g. Checking input data, until user wants to “quit” 3. To keep attempting for some operation E.g obtaining data from a remote computer over a network) until we succeed while Loop while Loop while(condition) { statement; } 1. 2. 3. If the condition is true, the statement is executed; then the condition is evaluated again … The statement is executed over and over until the condition becomes false. When the loop finishes, control passes to the next instruction in the program, following the closing curly brace of the loop. while while (loop-condition) { // loop-body; int count = 0; while (count < 100) { System.out.println("Welcome to Java!"); Statement(s); count++; } } count = 0; Loop Continuation Condition? true Statement(s) (loop body) (A) false (count < 100)? false true System.out.println("Welcome to Java!"); count++; (B) Trace Program Initialize count int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Trace Program (count < 2) is true int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Trace Program Print “Welcome to Java!” int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Trace Program int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } count is now 1. Trace Program (count < 2) is still true. int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Trace Program Print “Welcome to Java!” int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Trace Program int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } count is now 2. Trace Program (count < 2) is now false. int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Trace Program int count = 0; while (count < 2) { System.out.println("Welcome to Java!"); count++; } Loop exit Caution! No ; at the end of while No ; here! int count = 0; while (count < 100) { System.out.println("Welcome to Java!"); count++; } Caution! The body of a while loop must eventually make the condition false If not, it is an infinite loop, which will execute until the user interrupts the program! int count = 1; while (count > 0) { System.out.println("Welcome to Java!"); count++; } CAUTION! Don’t use floating-point values for equality checking in a loop control! Floating-point values are approximations double item = 1.0; while (item != 0.0) { // No guarantee item will be 0 item -= 0.1; } There is no guarantee that item will be exactly 0: actually an infinite loop! Tip Avoid using literals directly in code as much as possible (good programming style) int count = 0; final int REPEATS = 100; while (count < REPEATS ) { System.out.println(“Java"); count++; } System.out.println(REPEATS + “ Times” ); Tip To repeat a loop 10 times, you generally write a loop “not from 1 to 10”, but “from 0 to 9”. All counting in Java tends to start at zero rather than one. This is a convention that most Java programmers adopt. Again: good programming style Program Write a program that randomly generates an integer between 0 and 100, inclusive. The program prompts the user to enter a number continuously until the number matches the randomly generated number. For each user input, the program tells the user whether the input is too low or too high, so the user can choose the next input intelligently. GuessNumberOneTime Run GuessNumber Run Program Write a program that prompts the user to enter two positive integers and finds their greatest common divisor. GreatestCommonDivisor Run Program Suppose that the tuition for a university is $10,000 this year and tuition increases 7% every year. In how many years will the tuition be doubled? FutureTuition Run Program Often the number of times a loop is executed is not predetermined. You may use an input value to signify the end of the loop. Such a value is known as a sentinel value. Write a program that reads and calculates the sum of an unspecified number of integers. The input 0 signifies the end of the input. SentinelValue Run Program A sentinel-controlled loop can be implemented using a confirmation dialog. The answers Yes or No to continue or terminate the loop. The template of the loop may look as follows int option = 0; while (option == JOptionPane.YES_OPTION) { System.out.println("continue loop"); option = JOptionPane.showConfirmDialog(null, "Continue?"); } SentinelValueUsingConfirmationDialog Run Reading from file Input Redirection What if in previous example we wanted to enter a long list of numbers? We can store data (separated by whitespace) in a text file >java SentinelValue < input.txt This is called “input redirection” Output Redirection We can also send output to a file rather than displaying it on the console >java ClassName > output.txt This is called “output redirection” We can use “input redirection” and “output redirection” in the same command >java SentinelValue < input.txt >output.txt Agenda HW 3 is due today PA 2 is due today HW 4 posted today Previously … Loops while Input java redirection ClassFile <input.txt Output java redirection ClassFile >output.txt Today Loops do-while for Program This example gives a program that generates five questions and reports the number of the correct answers after a student answers all five questions. SubtractionQuizLoop Run do-while Loop do-while Will be executed at least once do { Statement(s) (loop body) // Loop body; Statement(s); } while (loop-condition); true Loop Continuation Condition? false Caution! ; at the end of while int count = 0; do { System.out.println("Welcome to Java!"); count++; } while (count < 100) ; ; here! CAUTION! Do NOT add a semicolon at the end of the while clause: int i=0; while (i < 10); { System.out.println(i); i++; } int i=0; do { System.out.println(i); i++; } while (i<10); BUT in