COMP 14 Introduction to Programming

COMP 14Introduction to Programming Adrian Ilie July 5, 2005

Writing Selection StatementsWhat Type of Construct To Use? • Only need to execute additional statements if condition is true • Need to execute separate additional statements based on if the condition is true or false • Need to execute different statements based on multiple conditions • Need to execute different statements based on an expression that evaluates to a char or int if if-else nested if-else switch

Writing Selection Statements • Write the outline of the selection statement • keywords • curly braces • case and break statements (for switch) • Write the expression • boolean expression, or condition • expression or variable evaluating to char or int • Write statements that execute based on the condition

Example Write a selection statement that sets grade to 'F' if score is less than 60 and sets grade to 'P' if score is greater than or equal to 60. 2) Write outline if ( ) { } else { } 1) Choose construct if-else

Example Write a selection statement that sets grade to 'F' if score is less than 60 and sets grade to 'P' if score is greater than or equal to 60. 4) Add statements if (score < 60) { grade = 'F'; } else { grade = 'P'; } 3) Add condition if (score < 60) { } else { }

Questions What type of selection statement should be used? • Print "Male" if gender is 'M' and "Female" if gender is 'F'. • Print the state associated with areaCode, where areaCode is an int. • Print the maximum of three integers. if-else switch nested if-else

Question Assume that the lengths of 3 sides of a triangle are given in the integer variables side1, side2, and side3. Write code that will print: • "Equilateral" if all three sides are equal • "Isosceles" if only two sides are equal • "Scalene" if no sides are equal equilateral isosceles scalene First, what selection construct should be used? nested if-else

Answer // side1, side2, and side3 are lengths // of the sides of a triangle if(side1 != side2 && side2 != side3 && side1 != side3) // no sides are equal System.out.println ("Scalene"); else if(side1 == side2 && side2 == side3) // all sides are equal System.out.println ("Equilateral"); else // two sides are equal System.out.println ("Isosceles");

Today • Repetition statements • while loops • for loops

Why Is Repetition Needed? • Want to add 5 integers to find their average? • declare a variable for each integer • initialize the sum • read in the user input one at a time • sum the numbers • take the average • Want to add 1000 integers to find their average? • declare one variable for input • initialize the sum • read in one line of user input • add to the sum • take the average repeat steps 3 and 4 1000 times

Repetition Statements • Allow us to execute a statement multiple times • Often referred to as loops • Controlled by boolean expressions • like selection, or conditional, statements • Java has three kinds of repetition statements: • the while loop • the for loop • the do loop

Loops • Must use a loop control variable • controls how many times to loop • 4 Parts to Every Loop • initialization - set loop control variable before condition • condition - when to stop • update - change the loop control variable • body - actions to repeat

Typical Uses of Loops • Repeat a section of code a specified number of times - counter-controlled • Repeat a section of code (reading input) until a specific value is read - sentinel-controlled • Repeat a section of code (reading input) until a valid value is entered - input validation • Repeat a section of code (reading from a file) until the end of the file is reached - EOF-controlled • Repeat a section of code until a boolean variable becomes false - flag-controlled

while is a reserved word If theconditionis true, theloop bodyis executed. Then theconditionis evaluated again. The while LoopSyntax while (condition) { loop body; } Theloop bodyis executed repeatedly until the conditionbecomes false.

The while Loop • Syntax while (expression) statement • Expression is always true in an infinite loop • Statements must change value of expression to false

The while LoopExample final int LIMIT = 3; int count = 0; while (count < LIMIT) { System.out.println (count); count++; } System.out.println (“All done!”); boolean condition Output: 0 1 2 All done! initialization loop body update

final int LIMIT = 3; int count = 0; while (count < LIMIT) { System.out.println (count); count++; } System.out.println (“All done!”); LIMITcount Output: The while LoopWhat's Going On? 3 0 1 2 3 0 1 2 All done!

The while Loop final int LIMIT = 3; int count = 0; while (count < LIMIT) { count++; System.out.println (count); } System.out.println (“All done!”); Output: 1 2 3 All done!

The while Loop Output: 0 0 0 0 0 0 0 0 0 0 ... final int LIMIT = 3; int count = 0; while (count < LIMIT) { System.out.println (count); } System.out.println (“All done!”);

The while Loop final int LIMIT = 3; int count = 0; while (count <= LIMIT) { System.out.println (count); count++; } System.out.println (“All done!”); Output: 0 1 2 3 All done!

The while Loop • If the condition of a while statement is false initially, the loop body is never executed • The body of a while loop will execute zero or more times

Counter-Controlled while Loop • Used when we know exactly the number of times to execute the loop • Basic Form: counter = 0; while(counter < N) { ... counter++; ... } N is the number of times the loop should execute

Example final int NUM_STUDENTS = 100; int sum = 0; double average; int i = 0; // initialize loop control var while (i < NUM_STUDENTS) { sum += Integer.parseInt(inFile.readLine()); i++; // update loop control variable } average = (double) sum / NUM_STUDENTS; Often we use i, j, k as counter variable names.

Reading From a File • How do you know how many items there will be? • ask the user • arrange the data file so that the first item is the number of items in the file • Other ways to read all of the items: • use sentinel value • read until end-of-file (EOF)

Sentinel-Controlled while Loop • Used when exact number of entry pieces is unknown but last entry (special / sentinel value) is known • Basic Form: input the first data item into variable while(variable != sentinel) { ... input a data item into variable }

Example int i = 0, sum = 0, score; final int SENTINEL = -99; double average; // initialize the loop control variable score = Integer.parseInt(inFile.readLine()); while (score != SENTINEL) { sum += score; i++; // update the loop control variable score = Integer.parseInt(inFile.readLine()); } average = (double) sum / i;

Average.java Example • Read in integers and print their sum until the user enters 0 • Print the average of the numbers entered

Input Validation while Loop • Used to ensure that the user enters valid input • Basic Form: input a data item into variable while(variable is not in valid range) { ask the user for valid input input a data item into variable }

Example int num; // initialize loop control variable System.out.print (“Enter a number [0-100]: "); num = Integer.parseInt(keyboard.readLine()); while ((num < 0) || (num > 100)) { System.out.println (num + " is not [0-100]"); System.out.print (“Enter a number: “); // update the loop control variable num = Integer.parseInt(keyboard.readLine()); } System.out.println (“You entered “ + num);

Flag-Controlled while Loop • Boolean value used to control loop • Basic Form: boolean found = false; while (!found) { ... if(expression) found = true; ... }

EOF-Controlled while Loop • Used when input is from a file • Sentinel value is not always appropriate • Basic Form: inputLine = inFile.readLine(); while (inputLine != null) { ... inputLine = inFile.readLine(); } null is a reserved word

Average.java Example • Modify example to read from a file

To do • Finish assignment 3 • Bring laptop. • Study for Quiz! • Writing a whole program • Variables and classes • String, StringTokenizer • GUI • Relational and boolean operators • if, if-else, switch while, for loops  not in the quiz!!!

COMP 14 Introduction to Programming