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UNC-Chapel Hill COMP 14 - COMP 14 Lecture Notes

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COMP 14 Introduction to ProgrammingWriting Selection Statements What Type of Construct To Use?Writing Selection StatementsExampleSlide 5QuestionsQuestionAnswerTodayWhy Is Repetition Needed?Repetition StatementsLoopsTypical Uses of LoopsThe while Loop SyntaxThe while LoopThe while Loop ExampleThe while Loop What's Going On?Slide 18Slide 19Slide 20Slide 21Counter-Controlled while LoopSlide 23Reading From a FileSentinel-Controlled while LoopSlide 26Average.java ExampleInput Validation while LoopSlide 29Flag-Controlled while LoopEOF-Controlled while LoopSlide 32To doCOMP 14Introduction to ProgrammingMiguel A. OtaduyMay 20, 2004Writing 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 intifif-elsenested if-elseswitchWriting 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 conditionExampleWrite 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 60.1) Choose constructif-else2) Write outlineif ( ){}else{}ExampleWrite 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 60.3) Add conditionif (score < 60){}else{}4) Add statementsif (score < 60){grade = 'F';}else{grade = 'P';}QuestionsWhat type of selection statement should be used?1. Print "Male" if gender is 'M' and "Female" if gender is 'F'.2. Print the state associated with areaCode, where areaCode is an int.3. Print the maximum of three integers.if-elseswitchnested if-elseQuestionAssume 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 equalequilateralisoscelesscaleneFirst, what selection construct should be used?nested if-elseAnswer// side1, side2, and side3 are lengths// of the sides of a triangleif (side1 != side2 && side2 != side3 && side1 != side3)// no sides are equalSystem.out.println ("Scalene");else if (side1 == side2 && side2 == side3)// all sides are equalSystem.out.println ("Equilateral");else// two sides are equalSystem.out.println ("Isosceles");Today •Repetition statements•while loops•for loopsWhy 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?repeat steps 3 and 4 1000 times1. declare one variable for input2. initialize the sum3. read in one line of user input4. add to the sum5. take the averageRepetition 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 loopLoops•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 repeatTypical 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-controlledThe while LoopSyntaxwhile ( condition ){ loop body;}while is areserved wordIf the condition is true, the loop body is executed.Then the condition is evaluated again.The loop body is executed repeatedly untilthe condition becomes false.The while Loop•Syntaxwhile (expression)statement•Expression is always true in an infinite loop•Statements must change value of expression to falseThe while LoopExamplefinal int LIMIT = 3;int count = 0;while (count < LIMIT){System.out.println (count);count++;}System.out.println (“All done!”);booleanconditionloopbodyupdateinitializationOutput:012All done!The while LoopWhat's Going On?final int LIMIT = 3;int count = 0;while (count < LIMIT){System.out.println (count);count++;}System.out.println (“All done!”); LIMIT countOutput:301012All done!23The while Loopfinal int LIMIT = 3;int count = 0;while (count < LIMIT) {count++;System.out.println (count);}System.out.println (“All done!”);Output:123All done!The while Loopfinal int LIMIT = 3;int count = 0;while (count < LIMIT) {System.out.println (count);}System.out.println (“All done!”);Output:0000000000...The while Loopfinal int LIMIT = 3;int count = 0;while (count <= LIMIT) {System.out.println (count);count++;}System.out.println (“All done!”);Output:0123All 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 timesCounter-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 numberof times theloop should executeExamplefinal int NUM_STUDENTS = 100;int sum = 0;double average;int i = 0; // initialize loop control varwhile (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?1. ask the user2. 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


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