case of do-while ; is required. for Loop for loop int count = 0; while (count < 10) { System.out.println("Welcome to Java!"); count++; } for(int count =0; count < 10; count ++) { System.out.println("Welcome to Java!"); } for loop for (initial-action; continuation-condition; action-after-each-iteration) { // loop body…. } int i; for (i = 0; i < 100; i++) { System.out.println("Welcome to Java!"); } Tip It is common to declare the loop variable at the start of the for loop itself: for( int Count = 0; Count < 10; Count++ ) { ....; } for loop Initial-Action Loop Continuation Condition? i=0 false (i < 100)? true Statement(s) (loop body) true System.out.println( "Welcome to Java"); Action-After-Each-Iteration i++ (A) (B) false More Tips The initial-action statement is carried out once only, at the start of the first time that the loop is entered. The continuation-condition is tested before each execution of the body of the loop, including a test before the very first execution of the loop. The third expression (i.e action-after-eachiteration) is a statement executed after every execution of the loop body, before the next test. It typically increments a counter. for Loop Declare i int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } for Loop Execute initializer i is now 0 int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } for Loop (i < 2) is true since i is 0 int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } for Loop int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Print Welcome to Java for Loop Execute adjustment statement i now is 1 int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } for Loop (i < 2) is still true since i is 1 int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } for Loop int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Print Welcome to Java for Loop Execute adjustment statement i now is 2 int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } for Loop (i < 2) is false since i is 2 int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } for Loop int i; for (i = 0; i < 2; i++) { System.out.println("Welcome to Java!"); } Exit the loop. Execute the next statement after the loop CAUTION! The initial-action in a for loop is a list of zero The action-after-each-iteration in a for loop is or more comma-separated expressions a list of zero or more comma-separated statements Therefore, the following two for loops are correct. They are rarely used in practice, however. for (int i = 1; i < 100; System.out.println(i++)); for (int i = 0, j = 0; (i + j < 10); i++, j++) { // Do something } CAUTION! If the loop-continuation-condition in a for loop is omitted, it is implicitly true. for ( ; ; ) { // Do something } (a) Equivalent while (true) { // Do something } (b) CAUTION! Do NOT add a semicolon at the end of the for clause: Don’t do this! for (int i=0; i<10; i++); { System.out.println("i is " + i); } (here ; is executed as for loop statement, therefore we won’t see any output) Program Write a program that sums a series that starts with 0.01 and ends with 1.0. The numbers in the series will increment by 0.01, as follows: 0.01 + 0.02 + 0.03 and so on. TestSum Run Program Write a program that uses nested for loops to print a multiplication table. MultiplicationTable Run Program The Monte Carlo simulation refers to a technique that uses random numbers and probability to solve problems. This method has a wide range of applications in computational mathematics, physics, chemistry, and finance. This section gives an example of using the Monto Carlo simulation for estimating . y circleArea / squareArea = / 4. 1 -1 1 -1 x can be approximated as 4 * numberOfHits / 1000000. Using circle equation: 𝑥 2 + 𝑦 2 ≤ 1 MonteCarloSimulation Run Which Loop? All loops are equal while (loop-continuation-condition) { // Loop body } Equivalent for ( ; loop-continuation-condition; ) { // Loop body } (a) for (initial-action; loop-continuation-condition; action-after-each-iteration) { // Loop body; } (a) (b) Equivalent initial-action; while (loop-continuation-condition) { // Loop body; action-after-each-iteration; } (b) Which Loop? Some recommendations 1. 2. 3. 4. Use the most intuitive loop If number of repetitions known for If number of repetitions unknown while If should be executed at least once (before testing the condition) do-while Break/continue break causes the loop to be abandoned, and execution continues following the closing curly brace. break while ( i > 0 ) { .... if ( j == .... ) break; // abandon the loop …. } // end of the loop body break will bring you here continue causes the rest of the current round of the loop to be skipped. continue "while" or "do" loop moves directly to the next condition test of the loop. "for" loop moves to the “action-after-eachiteration” expression, and then to the condition test. Continue/break causes the rest of the current round of the loop to be skipped. Break causes the loop to be skipped overall. continue Those two keywords might be useful sometimes But they make programs more difficult to read So use them rarely Use a boolean flag instead Program Examples keywords: for using the break and continue TestBreak.java TestBreak Run TestContinue.java TestContinue Run Program Here is a program for guessing a number. You can rewrite it using a break statement. GuessNumberUsingBreak Run Program Write a program that displays the first 50 prime numbers in five lines, each of which contains 10 numbers. An integer greater than 1 is prime if its only positive divisor is 1 or itself. For example, 2, 3, 5, and 7 are prime numbers, but 4, 6, 8, and 9 are not. PrimeNumber Run